Long travel suspension geometery

[Mark911]

When designing a suspension for a class 1 vehicle with about 25" of front wheel travel I'm running into the classic compromise between camber change and track width change (I'll call it scrub although that term is typically used to describe kingpin angle to tire centerline). I'm from a road circuit background so my inclination is to minimize "scrub" at the expense of excessive negative camber. In this way the suspension doesn't bind up on bump and so long as the wheels move somewhat in sequence the camber thrust shouldn't affect steering too much. Not sure what rolling onto the inside edge of the tire does for traction on the dirt, however. But then again, I'm only concerned with straight line suspension performance and steering control at this point. Are there other opinions and/or real experience out there?

 

[GregLocock]

experience:none

opinions: I agree that camber steer is a relatively small effect, at least with radial tires. You might like to work out the sideforce generated by a given lateral motion of the cp in a certain distance, compared with that cused by the camber gain for the same event. I haven't done it but at least that is a rational approach.

Incidentally I'd call it semitrack-gain-in-jounce. It is strongly controlled by your RCH.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

It's a matter picking the best compromises. Side scrub has not proven to be critical in the dirt. Camber change can usually make up for a good half of the scrub.

Unmodified scrub will be determined largely by the ratio of lower arm length to vertical travel, but other factors are axle height upon the upright, ground clearance at full bump (jounce), RCH as mentioned, and tire diameter. RCH may or may not tell you what you want. It is not uncommon for Class 1's (buggy) to have control arms which are at or near parallel to each other.

 

[BillyShope]

I'm not trying to give you a hard time, Greg, but isn't camber "steer" a bit misleading? I don't even like the more common "camber thrust." Both, in my opinion, tend to confuse rather than clarify. The presence of camber affects the patch load distribution and thereby affects the tire's cornering performance.

The following indicates what can happen when it is not recognized that camber effects can only be considered in relation to patch load distribution: About fifty years ago, I heard of a fellow who was building a car, probably for Bonneville, and he thought he could close up the front wheel openings and thereby decrease aero drag if he used "camber thrust" alone for steering. Surprise, surprise!! When he changed the camber, the car continued in a straight line. He could have saved a lot of time and money by attempting to ride a bicycle with the front wheel restrained from turning.

 

[GregLocock]

Well OK, I am a bit surprised by that as well, particularly with crossplies. Every tire I've got data for produces a sidethrust when cambered.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BillyShope]

Come to think of it, I've got a personal example of the limitations of camber thrust. My work experience wasn't entirely within the automotive industry. At one point, I was designing a new document handler for Generous Electric. This was a check sorter and it was necessary, of course, that the checks, as they came whizzing by, were driven down to the baseplate so the magnetic ink would be aligned with the reader. Previous models had used miniature rubber "tires" set at an angle to the horizontal, but I thought I'd be "different" and use camber thrust to force the checks down. I had the same experience as the guy with the camber-steered car!

 

[BioMax]

I recently had a similar question. A friend of mine told me that he concerns himself with side-scrub first, then camber and roll-canter. I concern myself with camber, then roll-center and no concern of side-scrub at all. I set up the lower a-arms to point down about 2” from the center out, so I tolerate how ever much side-scrub I end up with. My friend said that the trucks that he has built with side-scrub being his main concern are so much smoother driving than his other trucks. So when I finished the F-150 that I was building (24” travel front A-arm and 30” travel 3-link,) I paid particular attention to the front suspension in bumpy corners. What I found was that the rear suspension with the single point on the axle, was far twitchier in corners than the front suspension ever was. In fact the front suspension side-scrub was nearly undetectable.

Being a Class 1 car, you could be running a trailing arm style rear suspension that has little to no side-scrub, so the relatively negligible amount of side-scrub that my front suspension design had could be more of an issue for your project. I would be very interested in your findings.