Page:
Home > Help Needed / General Tech Chat > Tyre diameters.

Hedgemonkey

User Avatar

591 Posts
Member #: 360
Stu from Corwall aka Mr Jazz Piano, Love_Machine, kneegrow

12th Nov, 2004 at 09:05:52pm

Quick question, what diameter are typical 13" tyres?

I've got a useful spreadsheet of gear ratio/final drives for 10" wheels and I'll add the 13" ones and whack it for general perusing in technical. Would probably make a useful comparison.

cheers stu

Bugger off, I'm getting there.

adam1330

User Avatar

1014 Posts
Member #: 238
Post Whore

essex

14th Nov, 2004 at 06:11:06pm

i make mine, 1595mm.

““A turbo: exhaust gasses go into the turbocharger and spin it, witchcraft happens and you go faster.”” - Jeremy Clarkson


Hedgemonkey

User Avatar

591 Posts
Member #: 360
Stu from Corwall aka Mr Jazz Piano, Love_Machine, kneegrow

14th Nov, 2004 at 07:21:52pm

I assume that is the circumference, otherwise you must have a unimog!

Bugger off, I'm getting there.

nutter driver

User Avatar

969 Posts
Member #: 47
Post Whore

Not very sunny swanage

14th Nov, 2004 at 07:31:37pm

as a interesting point, i had three wheels lined up next to each other the other day 10 12 13 and they all ha pretty much the same rr, to within say 3-4mm so i am not sure how much difference it really makes???

And on the 7th day........... God created turbochargers!

billydude

291 Posts
Member #: 48
Senior Member

Leicester

14th Nov, 2004 at 07:46:09pm

http://www.miata.net/garage/tirecalc.html :)

AuthiMini

12 Posts
Member #: 44
Member

14th Nov, 2004 at 11:24:15pm

On 14/11/2004 19:31:37 nutter driver said:
as a interesting point, i had three wheels lined up next to each other the other day 10 12 13 and they all ha pretty much the same rr, to within say 3-4mm so i am not sure how much difference it really makes???



Dear NutterDriver,

The point is not so much the static, unloaded difference between them. Different size tyres with different aspect ratios, widths and almost identical "off the vehicle" diameters can have very different "rolling diameters", depending on how much that tyre deforms sligthly once fitted to the car, supporting all the weight and on the move. Needless to say, unless you are talking about very old bias-ply tires or big drag racing tires, rolling diameter is always smaller that the overall diameter as measured "off the car"...

A similar story: the difference between max static load and max dynamic load, which can lead to VERY interesting results at high speed if you confuse them when fitting tyres to, say, an M5.

Cheers

Hedgemonkey

User Avatar

591 Posts
Member #: 360
Stu from Corwall aka Mr Jazz Piano, Love_Machine, kneegrow

15th Nov, 2004 at 09:35:17pm

Very true, although looking at 165 falkens and 165 camacs, the dia is about 20mm different, maybe more. (on the rims)

Bugger off, I'm getting there.

AuthiMini

12 Posts
Member #: 44
Member

16th Nov, 2004 at 08:24:22am

Hedgemonkey, you have to bear in mind that for a given tyre size, rolling radii can vary quite a lot between different manufacturers. I seem to remember, for instance, that the rolling radius in 145/80 13 sizes of the old Pirelli P3 and Michelin ZX brands was different enough to be felt while driving a Fiat/Seat 127, for example.

Which is why ideally the actual rolling radius is the one quoted by the manufacturer in its catalogue - it just cannot be guesstimated.

A very good reason then to keep matching tyres ?not only same size but also same brand? in each axle.

What you say about the apparent difference between your Falkens and Camacs makes you think about there being so many possible variables in the actual performance of a tyre, like its internal structure and materials used. Possibly even two otherwise identical tyres of the same brand but with different speed rating have different rolling radii.

Incidentally, at least here in Spain 165/70 10 Camacs are not exactly renowned for being perfectly true, and the same has happened to me, I am sorry to say, with Dunlop rubber (which had cost an arm and leg at the time). Felt like a bloody roller coaster.

Regarding your idea of a table to compare tyres, gear ratios, FDs and so on I think it has already been done. Have a look at

http://www.ime.org.uk/ime/f/final_drive.html

Cheers

Home > Help Needed / General Tech Chat > Tyre diameters.
Users viewing this thread: none. (+ 1 Guests)  
To post messages you must be logged in!
Username: Password:
Page: