Selecting the correct tire pressure; How airing down motorcycle tires affects contact patch
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- เผยแพร่เมื่อ 27 ก.พ. 2023
- This is a deep dive into airing down tire pressures on large adventure bikes offroad and the effects it has on handling and tire footprint and contact area.
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As an engineer, I love these experiment videos. A lot of common motorcycle beliefs aren't rooted in science, so it's great to see people like Bret get down and dirty with real data.
Beliefs aren't rooted in science. A basic definition of belief is the absence of knowledge; it is a worldview that is chosen due to the social environment a person happens to find himself in. A prime example is religion.
@@nickthequick utter bullshit
@@nickthequick Beliefs can be rooted in truth, lies, science, or ignorance. I believe in good science, but then by your definition science is the absence of knowledge from my perspective.
Not gonna get into the semantic argument, but for sure it's GREAT to see some science done because as you said, a LOT of common knowledge for motorcycles is not at all rooted in science, just random explanations that people guessed at then told other people until everyone is repeating the same "seems plausible" but totally unfounded stuff.
This is the motorcycle content I want more than anything else.
@@nickthequick Most belief is absolutely based in science, or some kind of systematic observation, like belief in God is based in philosophical inquiry and observation of our universe. Whether that belief corresponds to truth, is another matter, which is why more scientific inquiry is employed to try to get closer to truth. Science should never really answer all of our questions absolutely, apart from the basic stuff, like how long an object is, thus ot can never really "be settled", a statement that is profoundly unscientific.
So, more scientific experiments are needed to try to better answer the questions this man here thinks he did. For example, he observed what he deems to be a small change in contact patch area. Cool. How much does that actually affect traction or floatation over a given type of surface? There was no attempt to test this scientifically, yet I feel like an attempt to draw such conclusions was made, which IMO was pseudoscientific. A 10% contact patch change could have a 50% impact on traction, if that could be measured. Another factor that was never considered here was the type of tire casing, sidewall thickness, compound, tread. What other unbeknown to us factors could there be? The more we know, the more questions we have. That is true science IMO.
Thank you for doing this, this must have been a lot of work and we all appreciate it.
Yep. The test tires were a roughly 70% pavement / 30% dirt design, with small grooves between large blocks, such that the surface of the tire is unlikely to shift around much when the internal pressure is reduced. I'd be interested in seeing a test with a more dirt-worthy tire, like a RallZ or Dual Venture. I've noticed that dropping the front tire pressure on a RallZ (from the KTM recommended solo 22 psi down to about 16-to-18 psi) made the front end of my 640 Adventure a lot less skittish on hard-pack single-track (hard-pack with a thin layer of dust on top). Given the marginal gains in footprint area, I wonder if the significant difference in "skittishness" (which is just the "feel" of losing traction) is primarily in the tire's ability to conform to small surface irregularities.
I'm an engineer [retired] and I've been saying this for years. The critical parameter in a tire is the sidewall. The engineers set the tire pressure recommendations to achieve a certain sidewall profile. When you mess with that, bad things happen [pinch flat]. But, Bret, the science won't matter... the engineering won't matter. They all watched Travis do it once.... once... and after that they won't listen to any calculations.
Absolutely. Suppleness. Compliance.
As a retired mechanic, I didn't have all the materials of measurement listed here, but I'd bought a 2020 Ducati Desert Sled, changed the front rim to 21" with an appropriate tire and slid the fork tubes up the triple clamps (head and crown if you will) and needed a system for initial setup. Having a 10 acre back yard of sand and mesquite and a (usually) dry creek bed with motorcycle trails well beat in through all of it I proceeded to testing. With front and rear at max suggested pressure it didn't take long to decide it was rolling like a rock. Those tire pressures were within 2 psi of each other and stayed at that difference for the whole test. Starting at suggested mfg. pressures I rode the track at an increasing pace until things were sliding around and the suspension was working vigorously. Back to the garage, let out 2psi, and ride to test parameters. 2 psi lower and test. Repeat. Eventually it was just wallowing around with little response to steering input. At that point I started adding pressure at the 2psi rate unti all responses were midrange acceptable. It was a well spent afternoon and I learned a lot. The Duc is running at expectations.
I struggle with this topic, for me there is a major difference in "feel" on sand when I air down about 50%. I can't say if its a larger foot print or what, but it makes the bike much easier for me to ride at low speeds on sand. R1200gs w/ 50/50 tires.
