Best aspect ratio? (Tyre size comparison)
Discussion
So, Im about to pick up another legacy spec.b, having sold my last one after quite a bit of bad luck. The new one will need new tyres soon, so Im looking for advice.
One of the things I disliked was the ride, it had been lowered and was on standard 215/45r18 tyres, and since then Ive owned a few other cars with smaller wheels and high profile tyres and the ride on those was much more comfortable. Therefore Ill be buying some 17" wheels for my subaru, I dont think 16" wheels would clear the breaks.
The standard tyres have an overall diameter of 650.7mm, I dont mind going for a SLIGHTLY larger tyre, but I dont want to upset things too much. Ideally I also want to go for a common size which is going to be easy to find if I get a puncture.
Im thinking 225/55r17 (would be 4.4% taller and 4.65% wider). A 225/50r17 is a closer match, but it seems to be quite an uncimmin size?
Your thoughts?
Also, a more general question, which aspect ratio gives most grip? Touring cars have low profile tyres, but most open-wheel racers dont? (Or is it because touring cars need big wheels to clear big brakes and then theres no room for a bigger tyre?)
Thanks!
One of the things I disliked was the ride, it had been lowered and was on standard 215/45r18 tyres, and since then Ive owned a few other cars with smaller wheels and high profile tyres and the ride on those was much more comfortable. Therefore Ill be buying some 17" wheels for my subaru, I dont think 16" wheels would clear the breaks.
The standard tyres have an overall diameter of 650.7mm, I dont mind going for a SLIGHTLY larger tyre, but I dont want to upset things too much. Ideally I also want to go for a common size which is going to be easy to find if I get a puncture.
Im thinking 225/55r17 (would be 4.4% taller and 4.65% wider). A 225/50r17 is a closer match, but it seems to be quite an uncimmin size?
Your thoughts?
Also, a more general question, which aspect ratio gives most grip? Touring cars have low profile tyres, but most open-wheel racers dont? (Or is it because touring cars need big wheels to clear big brakes and then theres no room for a bigger tyre?)
Thanks!
Edited by LotsOfLaughs on Tuesday 8th August 02:37
2.5 Forester SG9 XTs are on 215/55 x 17, standard size.
I have a set of 16" winter tyres/wheels which i kept from the previous Outback H6 and use during the season, no trouble clearing the brakes but its quite possible the Spec B's have bigger brakes, tyre size on those (correct and stated on the car's tyre label) is 215/60 x 16.
Don't know if thats of any use, the 17" are grippier but thats comparing apples and pears (summer/winter rubber), certainly harder riding too than the 16" which give such a pleasant ride i'm considering going all season 16" permanently.
I have a set of 16" winter tyres/wheels which i kept from the previous Outback H6 and use during the season, no trouble clearing the brakes but its quite possible the Spec B's have bigger brakes, tyre size on those (correct and stated on the car's tyre label) is 215/60 x 16.
Don't know if thats of any use, the 17" are grippier but thats comparing apples and pears (summer/winter rubber), certainly harder riding too than the 16" which give such a pleasant ride i'm considering going all season 16" permanently.
This is a great guide which I use all the time -
https://tiresize.com/tyre-size-calculator/
Personally I'd go something like 225/50r17 if you wanted to drop down into a 17" wheel size, and I'd also raise the suspension back up to standard ride height (guessing you're already planning on that)
https://tiresize.com/tyre-size-calculator/
Personally I'd go something like 225/50r17 if you wanted to drop down into a 17" wheel size, and I'd also raise the suspension back up to standard ride height (guessing you're already planning on that)
LotsOfLaughs said:
Also, a more general question, which aspect ratio gives most grip? Touring cars have low profile tyres, but most open-wheel racers dont? (Or is it because touring cars need big wheels to clear big brakes and then theres no room for a bigger tyre?)
Thanks!
Completely impossible to answer.Thanks!
Edited by LotsOfLaughs on Tuesday 8th August 02:37
Grip where, when ? under what circumstances ? Top fuel have huge sidewalls for maximum grip.....of course that would never apply to a road car
F1 have large sidewalls, however that's forced due to having to run 13" wheels
sidewalls may or may not give grip....but they absolutely give feel, steering feel, response, but can also then give a harsh ride. So sidewalls also give you comfort.
And every tyre will be different too, even for same size, just different brands are constructed differently.
They're just all different.
Not fussed about the speedo, they all under-read slightly from factory, and the increase is only minor.
Width = grip, because more surface area = more mechanical interlocking with the road surface = more grip
16" wheels wont clear spec.b brakes Im pretty sure, otherwise Id be tempted to try them out. But for a spec.b its 17" or 18"
Suspension on the new one is standard (its a sporty setup because its a sporty car, and thats fine, but the old one was just awful)
Im refering to grip when cornering. The only time I managed to "run out" of grip in my old one was when I was deliberately trying to find the limit. Was very very mild understeer. An excellent car, mine was ruined by the ride and the reliability.
Width = grip, because more surface area = more mechanical interlocking with the road surface = more grip
16" wheels wont clear spec.b brakes Im pretty sure, otherwise Id be tempted to try them out. But for a spec.b its 17" or 18"
Suspension on the new one is standard (its a sporty setup because its a sporty car, and thats fine, but the old one was just awful)
Im refering to grip when cornering. The only time I managed to "run out" of grip in my old one was when I was deliberately trying to find the limit. Was very very mild understeer. An excellent car, mine was ruined by the ride and the reliability.
