Eliminating bump steer

[Vovchandr]

Anybody ever try the Jack Webb solution?

 

It seems pretty praised online.

 

The alternative to the problem that is done by some is shimming the steering rack otherwise

 

https://jackwebbmotorsport.co.uk/Adjustable-Bump-Steer-Track-Rod-Ends-for-Caterham-7-Pair-p109852977

 

 

Seems like a clever solution

 

You can change the bump steer characteristics by shimming the outboard rod ends appropriately. Rack spacers do not allow for proper adjustment, since changing the height of the rack on one side effects the bump steer curve of the other side. It is always a compromise. This is the only method to effectively neutralise bump steer.

 

As well as the bump steer, you also benefit from lighter, more communicative steering, and reduced stiction/friction in the suspension system (aiding contact patch stability).

[xcarguy]

I used components from McMaster-Carr and Aircraft Spruce to achieve the same solution for my Stalker. Made a world of difference. Post #365... http://www.usa7s.net/vb/showthread.php?7852-Stalker-85-headed-back-to-Arkansas/page37

Edited August 7, 2020 by xcarguy

[Shortshift]

Given that your car is a S3, then if it follows the general trend for this type of Caterham it will likely need either the rack raising (9mm seems to be the oft-quoted amount) or, conversely, the outer TREs lowering. The effects of each method are not directly equal and opposite to each other but they are generally like that - so you can either raise the inner joints (the rack) or lower the outer joints (the TREs) to achieve broadly the same effect.

 

You're right to say that Jack's adjustable TREs do seem to have a good reputation and they offer superb ease of adjustability (compared to shimming the rack) but I'd check, before ordering, how much adjustment they offer in each direction either side of the nominal factory spec. They also look cool... I don't know for sure but I suspect that they offer more in the 'increase height' direction than they do in the 'lower height' sense. So you might be a bit tight if you need to lower them significantly (analogous to raising the rack, which seems to be the S3 norm). You might also check the steel specs; if the pin is longer than the standard TRE pin you might reasonably expect to see it subjected to higher levels of bending stress. I think that Jack's pins are EN8 (a conventional engineering grade carbon steel) but I also know that some people prefer to see an alloy steel being used in this type of application. And I think that Jack's TREs are not taper fit and they do require the control arm to be drilled out to take the non-taper pin (not a big deal, so long as you are aware).

 

Finally, from what I have found out on bump steer, if you're driving on road it may pay not to chase the oft-quoted holy grail of zero bump steer. This seems to be universally the way to go for circuit driving but, for road use, it seems to be generally considered that a small amount of toe-out on bump is fine (perhaps even preferred). It certainly appears to be the case that toe-in on bump (particularly for road use) should be avoided.

 

James

Edited August 22, 2020 by Shortshift

[ashyers]

Have you measured the current curves?

[Vovchandr]

Given that your car is a S3, then if it follows the general trend for this type of Caterham it will likely need either the rack raising (9mm seems to be the oft-quoted amount) or, conversely, the outer TREs lowering. The effects of each method are not directly equal and opposite to each other but they are generally like that - so you can either raise the inner joints (the rack) or lower the outer joints (the TREs) to achieve broadly the same effect.

 

You're right to say that Jack's adjustable TREs do seem to have a good reputation and they offer superb ease of adjustability (compared to shimming the rack) but I'd check, before ordering, how much adjustment they offer in each direction either side of the nominal factory spec. They also look cool... I don't know for sure but I suspect that they offer more in the 'increase height' direction than they do in the 'lower height' sense. So you might be a bit tight if you need to lower them significantly (analogous to raising the rack, which seems to be the S3 norm). You might also check the steel specs; if the pin is longer than the standard TRE pin you might reasonably expect to see it subjected to higher levels of bending stress. I think that Jack's pins are EN8 (a conventional engineering grade carbon steel) but I also know that some people prefer to see an alloy steel being used in this type of application. And I think that Jack's TREs are not taper fit and they do require the control arm to be drilled out to take the non-taper pin (not a big deal, so long as you are aware).

 

Finally, from what I have found out on bump steer, if you're driving on road it may pay not to chase the oft-quoted holy grail of zero bump steer. This seems to be universally the way to go for circuit driving but, for road use, it seems to be generally considered that a small amount of toe-out on bump is fine (perhaps even preferred). It certainly appears to be the case that toe-in on bump (particularly for road use) should be avoided.

 

James

 

Thank you. There is a bit to digest in that post, but it certainly seems to not be a "brainless cure on" bolt on solution as it would seem.

 

Have you measured the current curves?

 

Negative

[ashyers]

You may want to take the time to see where you're at before throwing $ at it. If you do end up making changes make sure you have your ride height dialed in. I don't know how sensitive the bump steer on a Caterham is, but it's possible you'll find what works at your "road" setting is not the hot ticket for your "track" setting.

[bball7754]

Some info on Caterham bump steer here:

 

http://www.fastgrandad.co.uk/documents/Measuring%20and%20Correcting%20Bump%20Steer%20on%20a%20Caterham%207.pdf

 

His webpage with other articles is here:

http://www.pauldeslandes.co.uk/page8.html

 

I originally found the bump steer article on Blatchat.

 

Steve

[Shortshift]

Measuring whatever bump steer characterstic you have on your car before doing any modifications is obviously sensible. It will give you a reference or datum point from which you can make and measure incremental adjustments.

 

Methods to do this at home include:

 

(1) Making a gauge (as per the Paul Deslandes pdf/Blatchat article above)

 

(2) Using a laser lamp and projecting onto a surface ahead of the car to plot the characteristic at that forward distance. These results then need to be scaled (schoolboy mathematics/trigonometry) to get back to bump steer at the wheel/hub (plenty on Blatchat about this)

 

(3) Using a laser lamp in conjunction with a mirror to measure the extent of bump steer directly at the hub (a general Google search will throw this up).

 

One small consideration to bear in mind with (2) above is that, strictly speaking, you also need to take into account the lateral movement that your hub will go through as it is swept from full droop (rebound) to full bump, even with zero bump steer (ie: with the wheel and hub always facing perfectly straight ahead). In a Caterham, with double wishbone front suspension, there will be something in the region of 3mm of lateral movement due to this geometric effect and you could argue that this needs to be factored into the results that you will otherwise get. The mirror method avoids this.

 

But, in any case, measuring to establish a base-line and to determine the effect of any changes you make is an essential part of the process.

 

James

Edited August 23, 2020 by Shortshift

[Pokey]

Anybody ever try the Jack Webb solution?

 

It seems pretty praised online.

 

The alternative to the problem that is done by some is shimming the steering rack otherwise

 

https://jackwebbmotorsport.co.uk/Adjustable-Bump-Steer-Track-Rod-Ends-for-Caterham-7-Pair-p109852977

 

Seems like a clever solution

 

I recently reached out to Jack for a different purpose and learned that he doesn't sell to the US because of UK product liability law.