What I'm getting from your video and then the comments below is that the science is telling us that while the actual change in footprint and/or flex is small, most comments show that airing down gives the rider a better "feel", and more confidence. I think I have heard you and other top experts teach that confidence is one of the main things a rider needs to become competent on a motorcycle. So if that tiny growth footprint and miniscule amount of flex result in a huge boost in confidence and feel - then airing down a bit is probably well worth it for many. And for competitive riders, every tiny bit of advantage can make a difference. Thanks for clarifying the science behind this, but the placebo effect seems to be much stronger than the science in this case.
May be, but riders get a boost in confidence because they have been brained washed to believe that they have more traction. It’s all in their mind. I am one who when ai bought my Tenere 700 about a year and a half ago started airing down my tires when riding in the rain. Until a couple of months ago I realized that I’ve been riding for 34 years, rain or shine, and I never aired down my tires on any of my sport bikes and never had any problems at all with traction in the rain. So I started asking myself why I was doing that now. I realized that I developed that mentality from watching videos about ADV bikes. It’s amazing how much others can influence us when you when we are hungry for knowledge. Which is the problem with social media, any fool can make himself or herself sound like an expert when they are not. Sadly someone will always be willing to believe them, I did. What I like about Bret is that he practices what he teaches and his skills are far what I see from others.
Seems like a good case of confirmation bias is at work here and potentially being confused as better "feel" and confidence.
As somebody who's sold tires for a long time, I can guarantee you, most people can't tell the difference in air pressure.
The "confidence" comes from believing they have done something that aids them.
@@4LowRocks YUP!
i think we also need to factor in that the low tire pressure is making the tire work like a suspension system, suspension systems keep the tire in contact with whats ridden on, therefore contact surface does not matter so much?
Comin from trials riding where we are running with almost no air in the tires, going to a 2005 model 1200GS with TKC-80`s going a lot gravel and also real offroad, i think I have some experience to share. For "normal" gravel riding reducing slightly from road pressure - lets say from 32PSI to 25PSI - makes the ride noticeable more smooth and noticeable more grip on loose surface. Its not only the tires that works better, but it also takes some of the load off the suspension. For "real off road" riding in slow speed (im a trials rider. Trials riders dont need speed to have fun ;) ) i go even lower to around 20PSI and this really makes a difference in grip over rocks, diagonal roots and so on.
20 PSI is around 45% below the pressure for the OEM tires. Of course if you are running an ADV knobbie then rise often have lower pressures by design including the pressure for max load
I think the small increased surface area of my tire when airing down pales in comparison to the massive increase in my skills that I get by some proper training. I always like to put my effort into where I get the best payback. Thanks for these videos.
The way you started that statement, ... I was getting ready to post "YOU CAN"T ARGUE WITH FACTS"
Based on the 'riding on air', rather than 'riding on the tire', principle, I calculated the ratio of running load (i.e. wet bike + me + space suite + luggage) TO maximum tire load. I multiplied that ratio times the oem (for KTM 390 Adventure) recommended psi. The result (22 psi front & 18 psi rear) was a more comfortable, less 'squirrely' ride - both on and off pavement. For challenging off-pavement riding, I found that BELOW 15 psi the bike got 'squirrely', again; BUT, 17 psi front and 15 psi rear felt very nice i.e. confidence inspiring.
Also, I noticed that KTM recommends 15 psi for MOST of their off-road bikes! All this was predicated on the assumption that tire manufacturer 'maximum load psi' is carefully (and conservatively?) calculated tor each specific tire model and size.
Thanks Bret! Insightful as always. In my limited experience, I have found that whatever traction issues I encounter are most likely due to a lack of riding skills, and not due to tire pressure.
Thanks! You creating extremely useful content. As your proud supporter on patreon, adding some here too.
I am currently in CA lining up to shoot a trail braking video that is only possible because of Patreon funds... Thank you
I really appreciate that you included how the tires form around obstacles in addition to surface area only. I would have thought it had a bigger difference, but this was great to see.
very interesting! On our 300cc dirt bikes we go from ~14psi on dry rocks down to ~9psi in wet/muddy terrain (on mousse) and it makes a huge difference because the carcasse is so soft that it basically wraps around rocks. In comparison on my 790 the carcasse is much more rigid, the compound is harder and the knobs larger and shallower. That's why airing down doesn't have that much of an effect. And indeed: in mudddy terrain no matter the pressure the grip is always terrible :D
This lines up with my experiences too. I have a KTM 500 Dual sport and decided to try to ride my local trails at street pressures. It was mostly OK but the steep loose climbs were much more difficult to keep the rear from spinning up, and the front was much less stable.
On a 300lbs or less bike even 10psi is a huge difference. Not only in feel but performance. This video doesn't apply to actual off-road bikes and only for large adv bikes with road focused tires.
