Ivan
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| posted on 27/9/11 at 08:32 AM |
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Tyre width and lateral grip?
Toying with the idea of changing the wheel size for my Cobra as there is a very poor selection of performance tyres in the 295 x 15 X 60 range (zero
available) so thought of going up to 17" or 18" for track days. However cost becomes a facto as well as wheel & tyre weight so was
looking for the real effect on handling by changes in tyre width.
Found this:
http://www.eng-tips.com/viewthread.cfm?qid=258842&page=2
which basically says:
Grip proportional to width ^0.15 ie go up from 200mm to 300 mm and gain 6% lateral Acc.
Stiffness Proportional to ^0.3 of width - ie from 200 to 300 gain 12% lateral force for given slip angle.
Haven't checked maths or theory but is interesting
Must scan my Milliken to see if there is corroboration.
Basically says to me that going from a no grip 295 X 15 to a high grip 255 x 18 tyre I should gain lots of grip with minimal loss in width effects.
[Edited on 27/9/11 by Ivan]
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MikeRJ
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| posted on 27/9/11 at 09:01 AM |
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quote:
Originally posted by Ivan
Basically says to me that going from a no grip 295 X 15 to a high grip 255 x 18 tyre I should gain lots of grip with minimal loss in width effects.
But then you have all the other negatives of huge wheels and low profile tyres. Greater unsprung and rotating weight, much harsher ride, less
progressive loss of grip, more easily damaged tyres and rims.
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multanen
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| posted on 27/9/11 at 09:30 AM |
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quote:
Originally posted by IvanGrip proportional to width ^0.15 ie go up from 200mm to 300 mm and gain 6% lateral Acc.
Stiffness Proportional to ^0.3 of width - ie from 200 to 300 gain 12% lateral force for given slip angle.
[Edited on 27/9/11 by Ivan]
Low profile 200mm tyre reaches it's maximal lateral force with smaller slip angle. Anyway you get better
lateral force with 300mm tyre, you just have to drive it with larger slip angle.
If it's for a track day there's probably many factors more important than absolute maximum lateral force.
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Neville Jones
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| posted on 27/9/11 at 09:48 AM |
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quote:
Originally posted by Ivan
Toying with the idea of changing the wheel size for my Cobra as there is a very poor selection of performance tyres in the 295 x 15 X 60 range (zero
available) so thought of going up to 17" or 18" for track days. However cost becomes a facto as well as wheel & tyre weight so was
looking for the real effect on handling by changes in tyre width.
Found this:
http://www.eng-tips.com/viewthread.cfm?qid=258842&page=2
which basically says:
Grip proportional to width ^0.15 ie go up from 200mm to 300 mm and gain 6% lateral Acc.
Stiffness Proportional to ^0.3 of width - ie from 200 to 300 gain 12% lateral force for given slip angle.
Haven't checked maths or theory but is interesting
Must scan my Milliken to see if there is corroboration.
Basically says to me that going from a no grip 295 X 15 to a high grip 255 x 18 tyre I should gain lots of grip with minimal loss in width effects.
[Edited on 27/9/11 by Ivan]
It's just not quite that simple.
The wider tyre has a narrower lengthwise contact patch for a given tyre pressure than a narrower tyre, which is more susceptible to roll and camber
change, which instantly loses all that mathematical extra grip.
And as said, the extra weight and rotating mass bring along their own extra problems.
You also have got to have the suspension designed to take full account of the behaviour of the wide tyre.
Dropping the profile then further adds its ramifications.
But, those 18" rims surely will look all 'bling'. The tyres will cost twice what the 15" do.
Spend the rim and tyre money on decent 15" rubber, and gain all the benefits with none of the downsides.
Cheers,
Nev.
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Steve Hignett
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| posted on 27/9/11 at 10:35 AM |
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Ignoring the obvious maths portion of this post - is going to a 2nd hand second set of wheels with slick tyres an option - they are cheap and readily
avail in the UK...
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Ivan
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| posted on 27/9/11 at 11:29 AM |
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I think the overall wheel weight will be the same as I currently run 15 X 10" steel rims and think 17 or 18 X 8" AL rims will pretty much
weigh the same and lower profile smaller tyres won't make much difference.
Good point re the 2nd hand racing tyres - but finding rims for them at 15" is a problem cost wise (about 1200 Pounds) due to 120.3 x 5 PCD
compared to 300 Pounds for 17 or 18" rims).
Another problem with current tyres are they are only rated for 180 kph and on a 230 to 250 kph car might not be such a good idea on track. If I could
find decent road tyres in the 275/295 X 60/55 X 15 size I would stick with them and not change wheels for track days as 4X those size wheels are a
storage problem in their own right.
Re the handling impact of lower profile tyres I believe the heavier weight of the Cobra compared to a Locost will lesson the impact of profile change
compared to a Locost.
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Fred W B
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| posted on 27/9/11 at 04:15 PM |
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Hi Ivan
Search on gt40s.com for some discussion on high perfomance 15" street tyres.
What I intend to do is import 15" tyres from the states, probably Hoosier DOT radials. You can import personally or get one of the V8 engine
parts suppliers to bring in in their weekly containers. I think you do have some companies that do that in Cape Town.
Look on Summit racing's website. They do all sorts of things you will want to buy.
http://www.summitracing.com/
Cheers
Fred W B
[Edited on 27/9/11 by Fred W B]
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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