Suspension Settings - Dialing Out Bump Steer
Discussion
deadscoob said:
Didn't Dave change the tyres at the same time as the wheels? So to get a true comparison we need a lback to back test with normal bs thinner wheels but both with same tyres
Yes he did. I was looking around last night and 305/30/19 tyres are plentiful & almost half the price of 335/30/18s. With so much choice in that size, it's making a wheel change look like a very interesting option.V8Dom said:
interesting reading
i have a few questions
if you are running 100mm fr and 160mm rear ride height, then the car is low at the front and the caster will be different to normal..... has this change helped or have you readjusted caster to compensate ride height differnces?
Since making the changes i notice you still have some bump steer... have you removed the rack again as altering caster and camber will alter the tseering rod angles too so the rack will need moving again?
interesting how you are running toe out at the front.... ive found on rear wheel drive cars, this tends to want the car to lead you in straight line rather than naturally want to go straight... may i ask why you have gone this way rather than toe in? what differences it made
also what tyres are you running...
thanks dom
Hi Dom... crikey, I have a week away on business and when I return the factory are offering longer wishbones and the forum is awash with handling chatter i have a few questions
if you are running 100mm fr and 160mm rear ride height, then the car is low at the front and the caster will be different to normal..... has this change helped or have you readjusted caster to compensate ride height differnces?
Since making the changes i notice you still have some bump steer... have you removed the rack again as altering caster and camber will alter the tseering rod angles too so the rack will need moving again?
interesting how you are running toe out at the front.... ive found on rear wheel drive cars, this tends to want the car to lead you in straight line rather than naturally want to go straight... may i ask why you have gone this way rather than toe in? what differences it made
also what tyres are you running...
thanks dom
My car is on the red bushes, so no opportunity to adjust caster, unfortunately. Rose jointed suspension was the only extra I never went for during the build and I kick myself over it frequently. If I do the longer wishbones, then I'll change the rear over to rose joints too.
The rack has been adjusted to minimise the bump steer as far as is possible, using a Longacre gauge. Since doing the geo changes noted above, I've not put it back on the Longacre, chiefly because we're going to do the track rod end mod, which will give simpler & finer adjustment. The hope is that with as much as possible done to the rack, the track rod mod should allow residual BS to be dialled out.
As far as the toe is concerned, I went with the setup recommended by Jonny, based on his previous trials. All together, his setup has made a big difference to the way the car handles & feels. It turns in nicely and feels less nervous at all times, than it ever has done. This setup will be further refined, Jonny has another idea to try when I go back in a few weeks...
I binned the Kumhos after too many near death experiences, on the road. The Pirelli P-Zeros made a massive difference to it's road manners (way less tramlining & wandering) and should be miles better in the damp. I'm not worried about track grip, as it's a 99% road car for me. I'm considering a wheel size change to get more tyre choice, but I think that'll be one for a different thread
Storer said:
So my repairs are done and it is now time to test different wheel/tyre combinations.
The car is not too wayward on the 335/245 combination but there is some movement on ridges.
This evening I popped the 285 Khumo track tyres on the front (still 335 Pirelli on the rear) and although the grip was much better (as expected) there was more movement and some strong 'tugging' of the wheel on uneven roads.
Tomorrow morning I will leave the 285s on the front and try 295s on the rear.
The final trial will be 245 front and 295 on the rear.
Interesting time!
Paul
The 245 Kumho's tramline so badly, that I imagine the 285s will be worse..... it'd be interesting to hear what a wider front tyre in an alternative brand is like. Also what width & offset vs std are your wider front wheels, as that'll have an impact too.The car is not too wayward on the 335/245 combination but there is some movement on ridges.
This evening I popped the 285 Khumo track tyres on the front (still 335 Pirelli on the rear) and although the grip was much better (as expected) there was more movement and some strong 'tugging' of the wheel on uneven roads.
Tomorrow morning I will leave the 285s on the front and try 295s on the rear.
The final trial will be 245 front and 295 on the rear.
Interesting time!
Paul
After a lot of work and questions from other owners, I thought it’s now time to close this thread out with some updates and conclusions.