Same here. I run Tubliss on my WR450F and it's amazing the increase in traction from dropping from 15 psi normal trail pressures to 8-10 psi. I only do that in the rear. Found low pressures in the front just kill my steering feel with no gain. I can't place the bike precisely with low pressure in the front.
@@ExpatonTwoWheels yeah front tire I run it a little higher forsure.
i think we also need to factor in that the low tire pressure is making the tire work like a suspension system, suspension systems keep the tire in contact with whats ridden on, therefore contact surface does not matter so much?
Left my home in eastern nc on brand new Dunlop trailmax mission tires on my 2017 f 800 gs. Rode the entire Trans American trail coast to coast and back on the same tires. Kept them at the operators manual recommended TP the entire trip. Tires performed magnificently on and off road. Tires held up even on big rocks etc. No damage to my rims or tires. Was comfortable riding at the recommended pressure.
Tyre construction plays a big part in how much flex you get from the tyre, stiffer side walls give less flex as opposed to softer side walls for the same air pressure.
Most tyres designed for overweight motorcycles have stiff side walls so will flex less than say a tyre designed for a mid weight bike, the difference is quite noticeable on an enduro bike like a KTM 450, 16 PSI on a stiff construction is a world away from that of a soft construction tyre when it comes to traction.
The best way to test if a tyre has a stiff side wall is, one, try and fit it to a rim or Two, let the air out and sit on the bike, a tyre with a stiff side wall will feel almost the same to reach the ground when seated, a soft side wall the bike will sit a lot lower.
I guess there is a reason why enduro riders and Paris/ Dakar riders run a mouse equivalent to around 14/16 PSI in the tyres and their bikes don’t weigh 250 Kg.
To get the tyre to flex more on a stiff side wall you need a fair pressure drop then one with a softer side wall but a stiffer side wall is needed as the weight gets higher.
All I know are dirt bikes. I have the “tubliss” system in the front and rear allowing me to run 4psi in the rear and 6-8 in the front. The most noticeable upgrade I’ve done on the bike. We call it cheating. Traction and control over wet, angled roots in the trail is amazing. Plus, the 110 psi core keeps the rim protected while hitting obstacles like in this video. It’s not a myth. On this, I absolutely know of what I speak.
I ordered a 23 890 r that’s not here yet. I know nothing about adventure bikes. Thx for the info although if “tubliss” ever makes a system for bigger bikes, I’m all in
Although not measurable by any criteria discussed, my anecdotal experience is about FEEL. When I air down the bikes feel planted, stable, and confident in marginal off-road surfaces. The difference in sand is remarkable. Nice analysis, thanks Brett.
Agreed. Try both and draw your own conclusions. I'm not going to run 8psi dirt bike pressures on my 600 lb tubeless ADV, but I'm also not going to run the manufacturer suggested *STREET* 36/42 pressures in the gnarly stuff either. I like Brett's conclusion of no more than 20% lower. 28/33 sounds about right, though I have gone down to 25/32.
I can't say for sure if I"m truly feeling a difference or if it's the placebo effect. After watching this video, I have no doubt Bret is right about the contact patch changes being minimal (like 1-3 mm) but there are many times when a few millimeters here and there make a meaningful change. Examples include seat height, handlebar height, rake & trail, preload, etc. So maybe a tiny change in contact patch is still substantial?
Sand, gravel, and mud are reasons to air down. You will have much better feel. These are 3 dimensional surfaces the tires sink into. The channel host's analysis was done on a hard 2 dimensional surface.
@@JohnnyFuelMotorcycleAdventures I don't think it's a placebo effect. When I had E10 front on my R1200 GSA it was better all the way around off road at 28psi than my road pressure. The front suspension wasn't as rough over obstacles, and the front was planted in corners.
@@stevestowell-virtue3781 Rob Pepper's analysis (on 4x4's) has similar conclusions as Bret's. Perhaps in terms of feel it *may* be different up to a certain pressure (as Bret noted) but the traction/surface area argument is near insignificant.
And a pinch flat avoided is worth any real or imagined gain! Thanks for such in-depth analysis.
Thanks for the years of great content Bret! I appreciate the effort you put into these videos, and I'm looking forward to taking one of your classes in person at some point in the future!
Awesome, thank you!
Wow... i've taken for granted that airing down my tires won't give me a lot of benefits based on your words.. And now you came out with this video supporting your words with experimental testing .. Well done .... you rock !! Hi from italy Bret..
To analyze properly, in my opinion , the contact area of the tread should be compared while in a rock bed, sand, or other obstacle, not on a flat surface.
It has to be taken into consideration of how the power and control is different if say, the rear tire of a GSA was dropped to 4 lbs and how that would allow for slower speed manoeuvres through and over rough terrain.
