Wheels and tyres - yet again

[Ian Macquarie]

I've read most of the past threads about wheel and tyre sizes and come to the conclusion that most seem to reccommend a change from my current 15" michelins to 13" somethings (probably AO21s as most of my driving is on the road).

 

 

I still have a few questions I'd like to ask.

 

1. George Polley and DT each has a list of Yokohama tyre sizes for AO21-R which would seem to imply that the optimum wheel width is 5.5" - why does everyone (including Caterham) seem to suggest using 6" rims?

 

2. The rolling circumference is less than pi X diameter (For a 185/70R13 it is 1790, diameter is given as 587, piX587 = 1844) I presume that this is due to the fact that the tyre is squashed under load. If I want to see the effect on ride height, is it reasonable to calculate "rolling radius" as rolling circ divide by two pi and then comparing for different tyres or is it not as simple as that?

 

3. AO21Rs are listed as competition tyres. Are there any issues with using them on the road - either with police or insurance companies in the event of an accident?

 

Ian

 

 

[james fillingham]

I cannot answer your precise technical queries, but have just done the same.

 

I had 15" prisoners and pilots as per normal - this gave me sump/ground clearance of 74mm. I swapped to 13" superlites with 60 profile and the clearance decreased to 50mm. This frightened me. Last weekend I put 70 profile Yoko 21rs onto the car and clearance is back up to c 70mm.

 

I guess it depends on your suspension set-up, existing clearance etc, but that is the type of variation I found.

 

James

[Ian Macquarie]

Thanks James,

 

So on your figures I should expect a 4 mm drop ground clearance going from 195/50 15s to 185/70 13s, everything else being left as it is.

 

Is this other peoples experience?

 

Ian

[murph7355]

Actual figures compared to mathematical ones aren't usually *that* far out. So for comparative purposes you'll be there or there abouts. I think they probably use some assumptions on tread depth etc (the 70 bit in the profile relates to width so doesn't account for variation in tread depth).

 

70 profile will help minimise the difference and will probably increase comfort levels by a fair degree (certainly over 45 or 50 profile 15"ers). I have no experience of them though as I use 60's myself. The higher the profile, the more likely it is that the tyre will add relatively uncontrollable effects to the whole suspension layout though.

 

You'd have to try both back to back to check if the differences were noticeable on either count. 70s are quite a bit cheaper I believe.

 

Adjustable platforms are a very good idea if you don't have them already. You can offset some of the ride height difference using these if you're really worried, but I'm sure I have nearer 50mm of ground clearence and, as long as you're alert, it doesn't cause issues. With adjustables you can also tune the set up to make best use of the avaiable tyre.

 

I have 6" rims with 185 tyres and it works fine. There's usually a range of widths that are OK for the various tyre widths. Wider rim "flattens" the tyre and v.v. If you go too far either way though it gets dangerous (for obvious reasons).

 

I don't believe there's an issue with using the 21Rs on the car, but have never been queried by the rozzers or had to claim. I *think* they're E marked and so should be fine.

[Drumster]

Ian, I also changed from 15" prisoners and michelin pilots to 13" superlites and 185/70/13 A021Rs. The main purpose being to maintain the same ground clearance and get some decent rubber

 

185/70/13 A021Rs have the added bonus of costing only £40 inc VAT per corner from George Polley and delivery is another £10 on top.

 

Chris Alston

 

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