Wider wheels tyres V's bump travel
Wider wheels tyres V's bump travel
(OP)
Hi
We usually run 235/45/17 R Spec (street legal race tyres). Recently we ran an event with a later model car (2008) which had bigger wheels and wider tyres and we ran 245/40/18. They are actually 17mm wider in the tread width. I didn't drive the car but the driver (a professional driver) said there was a significant improvement in grip, worthy of some further testing on the older cars (2005 + 2007). (These are Subaru AWD by the way)
We have 60mm of bump travel in the suspension. The next event allows me to run a wider wheel (not bigger) and tyre.
How wide is too wide?
I can run a 255/40/17 which is actually another 15mm wider than the 245's and 32mm wider than the original 235's, problem is the tyres will hit the guard if I allow more than 35mm bump travel.
These are tarmac rally cars and with current spring rates the bump travel will easily exceed the 35mm travel in normal loads and sometimes bottom out on bump stops in gutters and pot holes.
Question is: if I increase the spring rate until in normal use the travel is = or < 35mm will the extra lateral grip from wider tyres exceed the ability for the tyre to stay in contact with the road on the uneven surface with the higher spring rate.
I assume you will say it depends on how uneven the road surface is. However the roads aren't rubbish but do have bad spots and pace notes can identify potential problems.
Maybe I'm just talking out loud but does anyone have an incite.
Thanks
Dave
We usually run 235/45/17 R Spec (street legal race tyres). Recently we ran an event with a later model car (2008) which had bigger wheels and wider tyres and we ran 245/40/18. They are actually 17mm wider in the tread width. I didn't drive the car but the driver (a professional driver) said there was a significant improvement in grip, worthy of some further testing on the older cars (2005 + 2007). (These are Subaru AWD by the way)
We have 60mm of bump travel in the suspension. The next event allows me to run a wider wheel (not bigger) and tyre.
How wide is too wide?
I can run a 255/40/17 which is actually another 15mm wider than the 245's and 32mm wider than the original 235's, problem is the tyres will hit the guard if I allow more than 35mm bump travel.
These are tarmac rally cars and with current spring rates the bump travel will easily exceed the 35mm travel in normal loads and sometimes bottom out on bump stops in gutters and pot holes.
Question is: if I increase the spring rate until in normal use the travel is = or < 35mm will the extra lateral grip from wider tyres exceed the ability for the tyre to stay in contact with the road on the uneven surface with the higher spring rate.
I assume you will say it depends on how uneven the road surface is. However the roads aren't rubbish but do have bad spots and pace notes can identify potential problems.
Maybe I'm just talking out loud but does anyone have an incite.
Thanks
Dave





RE: Wider wheels tyres V's bump travel
Regards
Pat
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RE: Wider wheels tyres V's bump travel
Limiting bump travel to 35mm might be difficult when a significant amount of travel can be caused by wheel displacement over pavement bumps and irregularities. Stiffer springs and sta-bars are better at controlling travel caused by chassis inertial responses to control inputs.
I'm not sure what the term 'guard' is referring to. Can it be modified for greater clearance per sanctioning body rules, or can wheel offset and perhaps camber settings be used to gain a little more room?
Norm
RE: Wider wheels tyres V's bump travel
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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for site rules
RE: Wider wheels tyres V's bump travel
The off-set will only allow the wheel to move inwards 3mm before the tyre contacts the strut, the rest of the additional width will have to move out.
I am able to roll the lip on the guards/fender, it won't give me enough though, and I may be able to squeeze a bit more out of the guard by fitting it differently but I'm not allowed to modify the factory panel.
A little bit of looking and the Bridgestone tyre has a shorter radius again and will give me 5.5mm benefit.
I'm not sure what the wider tyres effect may be on optimal camber settings, we currently run 3.25F/2.5R I assume we would need somewhere around 0.5 less with stiffer springs and wider tyres ?
Is there enough potential benefit to give it a try or should I just use the 245's.
Dave
RE: Wider wheels tyres V's bump travel
That is an old saying, but it leaves of the extra message.
The cost increase is exponential for a linear increase in performance.
Only you can decide when you have got to the point where the return on investment becomes not worth it, but if it were me, I would stay with the 245s.
Sometimes drivers do not notice the the change in traction if you don't tell them you made a change or they notice an improvement when you tell them you made changes when in fact you did not.
Regards
Pat
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RE: Wider wheels tyres V's bump travel
The interplay between those two is very dependent on all sorts of details. Simplistically the question is whether the better grip around the corner offsets the worse traction into and out of it.
Cheers
Greg Locock
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RE: Wider wheels tyres V's bump travel
Other points to consider:
Wider tyres for the same sprung mass may take longer to get to temp and an increase (slight) in spring rate will help heat them up a bit quicker.
The wider rims and rubber will add weight in the worst possible spot but the gains may be better than the losses?