Hitting a curb with low tire pressure in the front tire is obvious that it will be jarring, but over a rock bed of 4-6” round rocks it would be a different result.
Having said this, I don’t air down my tires. There has been some challenging hill climbs and challenge courses that I wish I had thought of airing down. The individual who did air his rear tire to 4 lbs did win the challenge course over 40+ other participants.
The challenge is how to isolate each effect.
@@BretTkacs Of course it is. Actual Comparative Analysis. Otherwise the data is not credible.
Great job Bret. People can't argue against the facts. Everybody thinks they are an expert and will come up with their own convoluted reason to continue doing what they're doing, when the facts, so clearly presented, overwhelmingly demonstrate that their reasoning is illogical and inaccurate.
Bravo mate.
Thanks Bret for all the hard work and reasonsble explanations. It is no surprise that the Contact patch variation in size is neglectible when airing down about 10 to 20 %. The tire manufacturer of a 50/50 street legal tire must comply with a series of safety requs and warrant road performance even for fools like me forgetting to check the tire pressure regularly. Off road lower pressure reduces vibrations caused by rough surface better than the suspension is able to do in the first 3 mil of travel. Thats why it feels better. 20 % down is a good starting point for self testing.
Contact patch shape is an underrated topic. The longe the patch the more straight you go. A shorter patch shape will steer more sensitive resp. deflect much easier on rough terrain. The size of the patch
Outstanding data! Quite comprehensive study.
As always I love the way Bret breaks down everything in his videos and shows his work. That's how you make an argument that should convince all but the stubborn idiots in a room.
This issue is about scale. The reason it works on a 4x4 is weight and size which is substantially more than a motorcycle. (I've built and tuned professional-level competition machines.) That is really important.
Motorcyclists are trying to apply that effect to their motorcycles and it just doesn't scale the way they imagine. I don't air down at all unless I'm on really rough ground and that is specifically to reduce deflection. A rock garden can bounce a fully aired-up tire all over the place, but at 20% less air the bike is much more controllable. It's not a lot but sometimes not a lot is what you want. The suspension setup is more critical to bike performance than air pressure, air pressure is more like the fine-tuning knob after you get the suspension right. Suspension is more like a course adjustment. Anyone that tunes anything with an engine can tell you that is dead true at any scale.
Sidewall and tread section construction matter more when it comes to tire flex and contact patch than air pressure even on a monster truck, air pressure is still a fine adjustment.
There is little surprise that all the tires have similar contact patch sizes. That's a matter of physics. They are balancing rolling resistance and grip and that formula only has so much give. If you have a narrower tire you need a longer contact patch to get the same surface area. What matters more when you are talking about contact patch size is surface area, not so much the shape. The shape is more about rolling resistance as a wider tire has more resistance than a narrow tire with the exact same surface area. Softer tires have more rolling resistance than harder tires, but the softer tire has more traction given the same surface area. If you want more grip you actually want a softer tire with a more appropriate tread pattern. I run 70/30s on everything but soft and loose soil, when I know there is going to be sandy river bottom soil I switch to knobbies with large and deep voids. The 70/30s work well on compacted soil, river rock, and whatnot.
But, there is something else to this discussion.
The size of the tire matters. Taller tires climb more easily. Narrow tires dig more, and wide tires float more. Ask anyone how a 300 or 350 tire feels on a wet road versus a standard-size tire. It's why they make giant agricultural tires and very narrow tires, each has its purpose. But in the motorcycle world there is more to it than just float vs dig. Width also affects turn-in rate a long with some other geometry of the forks and wheelbase, and weight, it's complicated, but just know that tire width affects how the bike turns into a corner. That doesn't seem as obvious as to why off-road people should care, it matters when you put that same bike on pavement. That tire has to keep you on the road at 70 MPH and keep you from eating dirt on a trail. Believe it or not, there is a magic formula most tire companies use to arrive at that point. That is the real reason why the tires are so closely matched in performance, they don't have a lot of room to work with while designing a tire that can do both as well as they can.
Damn dude - there you have it. Excellent work Bret. I always thought the air down was for greater tread flexion to increase grip off road - not increased contact patch but I may be wrong. A deep subject indeed. Thank you.
Good to see the firmly held age old "truths" being looked at rationally rather than hearsay.
Compared to the myriad of motorbike videos flooding the world, I am very grateful for such factual posts with sensible content. The comparison of the different surfaces between 90/90 and 120/80 (as an example) was very amazing, how little difference it makes. But I am even more thrilled to learn that more than 20% less air actually makes all driving characteristics worse.
