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Increase tyre size = worse traction?

Posted: Wed Jan 14, 2015 8:45 pm
by daxtojeiro
Hi all,
I have a traction issue when accelerating hard. So I've looked into fitting larger rear tyres.
255/45/17 to 275/40/17 using Toyo R888s.

But Ive been told that this increase will throw out the camber of the car during acceleration and make traction worse. Does this make sense?
The rear end is Jag XJ6 based,
thanks
Phil

Posted: Thu Jan 15, 2015 7:30 pm
by volospian
My guess is that it could make traction worse, depending on the camber angle. If it's fairly high, I suppose the angle could mean that one edge isn't necessarily getting enough downforce to create much additional grip.

I've heard in the past that simply getting wider tread will change the contact profile, making it shorter and wider (obviously :)) but the potential variance in things like tyre pressure, camber angle, etc. can effect how the new profile actually contacts the surface, potentially reducing grip (or at least not necessarily increasing grip).

So, for example, if your standard profile is 3" wide by 4 " deep you'd get 12 sq in. of contact, but if you changed the profile to 4 x 3 instead, and say the outside inch doesn't grip properly because of pressure/camber/etc, it would effectively reduce the contact area to 3 x 3 or only 9 sq in.

Not sure if that's all just pub philosophy or not, but it sounds feasible to my simple mind. I'll maybe consult Mr Staniforth later:)

Re: Increase tyre size = worse traction?

Posted: Fri Jan 16, 2015 2:08 pm
by stevieturbo
daxtojeiro wrote:Hi all,
I have a traction issue when accelerating hard. So I've looked into fitting larger rear tyres.
255/45/17 to 275/40/17 using Toyo R888s.

But Ive been told that this increase will throw out the camber of the car during acceleration and make traction worse. Does this make sense?
The rear end is Jag XJ6 based,
thanks
Phil
Wheel width ? is tyre flat on the ground ? Both same tyre and compound ?

And just changing wheels, camber will be exactly the same as this is governed by suspension parts.

Posted: Fri Jan 16, 2015 3:48 pm
by volospian
Just to clarify my comments above, I can't see it "throwing camber out" just by firing wider rubber, but I think I can see why simply fitting wider tyres may not improve, and even potentially reduce, traction.

I edited my comment to make my point a bit clearer :)

Posted: Fri Jan 16, 2015 4:10 pm
by DaveEFI
Think everything being equal it would help on a good dry surface. But likely worse on a damp or wet one.

Posted: Fri Jan 16, 2015 6:30 pm
by kiwicar
Hi
the biggest issue in a light car like the cobra replica will not be camber but the ability to get enough heat into the tyre, in particular the tread blocks. Increasing the size of the tyre will just make this worse in that you need to get a bigger volume of tread hot enough to get the rubber gripping properly. What you probably need to look at is getting a similar or slightly narrower tyre in a softer compound, and if you can a lighter construction. Why do F1 cars have tyre warmers, why do I do a burn out before a run in the slingshot?
best regards
Mike

Posted: Fri Jan 16, 2015 6:42 pm
by stevieturbo
I dont really think you can compare a road car, with F1.

F1 tyres need around 100degC for their rubber to work correctly...hence why they need heaters before going on track.


Width on the rim, tyre contact, suspension all matter.

As well as actual usage. Is it only road ? track ? race ? drag ? or a combo of them all ?

Suspension will play a big part too in getting the power to the ground

Posted: Fri Jan 16, 2015 7:33 pm
by Darkspeed
Have google search on the subject of "coefficients of friction tyre rubber"

Change to "tire" for a more american view

It depends what you are trying to do 1/4 mile straight line acceleration needs a different set up to one for corning grip.

Hows the LSD set, what sort of weight transfer do you get - what camber change in squat do you get - is any of this adjustable? etc etc.

Posted: Fri Jan 16, 2015 8:18 pm
by DEVONMAN
As you are going for a lower profile then the straight line break away point is likely to be sooner. Cornering should be better but only in the dry. All subject to the tyre temperature of course. If you are upgrading to 888's then you should see an all round improvement but just new tyres compared to old will give better wet grip.
If the loss of traction is accompanied by tramping then it may be a suspension or tyre pressure issue.

Posted: Sun Jan 18, 2015 8:59 am
by Eliot
Just drop a 3.5L rover in - that will solve your traction issues.

Posted: Sun Jan 18, 2015 8:16 pm
by mgbv8
You have a supercharged 5.3 V8 in a car that has almost no weight over the rear tyres Phil.

What did you think would happen :)

I'm no expert on suspension geometry etc. So I cant comment on what the guys have said above. If this is a road car I would be looking at a tyre with the best footprint on the road and a reasonable compound. So something like a BFG Drag Radial tyre. But I dont think their DOT approval is valid here in the UK. Although I did use them them on the road here for several years before the MG became a track only car. The MOT guy didnt see to have a problem with them ?

Just my thoughts I hasten to add. !!

Perry

Posted: Sun Jan 18, 2015 8:35 pm
by stevieturbo
Some Nitto 555R's or the BFG's would probably be ok.


DOT means nothing here. BUT. It's only law/rule/nonsense that tyres SOLD for road use in the UK must carry the E mark.

Not that tyres used on the road must be E marked. So it is a little grey.

But both BFG and Nitto in those sizes should have more than adequate tread to be perfectly legal.

Not sure how available they are, but you can also get softer compounds than the medium GG 888.

I've used the Kumho V70's before and you can get them in various compounds too, but I find their tread design makes them go illegal very quickly.


888's are a great all rounder, but their days are limited because of stupid EU legislation.

Given lightweight, the R1R might be better as with a more open tread it should warm up a little quicker. These have been discontinued though, so any left will be old stock and once run out, that's it.