NicBowman

I would say yes. 0 to negative is more doable. You need to shorten the lower arms position and incline the sai/kpi, or change the wheels off-set. Does anyone have a foolproof way of measuring scrub radius? Just to ensue I am correct. To change on a TVR is tricky, if I change the wheel offset the wheels will look odd, 20mm inset. An increase in wheel diameter will give a small bonus, but as KPI is around 11 degrees, it is only 1mm scrubb change for every 5mm radius of wheel. So would need 23" rims to fully correct! Otherwise it is a remanufactured upright. Nic

Hi My 92 TVR Griffith has been discussed in this forum previously, symptom is steering gets pulled by road defects, eg potholes, etc. I have just measured the scrub radius, not a desperately easy task, and find it about 20mm positive. I recon the car drives as though it has too much positive scrub, the question is, is 20mm a lot? Too much? Ta Nic

Nic Very unlikely if you have two or more degrees of positive castor!.... I worked in the end stages of a well known kit car developer who had almost identical problems to you... They went down the logical path using additional castor to retain stability and steering return-ability. The chassis design is similar to yours "twin A arm"... Anyway my job was to see were the Geometry modifier had gone wrong? they had.. Castor +7d Camber -30' KPI 3d Basically the scrub radius was near to 0 so the KPI was worthless.... If you analyse the virtual power of the castor value to the mechanical presence of the KPI it's obvious a vertical pin is at rest, whereas a lower perpendicular pin angle adds resistance... meaning the lower perpendicular attitude by design will naturally resist deviation due to the vehicles weight..... Historically the lighter the vehicle the lower the attitude. From memory the modifications finished at (this is not your positions remember that!) Castor +2d 30' Camber - 40' KPI 13d Historically again most castor/KPI combinations =15d.. This particular development was quite stressful since the target car was not adjustable, so each step in developmentment meant re-casting and engineering.. Also check this thread:- http://www.wheels-inmotion.co.uk/forum/ind...p?showtopic=561 Hi Interesting. My castor is around 5 degrees, from measurement just taken, which would make sense if the norm is 4 d and I have increased mine. I measured looking at wheel rim movement, using a dial test indicator to find the point of least movement, then calculating its angle from vertical. If the history figures are to be believed, then I have a KPI of 13d 20m, a castor of 5d, and 0.75d camber. (The original Sierra castor was around 2 degrees). From your experience with the kit car, perhaps I should try reducing the castor, it will go down to around 3d. It will give more real bump steer, as the steering arm moves with the castor change. But worth a try?? How do you normally approach this type of problem, I assume change and test, and then do it again? Or more theoretically? Thanks Nic

But Nic we don't know what your KPI is?... Would it not also be possible to borrow some other wheels with the correct off-set just for testing purposes? Hi The castor is adjustable, but limited, moving spacers around. I may be able to borrow some other wheels to try. But, steering is relatively lacking in self centring, would lack of castor be a suspect? Nic

Some numbers from historic data, the Chimera had the same suspension, the figures I found are for the Chimera, castor 4 degrees, KPI 13 degrees 20 minutes. Nic

[ Nic How adjustable is the front camber and where is the adjuster... My reasoning is to effect the KPI and re-locate the scrub radius just for testing purposes. Unfortunately i am not familiar with the suspension design. Hi There is a good picture here :- http://www.thegriffithpages.com/modifications/ Also, my car and article are:- http://www.thegriffithpages.com/modificati...easurement.html The upper wishbones are bolted through the upper ball joint, so very adjustable camber. The spacing of the ball joint between the upper wishbones adjusts the castor, so not very adjustable, back and forth around +/-2mm. Thanks Nic

[ Hi Nic welcome to wim.... Lets see if we can help you with this problem. Do you have any actual data for the KPI sometimes also called SAI or SJI. And an obvious question when did this handling issue arise, meaning was there a time when things were ok and now... if yes what modifications were done to the car in between? Also the castor... when you say "increased" is this more toward positive? if yes how much more? Hi, King pin inclination unknown. The car was like it when I brought it, but worse, I have been trying to track it down all 2006. Started by simply removing all play. Castor increased, more positive, simply a small adjustment made by placing the spacers either side of the top balljoint on the front, instead of one front one rear. The steering is still quite light on the road, and okay at parking speeds. Car started almost undrivable, flew all over the road. In handling terms now, it turns in very well, is balanced towards slight oversteer with or without power, so will rear wheel drift at speed. Steering very responsive, but feels to light at speed, front feels less planted than I would like. Sorry a bit 'descriptive'. I could probably measure the castor and KPI if I had a think about it. Thanks Nic

I would be extremely interested to read some actual calibration data regarding this vehicle. Tony, thanks for your interest. The problem is not conventional bump steer. I have made sure everything is in good condition, i.e bushes, dampers, springs, rack, tr ends, steering uj's etc all new. I have measured the bump steer, and it is pretty good, only about 1mm of movement at the rack connection over the suspension travel. The symptoms do not feel like bump steer. If you hold the stering wheel still the car goes in a straight line regardless, again not a symptom of bump steer. But, the road surface and depressions etc pull on the steering, so you have to hold the steering wheel quite tight! The way to drive bumpy roads fast is hang on tight! My initial thought was that the wheels were too far offset, i.e centre of wheel contact patch outside the turning centre for the upright. The front wheels are 7x15 et25. The original Ford (the uprights are Ford) was et35. So the TVR wheel appears to be 10mm further out than the Ford was. This 10mm might cause the increased force at the steering wheel from wheel contact with road features? General setup, camber -0.75 degrees, tracking 0.5 degrees per wheel toe in. Castor is slightly increased from factory setting, (it was needed to dial out the bit of bump steer). Tyres Bridgestone S02's 205x55x15 at 23psi. Rear wheels are the same set up. Any advice appreciated. Nic