Low vs. High Profile tyres
Discussion
Does anyone have any concrete information on this?
As I understand it, the benefit of high profile tyres, small rims is that they are lighter than low profiles / large rims, hence the unsprung weight is lower. What's so good about unsprung weight anyway?
I believe that another advantage is that the lighter the rims/tyres, the easier it is to accelerate them as there's less inertia to overcome.
However, low profile tyres deform less and allow for more control over the contact patch, better steering feel, more control over damping (high profile tyres can cause undamped responses to bumps)...
So what are the reasons for most kits (obvious exceptions excepted) being fitted with small rims and high profile tyres?
As I understand it, the benefit of high profile tyres, small rims is that they are lighter than low profiles / large rims, hence the unsprung weight is lower. What's so good about unsprung weight anyway?
I believe that another advantage is that the lighter the rims/tyres, the easier it is to accelerate them as there's less inertia to overcome.
However, low profile tyres deform less and allow for more control over the contact patch, better steering feel, more control over damping (high profile tyres can cause undamped responses to bumps)...
So what are the reasons for most kits (obvious exceptions excepted) being fitted with small rims and high profile tyres?
One advantage of low profile tyres is that there is less deformation when cornering. However on a lightweight kit car this isn't really a problem, and the extra give of a higher profile helps ride/handling.
Light cars don't require monster brake disks either. Big wheels / tyres and disks increase the unsprung weight.
I was once tempted with 15's on my Fury, but after investigation decided to stick.
>> Edited by RichardD on Tuesday 11th November 14:41
>> Edited by RichardD on Tuesday 11th November 14:41
Light cars don't require monster brake disks either. Big wheels / tyres and disks increase the unsprung weight.
I was once tempted with 15's on my Fury, but after investigation decided to stick.
>> Edited by RichardD on Tuesday 11th November 14:41
>> Edited by RichardD on Tuesday 11th November 14:41
LexSport said:
What's so good about unsprung weight anyway?
The lighter the unsprung mass, the better the resaponse from spring and damper inputs. If the unsprung mass is heavy, the springs and dampers need to work harder, therefore the grip level will drop as the tyre is not on the surface of the road.
Cars with a high Sprung -to- unsprung mass ratio ride better (e.g. Jags, Rollers, Mercs). This should also help handling because, as has been said, the dampers don't have to work as hard to keep the tyres in contact with the road. Imagine a very heavy wheel, tyre and brake hitting a bump as you drive along. Their natural tendency will be to carry on upwards for a while after hitting the bump (effectively they "take-off" like a skiier going over a ski jump). A light wheel/tyre/brake combination will need less restoring force to "push" it back down on to the road surface where it can do something useful.
As far as low profile tyres go, they are less tolerant of suspension geometries where the camber changes a lot as the wheel moves upwards. A full-profile tyre has lots of deflection in the carcass so it keeps pretty much the same footprint as the wheel changes its camber angle. The "downside" is that the "footprint" isn't as big for a given rolling diameter. If you get a suspension set-up that keeps the tyres flat under all circumstances (difficult) low-profiles have the potential for offering better grip. Also, the deep, squishy sidewall of the full profile tyre acts against you when turning into a corner because it provides a bit of a delay between what the wheel rim is doing and what the contact patch is doing. In other words, as you start to turn the steering wheel and the road wheel starts to turn, the sidewall just flexes and the footprint stays where it is for a fraction of a second. This can make the car feel a bit vague when you first turn into a bend but should also make it feel much less nervous and twitchy to dirve. No easy answer then -there's advantages and disadvantages to both. Also, I've had problems with light cars an wide tyres tending to "aquaplane" in the wet. Most kits (I think) are ridiculously over-tyred for their weight but wide tyres are fashionable!
As far as low profile tyres go, they are less tolerant of suspension geometries where the camber changes a lot as the wheel moves upwards. A full-profile tyre has lots of deflection in the carcass so it keeps pretty much the same footprint as the wheel changes its camber angle. The "downside" is that the "footprint" isn't as big for a given rolling diameter. If you get a suspension set-up that keeps the tyres flat under all circumstances (difficult) low-profiles have the potential for offering better grip. Also, the deep, squishy sidewall of the full profile tyre acts against you when turning into a corner because it provides a bit of a delay between what the wheel rim is doing and what the contact patch is doing. In other words, as you start to turn the steering wheel and the road wheel starts to turn, the sidewall just flexes and the footprint stays where it is for a fraction of a second. This can make the car feel a bit vague when you first turn into a bend but should also make it feel much less nervous and twitchy to dirve. No easy answer then -there's advantages and disadvantages to both. Also, I've had problems with light cars an wide tyres tending to "aquaplane" in the wet. Most kits (I think) are ridiculously over-tyred for their weight but wide tyres are fashionable!
