Tyre size - will it make a difference??

[jonnie5]

I currently have standard 16" wheels on my Cerb with 225/45s on the front and 245/45s on the back.

I've just acquired a set of anthracite wheels to make my car look really evil, still 16" but the tyres on the front wheels are 225/50s (backs are the same as current wheels). The tyres are in good condition although different type, so I need to change at least one (one S02 and one brand new S03).

So my question is - What impact will changing the tyre size on the front from 225/45 to 225/50 have on the handling? Or should I consider putting my existing tyres from the front onto the new wheels as they are only 4,000 miles old.

I know that's two questions but it's been a long day

Cheers
Jon


>>> Edited by jonnie5 on Friday 2nd July 20:18

[pilmar]

I have 225/45 on the front and 245/50 on the back. They are the standard 16" wheels and I think that this is the standard configuration.

[carl_w]

Standard configuration for a 4.2 on 16"s is 225/45 on the front and 245/45 on the rear.

[jonnie5]

hmmmm, OK thanks guys. These wheels came off of a Cerb so that must have been running 225/50s on the front.

It's looking like I'll get my 225/45s swapped onto the new wheels...

cheers
Jon

[carl_w]

Well personally I can't see it making any difference mechanically. You're changing the rolling radius at the front, but the engine drives the rear so no change to gearing. The only think is you're effectively raising the nose of the car by about an inch (225*.45 is 101mm, 225*.5 is 113 mm, so 12mm bigger at the top and the bottom of the wheel = 24mm, about an inch). Might not be a good plan with a car with a VMax of around 190mph

[jonnie5]

carl_w said:

Well personally I can't see it making any difference mechanically. You're changing the rolling radius at the front, but the engine drives the rear so no change to gearing. The only think is you're effectively raising the nose of the car by about an inch (225*.45 is 101mm, 225*.5 is 113 mm, so 12mm bigger at the top and the bottom of the wheel = 24mm, about an inch). Might not be a good plan with a car with a VMax of around 190mph

That's great maths at this time on a Saturday night!! That's the sort of logic I was looking for - you are of course absolutely right, I'll be swapping my 45s!!!

[pilmar]

According to my manual it should be 235/50 on the back

[carl_w]

You're right -- it's the same in my manual. However, my rears are 245/45 and I bought a second set of rims from another PHer and the rears are also 245/45. I suspect that 235/50 are difficult to get. Of course, going from 235/50 (118mm height) to 245/45 (110mm height) involves an overall 16mm drop on the rear, which might explain why I can't get my trolley jack under the sills

[shpub]

carl_w said:

You're right -- it's the same in my manual. However, my rears are 245/45 and I bought a second set of rims from another PHer and the rears are also 245/45. I suspect that 235/50 are difficult to get. Of course, going from 235/50 (118mm height) to 245/45 (110mm height) involves an overall 16mm drop on the rear, which might explain why I can't get my trolley jack under the sills

I think you will find that the drop is only 8mm as the car's height is set by the distance from the axle to the ground and that means only one tyre width. As the tread pattern is about 8 mm deep anyway and will reduce, the difference is not that great.

There is a tyre size spead sheet that does all the calcs including speeds and %age differences in the tyre faq on my website.

Steve
www.tvrbooks.co.uk

[DustyC]

or...
www.miata.net/garage/tirecalc.html

will tell you the speed difference too.

[carl_w]

Good point.

Although, as I said earlier, there should be no gearing difference if you're only changing the front wheels.

[jonnie5]

Does anybody have an idea how this would effect the handling of the car. I'm running Nitrons all round and had the full set up by Topcats, so would putting 50s rather than 45s on the front be foolish, in that it would need re-setting up?

Thanks in advance
Jon

[olly]

I suspect the 50's on the front are just from a speed 6 car, so you might just notice the front is slightly softer than with your old tryes.....

Can't really see it affecting the handing that you'd notice on the road though!