Thank you for pointing out that straight down compression (both left & right at the same time) is very unlikely to occur in normal low speed crawling. Most videos miss that critical point.
TBH I'm still unsure if I see an improvement. There are two sources of bump steer in a pan-hard three link suspension with a chassis mounted servo. The one that gets the most attention is the panhard bar and drag link angles relative to each other. When these two bars are not parallel bump steer is created because you have two links moving through different arcs at the same rate. Since the chassis end of the two is fixed, this will cause the distance between the axle ends of the two to vary. The Panhard causes axle shift which pushes the axle to the side of the servo and if the drag link doesn't follow the same arc, then the wheels will turn depending on if the misalignment causes the ends of the two links to become further apart or closer together. We can mitigate and nearly eliminate this source by ensuring the pan hard bar and the drag link are in exactly parallel alignment and even better if they are the same length. This is what the mods of using a low profile servo horn and moving the steering/drag link lower achieves as it makes the drag link match the steeper angle of the pan hard bar. However, if this were the only source of bump steer then a four link with CMS would have no bump steer, and we know this to be false. The second source is the angle of the drag link when at ride height. In the case of the Gen8, this is a fairly steep angle before any mods. As the suspension cycles the drag link swings in an arc so when the axle is the furthest from the chassis the horizontal distance between the axle mount point and the servo horn is at it's shortest. When the suspension is fully compressed the link is closest to horizontal and therefore the distance between the axle mount point and the servo horn is the longest. Therefor, cycling the suspension causes the wheels to steer as this relative distance changes. This is exacerbated by the axle shift caused by a pan hard bar. This is why axle mounted servos don't have bump steer. The angle of the drag link is constant as both ends move with the axle. Ideally, the drag link would be horizontal at ride height so that half of the difference occurs at full extension and half occurs at full compression resulting in minimal deflection at either end. The free mods do the opposite of this and instead increase the angle of the drag link to match the steep angle of the pan hard bar. Thus we are mitigating bump steer from source one (the angle relative to the pan hard bar) but actually increasing the bump steer from source two. Whether or not the reduction of source one supersedes the increase from source two enough to result in less bump steer is debatable. I don't believe it does from my experience. The only truly effective way to remove the bump steer is to adjust the angle of the drag link to be as close to horizontal at ride height as possible. This will mitigate bump steer from the drag link alone. Then aligning the pan hard bar to be parallel to the now horizontal drag link would be required. This would have a second large benefit of mitigating axle shift which in tern limits the exacerbation of the drag link caused bump steer. Something like the Pan hard relocate kits as well as some adjustment to the servo horn offset (in the opposite direction of the low profile horn) are really what is needed here. I'll be doing a future video looking at that once I get my hands on the pan-hard relocate kit
Thank you for pointing out that straight down compression (both left & right at the same time) is very unlikely to occur in normal low speed crawling. Most videos miss that critical point.
Thank you for the info I’m looking for a fix our my Gen8
Check out my latest gen8 video for more details on my finalized steering mods!
cool vid!! big like!! 👍👍👍
TBH I'm still unsure if I see an improvement.
There are two sources of bump steer in a pan-hard three link suspension with a chassis mounted servo.
The one that gets the most attention is the panhard bar and drag link angles relative to each other. When these two bars are not parallel bump steer is created because you have two links moving through different arcs at the same rate. Since the chassis end of the two is fixed, this will cause the distance between the axle ends of the two to vary. The Panhard causes axle shift which pushes the axle to the side of the servo and if the drag link doesn't follow the same arc, then the wheels will turn depending on if the misalignment causes the ends of the two links to become further apart or closer together.
We can mitigate and nearly eliminate this source by ensuring the pan hard bar and the drag link are in exactly parallel alignment and even better if they are the same length. This is what the mods of using a low profile servo horn and moving the steering/drag link lower achieves as it makes the drag link match the steeper angle of the pan hard bar.
However, if this were the only source of bump steer then a four link with CMS would have no bump steer, and we know this to be false.
The second source is the angle of the drag link when at ride height. In the case of the Gen8, this is a fairly steep angle before any mods. As the suspension cycles the drag link swings in an arc so when the axle is the furthest from the chassis the horizontal distance between the axle mount point and the servo horn is at it's shortest. When the suspension is fully compressed the link is closest to horizontal and therefore the distance between the axle mount point and the servo horn is the longest. Therefor, cycling the suspension causes the wheels to steer as this relative distance changes. This is exacerbated by the axle shift caused by a pan hard bar. This is why axle mounted servos don't have bump steer. The angle of the drag link is constant as both ends move with the axle.
Ideally, the drag link would be horizontal at ride height so that half of the difference occurs at full extension and half occurs at full compression resulting in minimal deflection at either end.
The free mods do the opposite of this and instead increase the angle of the drag link to match the steep angle of the pan hard bar. Thus we are mitigating bump steer from source one (the angle relative to the pan hard bar) but actually increasing the bump steer from source two.
Whether or not the reduction of source one supersedes the increase from source two enough to result in less bump steer is debatable. I don't believe it does from my experience.
The only truly effective way to remove the bump steer is to adjust the angle of the drag link to be as close to horizontal at ride height as possible. This will mitigate bump steer from the drag link alone. Then aligning the pan hard bar to be parallel to the now horizontal drag link would be required. This would have a second large benefit of mitigating axle shift which in tern limits the exacerbation of the drag link caused bump steer.
Something like the Pan hard relocate kits as well as some adjustment to the servo horn offset (in the opposite direction of the low profile horn) are really what is needed here.
I'll be doing a future video looking at that once I get my hands on the pan-hard relocate kit