Thanks, Bret, for this valuable work. Regards, Henry
Great content. Coming from a mountain bike background, we air down primarily for increased traction in cornering off-road. I know from experience that the amount of "G"s you can pull on a corner goes up significantly at lower pressures. We basically air down until you risk a pinch flat, or until it feels too squirrelly.
I would like to suggest a follow-on video, again, data-driven: perhaps record max speed on a fixed turn radius before slipping vs. air pressure.
I too am curious about the contact patch footprint and cornering; more challenging to collect the data. Thank you for this data-driven study.
Thanks a lot!!! Lots of work and commitment put into this material.
A set of very important experiences.
Very solid and important exercise. Thanks for doing it.
Thanks very much. Very interesting. I know from experience that the 90/90 21" front wheel on my KTM 950 offers plenty of grip on-road, even compared with bikes that have traditionally sized road or track tyres. Normally 17".
I never air down when riding off-road; I dont think it's worth the faff. The biggest advantage is to learn to ride better.
Nothing better than knowledge from adventure riding guru, always learning something new.
This is a great format for discussion, thank you Bret!
I guess it is VERY important, after reading many comments, to keep this in mind and read well the video title.
This air down test its for ADV Bikes, and NOT for enduro, motocross or trial bikes.
Great job as always Bret, thank you so much for your effort and time.
Details matter
Thanks for fixing the title. My OCD was triggered.
Good job Bret.
Great information Bret! Appreciate your time & knowledge you put into your content and no ads or sponsorships, doing it yourself and that says alot.
I love your approach Bret and look forward to meeting you. Your effort, and straight forward explanation even in tutorial videos is another level. Much appreciated
.. the other aspect of airing down is the temperature. Under-inflated will generate heat and change rubber properties. This mostly impacts riding at higher speed on high grip surfaces.
Most likely the changes in behavior and between 140 vs 170 and 21 vs 19 are related to specific tire construction and there are substantial differences between different brands/models of tires
Fantastic work putting this all together.
Great investigation Brett.
Thanks for all the research on this! I still rarely air down, especially since the time I hit a pothole and blew the bead on my cast rim, necessitating a tube to keep going. Lesson learned.
The purpose of airing down is to keep more tire in contact with the ground when the ground is uneven. If you have a tire at 35 psi and it hits a little rock, that rock is going to make the rest of the tread lift off the surface. If you hit that same little rock with that same tire at 25 psi, the tire will give and allow the rest of the tire to stay in contact with the surface. Not talking about big obstacles like in your example, but small rocks and surface imperfections. It's a balance between protecting your wheel/avoiding flats, and improved grip.
You can sit here all day and explain why it doesn't make a significant difference in grip, but if you ride a bike down a trail at full pressure, then go again at lower pressure, there's a *clear* difference in the stability and grip of the motorcycle. It won't make up for rider skill or training. It's usually not *required* to get down a trail. But ride down a road that's hardpack with little marbles on top of it, airing down makes a monumental difference in available grip.
cool to debunk the surface area myth, though.
100% agree. This presentation is not a true representation of what actually happens on the trails.
@@philipraposo8324 BINGO
He mentions zero about Hysteresis losses.
Thank you for doing all this research. It's really appreciated!
Another absolutely fabulous work! 👏🏻
Great information!
I really appreciate all the hard work you put into giving us objective advice.
You have given us valuable information! Thank you so much! Keep up with this type of content!
Interesting! I'm learning to ride off-road, recently got a light dual-sport, and the whole concept of airing-down (along with much else) is new to me. My recommended highway pressures are 22/22, but I've been dropping to 10-12 off-road at an instructor's recommendation. At my level, I'm sure I can't perceive any difference, but I thought I was giving myself a little help-and I need all of that I can get. In fact I just got Tubliss installed, in part, so that I could experiment with _really_ low pressures in relative safety. Now I'm wondering if it's worth the bother, haha...maybe I should just leave them at 22.
I am appreciating the concept of "ride your ride" more as I watch different instructors. We ride based on experience, bike type, bike setup, and terrain.
As always excellent content and research. As a newer ADV rider the information you can get is mostly “this is how I do it”. My big Pan America in sand is difficult aired up or down. Biggest difference was when I went from the stock tires to the Anakee Wilds. HD wants those run at 36PSI and have found it to work well.
a couple of things i feel the need to pull you up on being an experience trail bike rider and 4wdriver.
airing down isn't about width of the tyre it's about length, it's about changing the tyre from more of a fixed round shape to turning the bottom section of the tyre into a tank track (figuratively) where the leading edge is placed on the ground stays in situ until it moves past the centreline of the rim and is lifted up. this does happen at road pressure too but your analysis of the increased length from airing down being basically insignificant is flawed. small changes can have big effects.