Before I kick off, I will caveat the post by saying that while a number of owners have approached me with the same issues, there are also those who have had no such problems. I won’t presume to guess why this impacts some cars more than others and just say, I hope my experience help those who have an issue with Bump Steer.
As discussed on this thread, after getting the car on the road and finding bump steer (BS for short, from here) to be an issue, the first thing I did was follow the factory’s advice by slotting the rack mount holes to allow the rack itself to be moved up/down. This had a marginal effect on proceedings, but didn’t resolve the BS.
While investigating the chassis geo, I decided to swap out the Kumho tyres I was running. On the road, these are really nervous, tramline terribly and generally exaggerate any handling issues. The replacement P-Zeros made a big difference to the way the car felt, but of course doesn’t fix BS. Oh, and they melt on track after 10mins….. but that could just be my driving
Following the tyre change, I decided to bring the steering rack’s inner track rod pivot point in line with the inner mount plane (the line drawn between the wishbone’s mounting points on the chassis). This meant shortening the rack itself and extending the track rods themselves. This was done by ZCars, although in retrospect, I’d suggest it’s more cost effective to have Titan build a custom rack from scratch instead. This change made a very positive difference to the steering feel and took another bite out of the BS problem.
As intimated in my 2015 posts, I did fit MacG Racing’s outer tie rod BS adjustment kit, along with their fast road / race springs. Together these both made a very sizable impact on the car’s handling, by further reducing the BS, but hugely increasing body control. With less roll in corners, less pitch under acceleration, less dive under breaking and therefore, more controlled suspension movement, the car felt a lot more planted…. But again, BS wasn’t eradicated.
Over the winter, I chose to take the leap on longer wishbones from MacG, together with some custom 19” wheels from BC Forged. The wishbones gave a wider track, ability to adjust caster & slightly reduce the effect of suspension movement on the steering, therefore, again reducing BS. The lightweight 19” wheels were spec’d to: accommodate the longer wishbones; allow me a full choice of Porsche GT/Turbo spec rubber (cheaper than std 18” Ultima sized tyres, much more plentiful and more modern specs / construction); and narrow the rear width slightly. That last point is another topic of discussion amongst some owners who feel there is a BS issue with the rear suspension too….. I have always felt that the rear wasn’t tracking perfectly, but felt this as a tramlining sensation from the 335 tyres. In taking 1.5” from the width of the rear wheels and using a 305 tyre, this sensation has significantly reduced. The longer wishbones, lighter wheels & more modern Michelin Super Sport tyres, all had a very positive effect on the handling, again.
This week the last piece of the puzzle slotted into place. I decided to fit MacG’s own spec of Quantum Zero-One dampers, to compliment their (already fitted) springs. Quite simply, these are a revelation. The car now rides superbly well, soaks up bumps with incredible ease, inspires confidence and finally feels like a cohesive package. Driving the car it’s night & day vs how it was when it first went on the road. I can send it down A-Roads at speed with only a gentle one-handed grasp on the steering wheel, without fear that a bump will wrench the wheel from my hand and pitch the car into a field! I need to do a few more miles down some broken B-Roads, but so far, so good and I think I’ve finally beaten this BS problem, as well as resolving a few other handling traits.
So there you have it. It’s taken a lot of work to get to this point, but I’m glad I’ve made the investment in the car. For those who have experienced the same issues, I hope this helps….. assuming you’ve read this far! I’ll continue to report on the car (mostly via my FaceBook build page and the Owners group) and as always am happy to hear from others, as they make similar updates or seek advise on their car’s handling (to the extent I can, of course!). For me, I’m happy that this BS chapter is closed and plan to enjoy the car to the fullest. Drive safe, folks!
Before I kick off, I will caveat the post by saying that while a number of owners have approached me with the same issues, there are also those who have had no such problems. I won’t presume to guess why this impacts some cars more than others and just say, I hope my experience help those who have an issue with Bump Steer.
As discussed on this thread, after getting the car on the road and finding bump steer (BS for short, from here) to be an issue, the first thing I did was follow the factory’s advice by slotting the rack mount holes to allow the rack itself to be moved up/down. This had a marginal effect on proceedings, but didn’t resolve the BS.