I used to run 195/45 x 14s on my Sylva Fury.
Looked nice but gave the car a terrible ride.
So I changed to Caterham R500 13" rims which look miles better which lesss than a gnats chuff and use Avon ACB10s which have a much fatter sidewall. The extra sidewall gives a more comfortable ride and more notice when the car is about to go sideways than the low profiles which were much less forgiving, so it is easier to go quicker since you feel more confidence in what the car is going to do.
My old Sylva Leader used 185/70 x 13s and that had superb handling would drift so easily, again due to the right tyres for the job.
Looked nice but gave the car a terrible ride.
So I changed to Caterham R500 13" rims which look miles better which lesss than a gnats chuff and use Avon ACB10s which have a much fatter sidewall. The extra sidewall gives a more comfortable ride and more notice when the car is about to go sideways than the low profiles which were much less forgiving, so it is easier to go quicker since you feel more confidence in what the car is going to do.
My old Sylva Leader used 185/70 x 13s and that had superb handling would drift so easily, again due to the right tyres for the job.
Sort of... I think! the trouble is I'm getting out of my depth here now! I think the honest answer is that there aren't that many tyres with a light enough weight rating to work well on most kit cars. I think Avon make some special tyres for Caterhams but beyond that, I'm not sure. Also, the suspension geometry has a lot to do with it. Many cars don't utilise their full contact patch properly and the lighter the car and the lower the profile, the worse the problem is because they car just isn't heavy enough to "squash" the sidewall into taking up the difference between the camber angle and the road surface. I'm guessing a bit here but if you went out on the track and found that your tyres were "going off" because they got too hot, you could probably use bigger ones. Until that happens, you'd probably gain much more grip by using a softer compound than by using a bigger tyre. I'll ask some mates who are well up on this sort of thing and get back to the forum. Might take a few days though.
Thanks for the reply! any info greatly appreciated!
I'm rebuilding a GT40 rep & can't dcide on wheel/tyre size! The car isn't all that light, weighing in at about 1100 kg.
There is also a vanity issue !! The cars look 'right' with 15" rims, but the tyre choice is very limited for the rear, 16" rims give more choice (up to 255 wide) & look similar to 15", whereas 17" rims give plenty of tyre choice (up to 335 wide) but change the look of the car dramatically.
Also I beleive the geo isn't all that clever & a wide 17" tyre would be very difficult to keep in full contact with the ground, but perhaps the wider choice of tyre & compounds may compensate......
I'd appreciate any input to help me decide, I can't afford to buy too many sets of wheels & tyres!!!
I'm rebuilding a GT40 rep & can't dcide on wheel/tyre size! The car isn't all that light, weighing in at about 1100 kg.
There is also a vanity issue !! The cars look 'right' with 15" rims, but the tyre choice is very limited for the rear, 16" rims give more choice (up to 255 wide) & look similar to 15", whereas 17" rims give plenty of tyre choice (up to 335 wide) but change the look of the car dramatically.
Also I beleive the geo isn't all that clever & a wide 17" tyre would be very difficult to keep in full contact with the ground, but perhaps the wider choice of tyre & compounds may compensate......
I'd appreciate any input to help me decide, I can't afford to buy too many sets of wheels & tyres!!!
Agree that a GT40 will look better and more authentic with smaller wheels and higher profile tyres. what about a set of sticky mickeys? See website.
www.mickeythompsontires.com/street.html
They are somehow hooked up with Avon I think, sure you could get tyres in UK. Can get big width on small wheel. Probably not advisable to venture out in wet weather with these boots on, but will do the biz at the Pod on a sunny day.
(edited cos I spelt tyre tire - oh no I am becoming an American!)
>> Edited by gdr on Thursday 27th November 11:24
www.mickeythompsontires.com/street.html
They are somehow hooked up with Avon I think, sure you could get tyres in UK. Can get big width on small wheel. Probably not advisable to venture out in wet weather with these boots on, but will do the biz at the Pod on a sunny day.
(edited cos I spelt tyre tire - oh no I am becoming an American!)
>> Edited by gdr on Thursday 27th November 11:24
Julian,
If still excited, try a US part importer like Ultimate Spares of America www.ultimatespares.co.uk/ or maybe Real Steel (01895 440505).
There's nothing for sale in the US (apart from serious weaponry) that you can't get here by some means. Most Ultima builders buy their engines from US.
>> Edited by gdr on Thursday 27th November 16:05
If still excited, try a US part importer like Ultimate Spares of America www.ultimatespares.co.uk/ or maybe Real Steel (01895 440505).
There's nothing for sale in the US (apart from serious weaponry) that you can't get here by some means. Most Ultima builders buy their engines from US.
>> Edited by gdr on Thursday 27th November 16:05
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