tyre conformity increases both traction and puncture resistance, if a tyre can mould around a sharp object some of the pressure on the point is spread onto the sides of that object or onto the ground. you can visualize this by getting two party balloons and inflating one as much as you can and half inflating the other make a pyramid out of blue tack or something similar with a toothpick sticking out of the top and then push the balloons onto it. the hard one won't deflect and will pop the half inflated one will take much more force before popping if it does. so too much pressure = punctures, too little pressure = pinch flats.
the right pressure ultimately comes down to weight, tyre stiffness, speed and terrain.
more weight = more pressure
stiffer sidewall = less pressure required to achieve sidewall flex
high speed = high pressure or sidewall flex can overheat the tyre
terrain is variable square edged rocks require pressures or speeds that prevent pinch flats
sand requires low pressures for flotation (the figurative tank tread)
tar roads have good traction and high speeds, high pressures allow maximum handling and braking.
there's no set answer for tyre pressure, but there is benefits to airing down and times where it is necessary.
Excellent information Professor, as a beginner on the off road, glad I’m getting good info as I def would prefer a possible fall as to a bent rim
Ooohhh the cruelty in destroying good myths and discussions with facts. This will video will shake the foundations in some communities 😆 Great work Bret, once again you deliver high quality content. Cheers ☕🇳🇴
Solid work Bret. I was pretty convinced you were going to completely stone wall airing down. I was pleasantly surprised. We’re racing adventure bikes in Kentucky and have been doing “research” in this area with low
Pressures, rim locks, and even mousses. It’s interesting to compare our notes to yours. Thanks again!
Years back after watching one of your tire videos I went from a 130 rear on my Dualsports to a 120 rear (Dunlop D606 always) and the 120 is so so much better off-road. The thinner tire is lighter and spins up faster and it seems to dig into the mud and bite better instead of floating on top. Proper pressure and a thinner tire help me a lot
Thanks for one more great video based on experimentation and experience. You help a lot we riders with those informations. Good video Bret!
Always excellent content - Thanks for doing all this work...incredibly helpful in this debate...keep the videos coming!!!
Love it! Thanks for a great, informative, and well done video. I know every motorcyclist has wondered about contact patch variations, both with regard tire cross-section/width and also with regard air pressure and you covered them both. Thanks again, David
I feel reminded of a simplified math assignment in high school: From a predefined diameter -> Calculating the pressure inside a rubber tube with given outside pressure and contact patch surface - or calculating the contact patch surface from the difference between inner and outer pressure - or calculating the outer pressure from contact patch surface and inner pressure.
In essence: Apart from the diameter (basically flattening/prolonguing the curvature segment touching the ground, and therefore the length) the tube width and height had only neglectable impact (at least in that simplified assignment scope)
Thank you Bret....nice and serious analysis!
Thanks Bret! This video is a must for new riders.
Not necessarily
Valeu! thanks so much for the hard work all those years you have been putting content on your channel!
Thank you for the thorough explanation. Very impressive
“Common knowledge” around the campfire is often rooted more in bombast and a tenuous grasp of folklore than genuine understanding and wisdom.
Well done Brett.
very impressive research and effort. Thank you for sharing👍👍
Wow you put a substantial amount of work into this one!
A stupid amount, I could have made a 2 hr documentary on the process and controls to isolate accurate results.
Wow. Thanks Bret! As if I needed another reason to listen to his knowledge
I’m always afraid of pinch flats that my biggest reason for not airing down. Great video!!
EXCELLENT! Thanks for presenting!
Thanks, Brett. A lot of work and analysis went into this, which is greatly appreciated. Looking forward to comments from the tire and bike manufacturers.
Thank you for your great work and reporting. Awesome knowledge 💪
I agree completely on everything thing that you stated. I consistently run 25 psi on my Africa Twin in our SoCal desert and mountains specifically for puncture resistance over severe rocky terrain. In my experience airing down in the rocks, gives the tires just enough give to resist punctures versus 33 - 38 psi. If you have a chance, test for puncture resistance at different tire pressures.
Thanks Bret. Cool video...
You are the best motorcycling video instructor/informational teacher out there! I have learned so much from your videos!
Wow, thanks!
Nice job Bret! Very scientific.
Brett good work! I appreciate your videos and good info!
Great information... I have personally never aired down
This is a very interesting presentation. Thank you for looking at all of this. I do believe that I can feel differences related to fairly small changes in tire pressures, at least on the street, and that range is worth exploring on a given bike/tire combination. It also seems to make sense in terms of what was discovered at upper-limit and lower-limit pressures - that if noticeable differences occur in a fairly narrow range of settings, big deviations may result in more significant effects, often accompanied by negative sensations and performance.
