Bumpsteer?

[Mark A]

You there Mark?

 

Graeme

 

 

If lowering a car, as Graeme said, you need to raise the rack to compensate, you can't raise it the full amount you lower by (if you went lower than 10mm) as the rack can only go up by 10mm. This isn't a huge problem but there is a noticeable difference, and as the plates cost a couple of quid and you need to geo anyway it is pointless not to do it.

 

What this does is move the zero point on the bumpsteer curve back towards where it should be, the curve it'self doesn't change. When you lower the car you are effectivley using up the first 20-30mm of suspension travel, thereby increasing the toe out accordingly, this is what probably accounts for the lightness, i.e. too much toe out. The Elise has a horrible bumpsteer curve, as it goes bump toe out, as you expect, but in rebound it goes slightly toe in before going toe out again. The reason for bump toe out is to make the car safer as it introduses a touch of understeer, bump toe in makes the front end very pointy and can unsettle the rear (rear bumpsteer is always toe-in, or at least on a properly designed car, as this also makes it safer, imagine bump toe out, and work out how long before you end up in a ditch)

 

With stiffer suspension what happens is instead of absorbing the bump you drive over it which changes the attitude of the car and makes it move around a lot more. It's the same with tyres as the lower sidewalls don't absorb the bumps.

 

Camber (negative i.e //----\\ as opposed to positive \\----// ) increases with bump and decreases with rebound, this is due to unequal length wishbones. The increased negative camber also produces more tramlining due to contact areas etc, which gets way to complicated to try and explain, just accept it'll happen .

 

Graeme,

Not sure what you mean about the shims for bumpsteer unless you mean the rear, if so this is the same situation as the front where you move the rack up, all you do is change the height at the shim stack on the outer joint, you could do the same at the front if it wasn't for the taper on the track-rod end (and the fact you'd actually want to lower the outer joint ).

 

 

juicetin,

 

The Toyos have a bit of flex in the sidewalls so are less precise in their handling characteristics, also what was the geo set at? The UJ problem doesn't sound too good, when you say 5-6mm of play, where exactly and in what direction? Tightening them up isn't usually a good idea as you don't really take up the wear properly as it will have high and low points as it won't wear evenly, so although it may feel tight it won't be for very long. I haven't actually looked too closely at the Elise ones but I believe they can be replaced, but it isn't an easy job.

[GCCK]

Here endeth the first lesson. Thanks Mark - lucid as ever.

 

Yes, I meant the rear toe link for bump steer adjustment shims. As a for instance, I'm lowering mine from 140F/140R to 110F/120R. If I use the maximum 10 1mm height plates to raise the rack, any idea how many shims I need to put in the rear for a proportional effect, or will it be determined empirically during a geo?

 

Graeme

[Mark H]

Great explanation Mark

 

With stiffer suspension what happens is instead of absorbing the bump you drive over it which changes the attitude of the car and makes it move around a lot more

 

That sums up the main difference I notice about the Exige compared to my old Elise, need two hands on the wheel at all times as you suddenly go following the contour of the road!

[Mark A]

Graeme,

 

Sorry, my original description was to discuss keeping things as per the standard situation.

 

However in reality Lotus suggest no change to the shims on the rear for lowering, and having seen the standard rear bumpsteer curve I'd agree, unless you notice it as a problem it's best left alone.

[Simon G]

Mark, I didn't have the rack plates fitted in the end. I haven't noticed any issues with bumpsteer (I'm not an expert as you know), uneven or bumpy surfaces don't seem to upset the car anymore than before. Am I likely to see increased wear in the steering rack by not fitting the plates?