Suspension Settings - Dialing Out Bump Steer

[deadscoob]

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.

[356Speedster]

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

[deadscoob]

I would have already if I hadn't had to refurb a gearbox that was apparently refurbed

[key750]

Hello Mark.

Watching your pics about steer system rods mod seems MacG mod is aim to change Ackermann and antiAckermann angle!

I think this is one of the bigger improvement (after longer wishbones and consecutive minor scrub radius).

What do you think?

[BobE]

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?

[Ultima IT]

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:

Rusti Evo said:

2001ultima - it's difficult to mount the kingpins/brakes deeply enough in the wheels to provide perfect geometry. The solution on my car was to source a 1 inch deep outer rim and mount the wheel inner on the outside of it. Looks as ugly as sin but seemed the only solution at the time.

Edited by Ultima IT on Saturday 21st January 06:52

[356Speedster]

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

Good to see this is still being discussed 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?