Nicely done!
Thanks Bret, been looking forward to this one.
Hope you liked it!
thanks Bret
I love your myth busting, Bret, done with data. Your episode also confirms that "feeling" is important, that a combined team of rider-bike has a certain "window" a feeling well. A bit like cooks - they don't weigh their spices exactly to the milligram ;-) Thanks a lot, much appreciated, Günter/Nürnberg, Germany
Great observations.
Airing down effect on a tire is directly related to the ratio of rim diamiter to tire diamiter. In a truck with 15 inch rims running 40 inch tires, the effect of airing down is dramatic, and you can get away with lower pressures before bad things happen. There is a lot of sidewall to deform an extend the contact patch both wider an longer before the rim get near danger. You have roughly 12 inches of deformation to play with. And that can wrap around some pretty big rocks. But a 30 inch tire on a 20 inch rim, airing down has much less of an effect. An you need more air in the tire to keep from cutting the sidewall or bending the rim. You only have about 4 inches of sidewall deformation to play with. There is just not as much sidewall to flex and not as much safety margin. For that reason, if you see really tall rims on a 4x4, you can safely assume its a pavement princess.
A motorcycle tire is the extreme end of the ratio. The rim is big with a tire that is only slightly bigger. Sidewwalls are tiny. So there is very little gains by airing down. You only have about 2 inches to play with. That's not even taking in to account that a truck with its tires aird down is usually going no faster than a human can run, an a motorcycle is usually hitting things a bit faster. Margin of error is small.
My farm truck running 32 inch tires on 16 inch rims, I have about 7 inches of derormation to play with. I rairly air down because the gains are not that much. They exist so if you are doing some timed hard core wheeling, its worth it. But I am not doing that. And I sure dont see me riding my bike in a way that less than a third of that deformation is worth it. So the air that got me to the trail is the same air that will get ge home. But thats just me.
Great video - looking forward to the follow up video(s)
I've been riding offroad for a while, and I've always ridden factory pressures. IMO, at low speed, you can pick a line, and if you can't pick a line that's not the tires, that's you. Tire pressure won't fix that. At higher speed, you need a harder tire to take the hits or you'll break a rim. My current bike is a 2017 Africa Twin, and that 21" front with a wire rim and factory pressures works decently in all conditions, including 200+ kph highway riding (that wasn't me) and 20- kph babyhead riding. It's entirely possible you can buy 5% footprint with low pressure, but you break shit on rocks instead. Not worth it, IMO. Leave the tire alone, the tire isn't the issue. Let's face it, most of us aren't riding in the top 5%, farking with your tire pressure isn't gonna make you an offroad Rossi. Leave it the fark alone.
Instead of farking with your tire pressure, learn to aim your bike. Like anything, practice and skill will make your day a whole lot more fun. Maybe take one of Bret's classes, he'll make you a better rider. I did, and I got pushed hard. Which is good, because you should train how you ride. Between Bret, Kramer, and Reg, I'm confident I'm a middling competent rider :P
And the science is correct. Absolutely proper observations.
Folks. Find a bit of technical (hill, rocky, or deep sand/gravel) terrain that you can barely ride without crashing at street tire pressures. Then lower your pressure to 17-20 psi and ride it again. You won't need to do anything else. The difference will be unbelievable. The reason Brett isn't seeing much difference in the length and width of the contact patch is that he is not airing down by a lot, and contact patch (surface area) is a function of length TIMES width. If you air down by 10% your contact patch will only be 11% bigger. And the length and width of it will only each only be about 5.4% bigger. That won't seem like a lot. I run about 45 on the street and air down to about 18 off road. That gives me a contact patch that is 2.5 times bigger, but still is only 1.6 times longer and wider. It won't look like that much if you look at a footprint. Now imagine going to the beach and having a 2 by 4 piece of lumber. Press it end wise into the sand and it will have an 8 inch contact patch (2 x 4 = 8). If you press down on it with 144 lbs that will be 18 pounds per square inch. Imagine how deep into the sand it would go. That's how deep my GS will dig with tires at 18 psi. Now press down on that two by four with 360 pounds. That's how deep my GS (and yours) will go with tires inflated to 45 psi. All you have to do to decide which pressure is best for you is ask yourself how deep you would like your tires to dig. It's huge pain in the ass to air back up, and Brett is an incredible rider, so he doesn't need the help and hence doesn't bother. If he rode right at the edge of his ability and frequently ran into terrain he couldn't handle I bet he would be airing down too.
keep it going, love the channel
I’m a believer in 15% air down for off-road riding then back up to normal when on street. It’s about feel, control and confidence. Also, anymore than that I see riders getting pinch flats on their ADVs (500+ lbs).
