Wider wheels tyres V's bump travel

[TMcRally]

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

 

[patprimmer]

If the extra width also increases track width, there might be some undesirable consequences re traction and durability. The extra spring rate might also have undesirable consequences re traction and durability. Unless the travel is limited to <35mm if the car feels better, there is always the risk that the driver will go harder and damage a tyre or guard.

Regards
Pat
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[NormPeterson]

255/40 should be slightly shorter than 235/45, perhaps 4mm less radius. That's not much, but it would seem to be in your favor.

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

 

[patprimmer]

Mud guard = fender

Regards
Pat
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[TMcRally]

Hi all

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

 

[patprimmer]

Speed costs money. How fast do you want to go.

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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[GregLocock]

The problem you've got is that you have two changes simultaneously - one in the right direction - wider tires, and one in the wrong direction - stiffer springs/less 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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[TravWood]

For less of a trade off, what about a more progressive spring, dual rate spring or longer bump stops. This will keep the ride spring rate closer to what you have but give you extra force as you compress beyond ride height.
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?