While investigating the chassis geo, I decided to swap out the Kumho tyres I was running. On the road, these are really nervous, tramline terribly and generally exaggerate any handling issues. The replacement P-Zeros made a big difference to the way the car felt, but of course doesn’t fix BS. Oh, and they melt on track after 10mins….. but that could just be my driving
Following the tyre change, I decided to bring the steering rack’s inner track rod pivot point in line with the inner mount plane (the line drawn between the wishbone’s mounting points on the chassis). This meant shortening the rack itself and extending the track rods themselves. This was done by ZCars, although in retrospect, I’d suggest it’s more cost effective to have Titan build a custom rack from scratch instead. This change made a very positive difference to the steering feel and took another bite out of the BS problem.
As intimated in my 2015 posts, I did fit MacG Racing’s outer tie rod BS adjustment kit, along with their fast road / race springs. Together these both made a very sizable impact on the car’s handling, by further reducing the BS, but hugely increasing body control. With less roll in corners, less pitch under acceleration, less dive under breaking and therefore, more controlled suspension movement, the car felt a lot more planted…. But again, BS wasn’t eradicated.
Over the winter, I chose to take the leap on longer wishbones from MacG, together with some custom 19” wheels from BC Forged. The wishbones gave a wider track, ability to adjust caster & slightly reduce the effect of suspension movement on the steering, therefore, again reducing BS. The lightweight 19” wheels were spec’d to: accommodate the longer wishbones; allow me a full choice of Porsche GT/Turbo spec rubber (cheaper than std 18” Ultima sized tyres, much more plentiful and more modern specs / construction); and narrow the rear width slightly. That last point is another topic of discussion amongst some owners who feel there is a BS issue with the rear suspension too….. I have always felt that the rear wasn’t tracking perfectly, but felt this as a tramlining sensation from the 335 tyres. In taking 1.5” from the width of the rear wheels and using a 305 tyre, this sensation has significantly reduced. The longer wishbones, lighter wheels & more modern Michelin Super Sport tyres, all had a very positive effect on the handling, again.
This week the last piece of the puzzle slotted into place. I decided to fit MacG’s own spec of Quantum Zero-One dampers, to compliment their (already fitted) springs. Quite simply, these are a revelation. The car now rides superbly well, soaks up bumps with incredible ease, inspires confidence and finally feels like a cohesive package. Driving the car it’s night & day vs how it was when it first went on the road. I can send it down A-Roads at speed with only a gentle one-handed grasp on the steering wheel, without fear that a bump will wrench the wheel from my hand and pitch the car into a field! I need to do a few more miles down some broken B-Roads, but so far, so good and I think I’ve finally beaten this BS problem, as well as resolving a few other handling traits.
So there you have it. It’s taken a lot of work to get to this point, but I’m glad I’ve made the investment in the car. For those who have experienced the same issues, I hope this helps….. assuming you’ve read this far! I’ll continue to report on the car (mostly via my FaceBook build page and the Owners group) and as always am happy to hear from others, as they make similar updates or seek advise on their car’s handling (to the extent I can, of course!). For me, I’m happy that this BS chapter is closed and plan to enjoy the car to the fullest. Drive safe, folks!
The BS issue is a very odd one..... I can only conclude that some cars / people are more sensitive to the problem than others. Having had lots of discussions on it with other owners experiencing the same issues, there are of course those who have had no such challenges. Odd.
Regarding the Pilot SS tyre choice, I decided to give them another shot having continued to read positive reviews on them. Perhaps my experiences with them on the RS was a one off? I was considering Cup 2s, but thought I'd build upto that one, given the other works that I was planning (i.e. Quantums). Certainly, I can say so far so good anyway!
You're spot on with your comments on wheel / tyre combos. I don't "do" track days.... I drive 99% on the road (British A/B roads at that ) and then have a nibble at a track once every 6mths for 20mins, for a bit of fun. I'll soon be going to Switzerland with the car to enjoy some mountain passes and return via the Ring for a lap or two. No way I can take two sets of wheels for that one, LOL!
