Help with Tyre sizes
Discussion
SBN said:
Got a 2009 m3 with 18" alloy set up
The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
That's a significant difference in rolling radiusThe summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
The 40 is the height of the tyre expressed as a % of the width
so 245/40 is 98mm height
235/40 is 94mm height
Edit to add actually it's closer than I thought
Edited by B'stard Child on Monday 22 December 15:54
Thanks, called BMW they were kind of helpful but wouldn't couldn't recommend It. Same with Ats as well.
Thing is a 8" BMW alloy on the e90 takes a 225 as standard. This is a 8.5 so why jump up to a 245 surely a 235 would work??? But then BMW puts a 245 on for a reason and must contribute to handling, turn in etc having the 245 vs 235
Thing is a 8" BMW alloy on the e90 takes a 225 as standard. This is a 8.5 so why jump up to a 245 surely a 235 would work??? But then BMW puts a 245 on for a reason and must contribute to handling, turn in etc having the 245 vs 235
B'stard Child said:
SBN said:
Got a 2009 m3 with 18" alloy set up
The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
That's a significant difference in rolling radiusThe summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
I'd be looking more for 235/45 18
Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?
SBN said:
B'stard Child said:
SBN said:
Got a 2009 m3 with 18" alloy set up
The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
That's a significant difference in rolling radiusThe summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
I'd be looking more for 235/45 18
Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?
40 is profile (or percentage of the width)
18 is rim width
total rolling radius is a combination of tyre and rim width but you can just as easily work out the difference on just the tyre (if the rim diameter doesn't change)
Try this link for tyre sizes vs rim sizes: http://www.tyresizecalculator.com/tyre-wheel-calcu...
You should be fine with a 235.
And this for calculating the tyre circumference:
http://www.csgnetwork.com/tiresizescalc.html
Looks like you lose an inch of circumference with the smaller 235/40, but you gain nearly 2 inches with the alternative 235/45. I would therefore stick with the 235/40, which after all is the winter wheel for your car anyway (if i'm reading your post right).
Remember a slightly narrower front tyre may mean your car transitions to understeer more readily.
You should be fine with a 235.
And this for calculating the tyre circumference:
http://www.csgnetwork.com/tiresizescalc.html
Looks like you lose an inch of circumference with the smaller 235/40, but you gain nearly 2 inches with the alternative 235/45. I would therefore stick with the 235/40, which after all is the winter wheel for your car anyway (if i'm reading your post right).
Remember a slightly narrower front tyre may mean your car transitions to understeer more readily.
kambites said:
lufbramatt said:
Don't forget a tyre has about 6mm of wear between new and worn out, so the 4mm difference in height mentioned above is hardly worth worrying about IMO.
This is true, and even that's assuming you keep your tyre pressures perfect all the time. SBN said:
I'll have a look at the handbook later but I did speak to BMW and they would only specify the tyres I have on the car without another option being available.
Predictable - they didn't say that it wasn't OK though. You'll be running with the same radius that you run when the 245s are quite worn (but still legal) and a width which won't bother you unless you're at more than 9/10s of the handling capability. Unless there's some weird electronic sensor issue or something you should be OK - that's why you could just do with a BMW expert to confirm it. You need a good independent BMW specialist or a friendly tyre company engineer that knows the car to remove that 1% of doubt; I guess that most people would just try the tyre.B'stard Child said:
SBN said:
B'stard Child said:
SBN said:
Got a 2009 m3 with 18" alloy set up
The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
That's a significant difference in rolling radiusThe summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
I'd be looking more for 235/45 18
Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?
40 is profile (or percentage of the width)
18 is rim width
total rolling radius is a combination of tyre and rim width but you can just as easily work out the difference on just the tyre (if the rim diameter doesn't change)
Wheel width will be a number followed by the letter J.
TA14 said:
SBN said:
I'll have a look at the handbook later but I did speak to BMW and they would only specify the tyres I have on the car without another option being available.
Predictable - they didn't say that it wasn't OK though. You'll be running with the same radius that you run when the 245s are quite worn (but still legal) and a width which won't bother you unless you're at more than 9/10s of the handling capability. Unless there's some weird electronic sensor issue or something you should be OK - that's why you could just do with a BMW expert to confirm it. You need a good independent BMW specialist or a friendly tyre company engineer that knows the car to remove that 1% of doubt; I guess that most people would just try the tyre.Debaser said:
B'stard Child said:
SBN said:
B'stard Child said:
SBN said:
Got a 2009 m3 with 18" alloy set up
The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
That's a significant difference in rolling radiusThe summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?
I'd be looking more for 235/45 18
Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?
40 is profile (or percentage of the width)
18 is rim width
total rolling radius is a combination of tyre and rim width but you can just as easily work out the difference on just the tyre (if the rim diameter doesn't change)
Wheel width will be a number followed by the letter J.
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