Sheepshanks said:
Is that true?
I recall reading that the footprint stays the same - you can work out the weight of a car if you know the tyre footprint and pressure.
Correct, surface contact area is a simple function of weight divided by tyre pressure. If you increase the width of the tyre the length of the contact patch will be shorter if you leave the pressure the same.I recall reading that the footprint stays the same - you can work out the weight of a car if you know the tyre footprint and pressure.
Keeping things simple, if you have a 3000lbs car, with tyre pressures of 30psi you end up width a contact area of 100 square inches between the four tyres.
Sheepshanks said:
Is that true?
I recall reading that the footprint stays the same - you can work out the weight of a car if you know the tyre footprint and pressure.
Yes, thats correct, but you can also lower the tyre pressure for a wider tyre. Also, the weight transfer when cornering loads the tyre more because theres more sidewall to deform and so more surface area so more grip so more load so more deformation etc.I recall reading that the footprint stays the same - you can work out the weight of a car if you know the tyre footprint and pressure.
At least, I think? I assume there becomes a point where the floppyness of the sidewall becomes detrimental to the handling etc etc.
stevieturbo said:
Not really true, and subject to many variables.
If its not true, then why do M3s have wider tyres than 320Ds? On a smooth surface (in mechanics/physics terms) the surface area makes no difference, but a tyre interlocks woth the road surface, so surface area does matter.
LotsOfLaughs said:
If its not true, then why do M3s have wider tyres than 320Ds?
On a smooth surface (in mechanics/physics terms) the surface area makes no difference, but a tyre interlocks woth the road surface, so surface area does matter.
It is only one factor and, without considering other factors, it’s easy to ruin a car’s handling and road holding by just fitting wider tyres. There are lots of clued up people on this forum, but if you want to ignored well intentioned advice, go ahead. But, do yourself a favour and have a read about it. On a smooth surface (in mechanics/physics terms) the surface area makes no difference, but a tyre interlocks woth the road surface, so surface area does matter.
And if you ever find a road in the U.K. that is smooth, bump-free, defect free, never gets wet or greasy, and is great fun… the rest of us would like to know!
Yes, well thats exactly why Im asking!
Its very easy to go out and buy wider tyres, and its almost always the case that theyll give more grip, but surely if wider tyres were a good idea, they would have left the factory like that?
Dozens of engineers worked very hard to make sure the handling was good, so it would be a shame for some plonker [me] to ruin it by slapping wider tyres on if indeed it will make it worse
Its very easy to go out and buy wider tyres, and its almost always the case that theyll give more grip, but surely if wider tyres were a good idea, they would have left the factory like that?
Dozens of engineers worked very hard to make sure the handling was good, so it would be a shame for some plonker [me] to ruin it by slapping wider tyres on if indeed it will make it worse
LotsOfLaughs said:
Yes, well thats exactly why Im asking!
Its very easy to go out and buy wider tyres, and its almost always the case that theyll give more grip, but surely if wider tyres were a good idea, they would have left the factory like that?
Dozens of engineers worked very hard to make sure the handling was good, so it would be a shame for some plonker [me] to ruin it by slapping wider tyres on if indeed it will make it worse
Well, the engineers respond to requests from other departments, I’d guess, and the styling/marketing people have a big say. As many here know, driving on the UK’s terrible roads with 40 profile tyres on ‘sports’ springs and dampers can make every single drive unpleasant, but that’s what they seem to throw on lots of fairly mundane cars. Its very easy to go out and buy wider tyres, and its almost always the case that theyll give more grip, but surely if wider tyres were a good idea, they would have left the factory like that?
Dozens of engineers worked very hard to make sure the handling was good, so it would be a shame for some plonker [me] to ruin it by slapping wider tyres on if indeed it will make it worse
Lots here ask about making their cars more comfortable. Wider tyres pick up more bumps and pot holes BECAUSE they’re wider.
LotsOfLaughs said:
If its not true, then why do M3s have wider tyres than 320Ds?
On a smooth surface (in mechanics/physics terms) the surface area makes no difference, but a tyre interlocks woth the road surface, so surface area does matter.
Because they would look stupid with smaller wheels/tyresOn a smooth surface (in mechanics/physics terms) the surface area makes no difference, but a tyre interlocks woth the road surface, so surface area does matter.
Tony1963 said:
Well, the engineers respond to requests from other departments, I’d guess, and the styling/marketing people have a big say. As many here know, driving on the UK’s terrible roads with 40 profile tyres on ‘sports’ springs and dampers can make every single drive unpleasant, but that’s what they seem to throw on lots of fairly mundane cars.
Lots here ask about making their cars more comfortable. Wider tyres pick up more bumps and pot holes BECAUSE they’re wider.
Yes, going to a smaller wheel and a higher profile will make it much more comfortable. The question is; should I go for a wider tyre for more grip, or is it going to upset the handling? Of course all 4 tyres will be the same size. The boring legacy comes with 17" wheels as standard (215/50r17 iirc)Lots here ask about making their cars more comfortable. Wider tyres pick up more bumps and pot holes BECAUSE they’re wider.
LotsOfLaughs said:
Yes, going to a smaller wheel and a higher profile will make it much more comfortable. The question is; should I go for a wider tyre for more grip, or is it going to upset the handling? Of course all 4 tyres will be the same size. The boring legacy comes with 17" wheels as standard (215/50r17 iirc)
Once again, fitting a wider tyre does not mean more grip unless you change other settings as well.Gassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff