CSR bump steer

[p.mole1]

Hi James,

I fitted female rose joints and tapper pins ( from CHV I think ) as my rack required lowering. I have seen some kits which require the steering arms to be drilled! horrible. I have been running my rose joints for 2 years and no wear as of yet. The advantage of adjusting the track rods ends is, no enlarging the body work, it is quick and easy to adjust the height and you can adjust either side independently. My bump steer was horrendous, loads of advice on internet about putting spacers under the rack without any evidence of measurements.

One issue you need to be careful of is binding of the joint at full, droop and compression. 

 

[Tazio]

Have you a link for those?

[Shortshift]

Hi Philip - those joints look very good.  Neil (7WOTW) has a similar arrangement and Jack at JWM also offers some rather sexy looking pieces.

Interesting that you say your rack needed lowering as I think that, in general, S3 cars need the rack raising to reduce bump steer tendencies. As you say, an alternative solution is to tackle this at the track rod ends where my understanding is that a lowering of the TRE joint (that is, bringing the centre of the ball or Rose joint closer to the steering control arm) will have broadly the same effect as raising the rack.  A case of raise the rack or lower the TRE joint (or vice-versa depending on the nature of the bump steer characteristic). 

It rather looks as though your Seven has bucked the general trend of other S3 cars.  You mentioned the need to lower the rack (unusual in itself) but this is borne out by what looks like an extended pin length on your adjustable TRE's - which would have the effect of increasing (raising) the distance of the end joint from the control arm.

The purists will point out that moving the rack up or down is not directly equivalent to lowering or raising the TREs but, in practice, I think that adjusting one or the other is likely to give good results on our cars.  If you need perfection then I guess that a combination of optimised rack height and tweaked TRE height will deliver the result - but for most people, most of the time, I think one or the other will suffice.

Should add that measuring toe change accurately (in my case using a cheapo laser that was fixed to the hub, pointing to a board placed nearly 3m in front of the car) was not at all difficult.  An interesting exercise, in fact - so no excuse for not doing the job properly.

For me, having restored (lowered) the rack on my CSR to the 'design intent' position, I found that the resultant bump steer was so close to ideal that I can avoid shelling out even more beer tokens to get what would be a marginal further improvement.

James

[Jonathan Kay]

 

[Shortshift]

Brill - you have a job for life here, Jonathan!

James
 

[Bricol]

If you take a look here

https://www.lscd.de/images/WIKI_Files/NonCaterham/Achsgeometrie_OConnell.pdf

you may consider design intent was to have some bump steer.

[Shortshift]

Good point, Bricol.  Your link isn't working (for me, anyway) but I did find and file this technical paper (from Multimatic dating back to 2003 when the CSR was in development) several years ago so I have dug it out.

You're right that the stated development objective for bump steer was 3.0-5.0 degrees/m (toe out, going into bump) rather than aiming for zero or close to that.  We can't be sure but it's reasonable to assume that this objective was carried over from the development phase into the production CSR specifcation.  So, looking at the data from my car at nominal ride height and converting from my graphs to give degrees per metre of wheel travel I find that:

• Car with 5mm spacers under rack (as built by CC):  9.2 degrees/m (toe out)
• Car with spacers removed (as it now is):  0.4 degrees/m (toe out)

So my adjustments have moved the car from a condition that originally gave toe deviation (out) of approximately twice or three times the design objective down to a new value which is many times smaller than the target (close to zero).  It will be interesting to see how this works out in practice; I think I'd rather have less bump steer (compared to spec) than more...

Interestingly, though, I did also take some measurements with a 1.5mm shim under the rack and that puts back a bit more toe out, as you'd expect, of around 1.5 degrees/m (toe out) per wheel.  Interpolation suggests that a 2.5mm spacer may actually produce the spec/target amount of toe out on bump.  So maybe I will try that after assessing the rack without spacers.  Or maybe I won't, depending on how much the driving experience is improved without any spacers (close to zero toe deviation), and my remaining interest and energy levels!

James

[7 wonders of the world]

nice document, I hadn't seen that before,

Yes bump can be advantageous, it's a case of looking what and when it occurs, I found in certain conditions you could provoke a sudden spike in geometry change naturally this would make a B road somewhat of a challenge !

From full rebound to full bump mine is always increasing toe out, albeit slightly, 'feel' is ultimately te most important area and following a very quick shakedown mine is certainly an improvement over the previous rack shimming.

I will try to post figures later.

[p.mole1]

I don't think the amount of bump steer I had was engineered! I can't remember how much I had but the car would try and throw you into a ditch on a bumpy road!. I think the reasons are two fold, the chassis tolerances are probably not that great, the chassis are quite weak, I think they flex quite a bit. At least on early cars like mine.

Also I think the track rod ends are quite poorly made with a lot of scope for variable  tolerances, which is why most cars require the rack to be lifted. I think they where designed that way to enable an easy solution for the owner to adjust the bump steer 

[Bricol]

Remember too, that any static toe out will cause the car to pull to the side that hits a bump, whereas toe-in will tend to increase stability.  I used some extremely crap roads on my commute (when I'm not working from home that is) and running less toe out and lower tyre pressure worked for me.

If you have ever raced a radio control buggy in the last 40 yrs, the lack of adjustment on a full-size car compared to the many many variations of suspension pivot points, upright pivot points, upper arm attachments, rack height, inner toe link height adjustment, out toe link position, shock mounting point top and bottom might be pleasing, or annoying, depending on your point of view :)

 

[ECR]

BUMP steer can be a bit of a confusing name because it doesn't just happen when you go over a bump. It happens when the suspension moves through part of its range. It happens for instance when you brake because the suspension is moving relative to the chassis in the same way as when you hit a bump. This can be an advantage on track because when you brake for a corner the suspension is already trying to turn the car. It's not so nice when that happens on the road...,This is why a lot of race cars are set up with a touch of static toe out. They're prepared to accept a bit of straight line instability for sharper turn in into corners

[p.mole1]

I set my tracking to parallel just to increase stability, I do track days but I prefer the feel of the car. It probably won't be a problem if it wasn't for the state of our roads.