Rear axle bump steer: how there is probably minimal amounts despite very unequal length lateral links

[Niels007007]

Looking at some images of the FL5 rear suspension, there appear to be two lateral lower links. A long one carrying the spring, and a short one placed in front of it.

Initially, this seemed to me to cause some bump steer, as the unequal length means the shorter one covers more of an arc than the long arm as the suspension moves up/down. However, just as I was going to ask, I think I figured it out.

The trailing arm rigidly mounts to the upright. That means the lateral link that is mounted nearest to the chassis mount of the trailing arm, moves up and down less than the rearward placed lateral link, the one where the spring is situated. See my amazing drawing.

To have no bump steer, the lateral movement caused by the arc the lateral arms make as the suspension travels, needs to be the same for front and rear lateral link. Since the front one moves up/down less, it needs to be a shorter length arm to have the same lateral movement.

You can draw a triangle from the chassis mounts, and you can see that the length of the forward lateral link would basically make it fall on the diagonal line, once we account for the fact that the forward link is placed outward a bit more.

So my question is answered but I thought I'd post it anyway since maybe that is interesting.
Cheers,
Niels

Of course, bump steer is also a part of suspension design. It is very likely Honda engineered the mounting points so that they get a certain bump steer versus wheel travel curve, it may very well not be exactly zero. Bump toe in would add stability, bump toe out would improve turn-in, at the expense of stability. If anyone has the full kinematics curves of the suspension feel free to post them!

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[AZCWTypeR]

Hopefully the FL5 has less bump steer than my 2004 Acura TL. The TL wore out the rear tires on a 1500 mile road trip with 2 teens in the backseat and light luggage.
An Acura SB revised rear toe adjustment and they gave me a new pair of OEM tires. But in other words, Honda engineers don't always get it right on the 1st go. I won't ever buy a first year model from any manufacturer again. My last 2 cars have been second year production.

 

[tezzasaurusrex]

The difference in length of the forward most arm vs the main lower arm is engineered bump steer, to increase rear end stability.
The shorter forward arm travels a tighter arc, which induces toe in on both bump and droop. The geo is obviously affected once the car is lowered, which then causes some toe out under the first ~20-25mm of droop, but I digress.
The purpose is to optimise tyre wear by minimising required toe angles during static loading, but dynamically inducing toe in when the vehicle is under varying suspension loads.
It is 100% not zero, and you don't want it to be.

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