Tyre runout?

[Paul TVR]

Did I see a topic about pairing tyres on here using the lines painted in the tyres? Thought I did but can't find it in the search.

[Tony]

Did I see a topic about pairing tyres on here using the lines painted in the tyres? Thought I did but can't find it in the search.

Yes we did....Here you go. http://www.wheels-inmotion.co.uk/forum/ind...p?showtopic=165 Are you having problems?

[Paul TVR]

Not my car but someone from the Griffith forum is having problems on b roads of the car following the camber excessively and getting bump steer with pot holes, apparently feels very unsteady, has new bushes etc in the suspension so looking for other options

[Tony]

Not my car but someone from the Griffith forum is having problems on b roads of the car following the camber excessively and getting bump steer with pot holes, apparently feels very unsteady, has new bushes etc in the suspension so looking for other options

Tyre drift since compressive needs nothing to activate it so it would drift regardless of any road trauma.. The "bump-steer" reads concerning has the car been lowered or lowered further recently?

[Paul TVR]

No the car is a TVR Griffith, running standard shocks and springs, upgraded bushes have been fitted, the geometry has been done and not running anything heavy, just .75 neg camber on the front.

[Tony]

No the car is a TVR Griffith, running standard shocks and springs, upgraded bushes have been fitted, the geometry has been done and not running anything heavy, just .75 neg camber on the front.

Most times it's necessary to back track in an effort to identify the criminal.... The car has not been lowered/further so we have to assume the front wishbone and steering arm have the same radii. And after the bushings were fitted the Geometry calibration was corrected.... As you say -75' in not aggressive.... But the symptoms to me still reads Geometry calibration!

Sam@tdi could explain this more clearly than me..... but I'll have a stab at it! I think the calibration is at fault and this is why.... "Bump-steer" can easily be confused with pneumatic saturation. This condition is when the Yaw command exceeds the pneumatic ability, in this case during the bump-steer event. If the Geometric calibration is incorrect then the pneumatic ability is compromised in particular as the tyre lifts off the bump or if friction is suddenly reduced (drain cover or whatever). I would be extremely interested to read some actual calibration data regarding this vehicle.

[NicBowman]

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

[Tony]

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

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?

[NicBowman]

[

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

[Tony]

[

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

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.

[NicBowman]

[

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

[Tony]

[

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

Ok... To be honest i am not entirely comfortable with the maths conversions in the second site? Anyway since the camber adjuster is on the upper A arm then the influence to the "weighted" lower pin is near to non existent. Obviously the camber could be reduced to some ridicules position of something like -2d 30' in an attempt to manufacture stability but this will miss the point. I truly feel the scrub radius is incorrectly placed by the wheels off-set, but without actual data for the KPI i cannot maths this. For your type of chassis the KPI should be low in inclination so high figuratively speaking, 9 to 12d seems reasonable with a -.75' camber.... I bet the castor is +2d?

Since this has been an on going problem for you, wouldn't it be wise to have the full Geometry measured?

[Paul TVR]

Peninsula were selling adjustable bottom arms a few years ago, also had longer one to allow more camber. not sure what effect that would have though.

[Tony]

Peninsula were selling adjustable bottom arms a few years ago, also had longer one to allow more camber. not sure what effect that would have though.

The lower adjuster would be great but only serve as a proving ground with this particular problem. Manufactured additional adjusters would be intended for the race enthusiast "probably". The KPI's design is difficult to explain and indeed understand but fundamentally it replies to YAW by lifting it's perpendicular attitude thus suspending more weight. This weight upsets the equilibtium and is replied buy the absolute need to return via the steering rack since both wheels are connected. If the destination is outside of the scrub radius then the reply will be lazy, non-existent or need direct YAW commands from the driver... seemingly this is the case? Also as in this case the steering will become light at speed since the castor value is so low.. A balance between the -.75' camber a very declined KPI, 2 or 3 degrees of castor (probably 2) add a smattering of aoerodynamics then a very light front end should be stable assuming the contact patch is correct?

[NicBowman]

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

[Tony]

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

Is this your cars actual data? or suggested target positions.

[Paul TVR]

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

Is this your cars actual data? or suggested target positions.

 

On these cars the caster etc isn't adjustable so should be fixed.

[Tony]

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

 

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?

[NicBowman]

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

[Tony]

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

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

[NicBowman]

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

[Tony]

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

I strongly recommend you simply change the wheels and test first!!! The image of the car from the link suggests a visually healthy KPI and seemingly you have a long castor trail... A -.75 camber reads good, i assume positive toe 10' to 30' range.

My approach assisting development diagnostics is identify the criminall. To be honest we have very little data regarding your car but there are some immediate misgivings.

I do not like the wheel off-set and this displaces the scrub radius causing much distress in the dynamics... this is easily tested and costs next to nothing!! If after testing the problem still exists then a whole new chapter will need to be realised regarding how the Geometry can assist with a non-compliant chassis.

The "history figures" are examples from past proven positions.... The realm of the kit car needs guidance nothing else was intended from those figures! I did say "not your car"

Areas like the bump-steer can be addressed later, our main concern here is to capture the control issues you currently have. Change the wheels... If you can get a full Geometry image then we can assess the problem... evolution from there.