Thanks for the detailed analysis.
Interesting video, and spot on, IMO. I’ve always stuck with recommended pressure, and got where I was going just fine, on or off road. Tire type and rider skill trump pressure tweaks every time.
Great informative video. Thank you! I am pretty lazy so run my 890r at around 30 psi. On road is 35. Off road is 26. So kind of splitting the difference. Sounds OK after watching your video.
Super interesting. As a very average adv rider on a T7 what I am most interested in is how I smooth out my ride on dirt roads, when riding over wash-board and potholes (I hate standing up!), when going between 30-50 kmh. I've sometimes convinced myself that by airing down 10-15 percent that my ride is a little smoother, that the lower pressure is magically eating some of those little bumps. But it -- the smoother ride -- could very well be in my imagination! Adjusting preload, rebound and air pressure seems like a voodoo art to me!
I find airing down for a softer "catch" on impacts to be a common misconception in 4-wheeling, as well. The part of your vehicle that absorbs large impacts in a controlled manner is the suspension, not the tire. A tire is, effectively, and undamped spring. If one has any doubts, they need only drop a tire in a garage and see how high it bounces back. Increasing the deflection of the tire by airing down just increases the amount of work for the suspension to do. Airing down in the 4-wheeling world works at low speeds, but there is a reason we all air back up for any rapid travel.
Fascinating! It's good to see some explanation to things I've experienced, but lacked the knowledge to really understand. I found dropping 3-6 psi (Tenere 700, 300lb rider) maybe helped a little, but going beyond that (despite people telling me it should help) made the bike wallow and felt *bad* at high speed on dirt and gravel. Maybe that matters less when bike+rider isn't 750lbs, but it certainly mattered for me.
Not gonna lie. I REALLY thought there'd be a bigger contact patch size delta. While I already understood bigger contact patch doesn't inherently mean more traction (see fingertip vs whole hand on table traction experiment) I thought the conformance/give would do more... But it clearly doesn't until pressures are so low as to cause other problems.
Yeah. Fascinating. Thank you for this incredibly informative video!
That is a super interesting video. I'm not experienced with off road bikes, but I've done a fair bit of 4×4 offroading. There's two big effects there with air down, and I'd like to know how much they apply to bikes:
1. paddling- the tyre distortion gives you large scale dishing of the contact patch, which helps in sand and mud.
2. lug flex- the gap between the lugs opens at the edge and closes in the middle, giving grip edges, and clearing out some mud.
Lugs are shorter and tightly gapped, there is less area to allow distortion and bikes travel at higher speeds than those of 4x4's when they are aired down
As a suspension workshop - we greet the interesting video.
As a biker - it is quite subjective, what air pressure You like to ride. But! There is a "correct" pressure for the tyres. Has been so for the cars and is so also for bikes. It usually is a range of about 20percent of the overall pressure. It depends on so many things and will be different from rider to rider and place to place. But in any case - the manufacturers are running the so-called safe numbers on the bikes. Ktm 690 with a 65kg beginner rider will have a different pressure for good off-road characteristics than the same bike with 100kg pro agressive rider. But the pressure will not be more than 10-15% different.
As a workshop - AIR DOWN - but not too much. Rebuilding wheels is not cheap. air still is.
The tire acts multidimensionally, and your explanation with footprints, in that way, is only applicable on asphalt. The hardness of the tire as a result of a high pressure will only generate the effect of removing the loose terrain, while a soft tire will compact the contact surface on a micro level, generating a pushing force. Everything is add up.
Of course, too little pressure creates danger for the rim.
I don’t ride the big heavy bikes. I reserve judgment on the “science” of this experiment. One problem I see, however, is that the video does not discuss methodology. It does not state under what conditions the footprint was measured. But I get the feeling it was measured under static (stand still) conditions rather than specific dynamic conditions. For example, I sometimes ride up or down a steep rock surface. When going down, I believe there is considerable advantage to having low pressure and weighting the front end by moving all the way forward. As the brake is applied, the suspension compresses and I believe the tire flattens giving me a substantially bigger patch to slow the bike and prevent skid. Similarly, when riding up the face, the weight is shifted to the rear. The lower pressure allows the tire to flatten more and provide the greater contact, giving one more traction.
The video shows the tire will flatten more easily with lower pressure in a dynamic situation (going over obstacle), but only discusses this as a disadvantage (hitting the obstacle harder with the rim because the tire folded more). This is true in that situation, but this is why you would deweight the front of the bike going over such an obstacle. But he states they just ran over it. So I suggest this may be a case where the author just does not recognize the weakness of his testing methodology. 17:33