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MG MGB GT V8 Factory Originals Technical - Wheel/tyre size help

I could do with some help too on wheel/tyre sizes. I have a V8 under restoration/conversion. It is going to have Sebring type wings fitted. Can anyone advise the wheel/tyre size that will fit and fill the arches properly. Rim width and offset would be really useful. I anticipate 15" diameter and not using a low profile tyre.
Thanks
Alan Boother

Alan,
I can't say that I know the Sebring track or guard width but you may have trouble filling the arches without going to lower profile. ie. the wider the tyre the less likely it is to have high profile.

If you like the look of Superlites then my opinion, and taste, would have me putting on 15x8 front and rear with say 245/50/15 front and 265/50/15 rear. Do not take my word for it because I have not measured a Sebring car but estimate that this would be in the ball park.

15x8 Superlites are a 4 negative wheel.

By the way, a 265/50/15 tyre has the same sidewall height as a standard 14 inch tyre.

HTH

Stainzy
David Staines

I'm hoping to get a bit of a "gear-up" by having larger diameter wheels plus they fill the wings slightly better in my opinion. Hopefully 152 will be okay as thisi s the standard MGC diameter. Anyone know for sure what the racing lightweights used or is anyone racing a B/C/ with Sebring wings?

265, now that is big! ... just let me check my bank balance :->

I'm worried the steering might become too heavy if the wheels are too wide and offset too much.
Alan Boother

Alan,

I used 225/50/15 tyres on 8" rims on my Sebring, they fill the arches quite nicely. Wheels were Weller Racing steel rims, look quite nice but are very heavy.

George
George Bowick

George,
What offset do your wheels have?
Alan
Alan Boother

David you sure about that 265/50-14 matching stock?? It is way larg fromt he numbers I have...

Stock: 175/70-14 = 23.65"diam
215/50-15 = 23.46"diam
265/50-15 = 25.43"diam

There is a excel file on my website which has a gearing calculator..
Larry Embrey

I thought the stock tire size was 165R14? (24.13" outside diameter)

Of course, in that case, 215 would be too small, 265 to large.

235/50R15 is pretty close though.

I got tired of doing the math by had, so I built a nice web form to do this for me. if you are interested, it is at http://www.pharmacy.wsu.edu/users/awhitema/MG/tyre_conv.php

It does nothing near what your excel file does (IIRC), but it only took a couple minutes to write...
Aaron Whiteman

he, he, he

You got tired...
Alan Boother

yeah, for values of tired that = "lazy"
Aaron Whiteman

urk. just realized the joke.

n <- keyboard works, must have been me.
Aaron Whiteman

Alan,

Sorry, can't remember what the offset was and I've just sold the car so can't measure it! The wheel was a stock Weller item so they may be able to help you.

Weller Wheels Ltd, Wyvern House, Longfield Road, North Farm Estate, Tunbridge Wells, Kent TN2 3EY

Regards,

George
George Bowick

Alan,
Apologies...I thought by the sound of your post that you wanted to retain a higher profile for appearance purposes. Larry is right in saying that the tyre diameter would be big. Having just bought tyres for mine I chose 205/55/15 diameter 23.875, which put me close to standard.

Also, I could be wrong, but if you have a standard 3.9 rear end, tyres like a 265/50/15 will give you an equivalent rear end ratio (if you used a standard tyre) of about 3.65:1 . It wouldn't half screw up the speedo though.

HTH
Stainzy
David Staines

Anyone know what the racing Cs use?

(I'm counting on having to get the speedo sorted out and I may be looking for the 3.07 back axle anyway.)
Alan Boother

At the risk of being a killjoy beside ride height there is also a question of road-holding or at least of revised handling.

I am thinking back to an article about five years ago in the club rag which was quite clear about reduced grip for fat tyres. Although it wasn't totally convincing, the club obviously put its seal of approval on it.

Is there any authoritative data about handling changes on sizes greater than 195 (many of us are on 195s) - notably in the wet ? Or has anyone here raced on fatter tyres ?

Good wishes RW


Roger

Aquaplaning is going to be one problem with wide tyres. I have tyres recently fitted to an Astra Sport with what appears to be much wider rain grooves than usual. If I use these it should help with water clearance and improve/increase the loading per sq inch of contact area which will help too. Grips depends a lot on the rubber folding around the imperfections on the road and this happens more when with higher loading.

Another problem will be "kick back" or the turning effect of hitting a bump when the centre of the bump is not in-line with the instantaneous point of rotation around the king pin intersection with the road. This is not the same as bump steer ofcourse. A wider tyre has more chance of the distance away from the rotation point being greater and hence the kick back greater.

Wide tyres, especially as the profile gets lower don't sit so flat on the road as the suspension goes up and down and the car rolls. An optimised solution may require changing the geometry... negative camber.

Wider and larger diameter tyres put more rubber on the road and can be hardwork to turn.
Alan Boother

No one seems to answer the question about offset - although that seems to be a critical dimension. I use 7" rims with offset 30 from a SAAB 9000 - pre-1987 I think. Minimal mods needed - mere adjustment, really.

Lots about this in the arhive.
Marc

Mark,

Which offset King Pin when Alan refers to Kick Back or Rim (the B standard offset should be 22). The wider the track the more KPO you will have and therefore kick back - Racers can live with it as nice smooth tarmac and faster laps, not so much fun on the road. Anyone know the KPO for standard B. With your offset Mark you may have moved the KPO inside but would still be close to instantaneous point of rotation.

Paul

Paul

Alan's reference to variations to the geometry. (TY for yr response) I am fairly confident that the racing Cs all use negative camber wishbones.

Any comments on handling effectivess - especially for all those US RB muscle cars ?

Best wishes RW
Roger

Paul
I'm sorry but I could scarcely understand a word of your posting. I've got my head around Offset and now you are talking about Kick back or king pin offset, rim etc. I give up. Translations please. (Simple English)
Marc
Marc

Offset of 3-1/2 to 4" (sorry, wheel's on the car and hard to measure) on a 7" wheel seems to work well and will put a 265/50-14 (right close to 24" dia) as close as you can get to the inner fender on a CB without hitting it under heavy compression. This is a fairly standard offset and keeps kickback and steering effort at an acceptable level. As for grip, for racing go with a somewhat narrower tire with a softer tread compound (racing tires) but for the street this size works fine provided it will fit inside the bodywork. Hydroplaning will be a distinct problem below 1/4 to 1/3 tread depth and will require either a slower speed or new tires. Before that it will also rear it's ugly head with higher speeds and deeper puddles. This combination is not well suited to wet racing and should not be considered for such regardless of the provocation.
Jim Blackwood

The right wheel back spacing with Flares (wings) should be; rear 4" backspace on a 8" wheel and 5" backspace on the front on a 8" wheel
The tire should be 225X50X15 on the rear 23.3 dia on a BFG tire and 215X55X15 on the front 23.3 dia
The dia. varies from manufactures. the Best tire on those sizes can also be found on Dumlops.
That's my set up on my GT. No rubs anywhere.

R/Bill
Bill Guzman

He, he, he.
Sounds like you've got a smart car... except for those tyres
Alan Boother

Bill,

That seams the ideal combination - what other mods have you done to suspension?

Paul
Paul

This thread was discussed between 01/12/2001 and 10/12/2001

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