Not your ordinary tyre question...

[kwacker]

Is there a technical reason why you should run different size tyres front and rear (ie 205/50-15 front and 225/55-16 rear).

The reason I ask is that the last time round I wanted the same tyres (SO2's) all round. I therefore had to go 225/50-15 on the front and 245/45-16 on the rear. BUT this caused the speedo to go out by approx 10%. I am now back in the market for a new set all round and would like the spedo to read correctly. So back to the question....

Is there a technical reason why I can not have 225/50-15 on the front and 225/55-16 on the rear?

Or should I just shut up and quit whinging? after all not everyone has a TVR!

Kwacker

[scruff400]

Yes, your speedo will be wrong.

Although it'll be OK when stationary.

[richb]

Well technically I guess TVR decided the car needed a wider tyre to give the handling some balance. What does the car have as original fit? I am sure the Chimaera had 245x45 16 originally so in fact the speedo will be correct with these and wrong on 225's. Unlessof course your talking about your other car?

[ianwayne]

Chimaeras have had a variety of tyres fitted throughout their life. Full details in SH's bible, mine has 225/50 16 on rear but post '98 models have 245/45 16. Change in ratio means onlt slight change in speedo reading, about 2% I think.

[craigalsop]

quote:
Chimaeras have had a variety of tyres fitted throughout their life. Full details in SH's bible, mine has 225/50 16 on rear but post '98 models have 245/45 16. Change in ratio means onlt slight change in speedo reading, about 2% I think.

My 2000 year Chimaera 450 had 225/50 16s on the back, so I think it must have changed again after '98.....

[Bob C]

Lets make sure we are all talking the same language. A tyre size of say 225/50/ZR16 is read as follows:

225 - tyre width in mm.
50 - aspect ratio (of the side wall to the width), so for a 225 wide tyre the side wall would be 50% of 225 i.e. 112.5 mm tall
ZR - speed rating
16 - diameter of the wheel in inches.

The speedo accuracy would be altered if the circumference of the tyre were changed. Assuming you keep the same size wheels the only thing that would change the circumference would be the aspect ratio changing i.e. 225/55.... would give a side wall height of 123.75 mm. BUT if you go to a 215 wide tyre with a 55 aspect ratio the side wall height would be 118.25 mm. 205/55 gives 112.75 which is vitually the same as a 225/50.

Hope this helps.

[douglasr]

The Elise S2 has much narrower tyres on the front to improve the handling balance - I take this to mean less grip at the front means slightly more understeer. On a car without PAS like the S2, then I suppose it may reduce the steering effort required as well.

There is also the practicalities of having a decent steering lock and turning circle.

[shpub]

quote:
My 2000 year Chimaera 450 had 225/50 16s on the back, so I think it must have changed again after '98.....

Must have happened when the black rubber harvest failed in Japan and tyre widths were rationed as a result making it easier to get 225s rather than 245s... Some have poopoohed this explanation and prefer the fact that TVR probably found some of the 225s and used them. Could you believe that?

Steve
www.tvrbooks.co.uk

[richb]

quote:
Lets make sure we are all talking the same language.

I expect we are...

[MajorClanger]

How much variation in tyre size can a wheel with a given rim width actually take?

MC

[ianwayne]

Some tyre manufature websites list what range of tyres are recommended for what size rims. I think Avon's website lists them but their site is donald ducked at the moment.
From memory, I think a 7 inch rim (fronts) can take from 195 to 225 without due distress and a 7 1/2 rim 205 to 245 (rears).

[shpub]

There's a tyre size/wheel speed per rpm excel spreadsheet on my website which calculates speedo errors...

Steve
www.tvrbooks.co.uk
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