The move to 19s was more than just this tho', as I mentioned above, the rear wheel steer effect had bugged me for a while too, so needed resolving. I expect there's still some compliance in the rear wishbone bushes that will be having a small effect, but they'll be swapped out in due course too. Thankfully this is minor and is a nice to have rather than a necessity, as the other things were.
Each change I've made has had a positive effect on the car, so I wouldn't want to take anything off. The rack change was a good one in that it wasn't just the handling benefit it gave, but also feel & weight were improved. Together with the new springs and shocks, the difference in the car is huge vs when I started this thread.
More feedback to come as the miles rack up, but so far I'm really pleased with the changes and can feel the confidence coming back into the car
Regarding the Pilot SS tyre choice, I decided to give them another shot having continued to read positive reviews on them. Perhaps my experiences with them on the RS was a one off? I was considering Cup 2s, but thought I'd build upto that one, given the other works that I was planning (i.e. Quantums). Certainly, I can say so far so good anyway!
You're spot on with your comments on wheel / tyre combos. I don't "do" track days.... I drive 99% on the road (British A/B roads at that
) and then have a nibble at a track once every 6mths for 20mins, for a bit of fun. I'll soon be going to Switzerland with the car to enjoy some mountain passes and return via the Ring for a lap or two. No way I can take two sets of wheels for that one, LOL!The move to 19s was more than just this tho', as I mentioned above, the rear wheel steer effect had bugged me for a while too, so needed resolving. I expect there's still some compliance in the rear wishbone bushes that will be having a small effect, but they'll be swapped out in due course too. Thankfully this is minor and is a nice to have rather than a necessity, as the other things were.
Each change I've made has had a positive effect on the car, so I wouldn't want to take anything off. The rack change was a good one in that it wasn't just the handling benefit it gave, but also feel & weight were improved. Together with the new springs and shocks, the difference in the car is huge vs when I started this thread.
More feedback to come as the miles rack up, but so far I'm really pleased with the changes and can feel the confidence coming back into the car
deadscoob said:
Cool, great you're getting it how you like it.
Am weighing up whether to sell the forgelines and change to 19s, have been procrastinating over it for about a year lol.
Good on the Supersports, we have them on the gt3 RS and they're a fantastic road tyre and also pretty good on track too, miles better than ps2s.
Sounds like you're looking for an excuse to go for the 19s, LOL! Just do it Am weighing up whether to sell the forgelines and change to 19s, have been procrastinating over it for about a year lol.
Good on the Supersports, we have them on the gt3 RS and they're a fantastic road tyre and also pretty good on track too, miles better than ps2s.
Ultima IT said:
BobE said:
Is everyone sure that the issue is just bump steer? I'm coming to GTR ownership via a TVR Tuscan and that car even with everything set up perfectly in terms of ride height, wheel alignment and rack position was still extremely nervous on the road. Someone on the Tuscan forum had been looking in some detail at the front suspension design and kinematics and came to the conclusion that the steering axis relative to the wheel centreline/ contact patch was not optimum. In other words the scrub radius was wrong. By using a 3mm wheel spacer on each front wheel the situation was transformed. I couldn't believe the difference this - what seems like a minor change - made to the stability of the car down a rough road. From hanging on for dear life you could literally take your hands off the wheel the car was so stable. Has anyone ever tried altering the wheel offsets very slightly?
I think you have hit the targhet.A lot of people previously have fitted longer wishbones and used different rims (increased ET).
Some people have mantain central section of the original rims and used different rim lip/halve/dish to translate steering axle. Some other have fitted central rim section inverted on halves
From all the analysis I've done on this and the conversations I've had with people much more informed on this topic that I am, I agree with the above. It's not simply one thing, it's a combination of many different factors.Of all the changes I've made on my car, each tweak has made a difference. Rack height in the chassis, adjustable track rod end heights, steering rack & track rod lengths, wishbone lengths, spring rates, custom shocks, 19" wheels (custom ETs & widths) and Mich SuperSport tyres. The way the car drives now is unrecognisable from how it was when it first hit the road 3.5yrs ago. I can drive down an A-road at a decent lick, 1-handed, in a calm manner that previously wasn't even possible on a motorway.
Suspension setup is a real dark art, perhaps those who have analysed this in detail could give us their findings with facts / measurements?
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