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Wheels and tires, measurements, and hard decisions - a brief case study

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58custom Avatar
58custom Tom Crocker
Santa Clarita, SoCal, USA   USA
Dear Forum,

The recent resurfacing of one of the many threads discussing the availability of 13" performance tires has got me thinking of buying a set of... something. I have been using 155/80-13 tires since i bought this crate, I have used Milestar, Douglass, and one other brand. They are all "all season" tires and offer little in the way of grip under good conditions and nearly none in wet conditions. (One particular example: there is a curve on Bouquet Canyon Road near the Big Oaks Lodge that, when moist, will break the rear end loose suddenly under moderate speed. I have to creep through to maintain control. It's shaded often so some amount of thin algae may be a contributing factor.) I have wiped them bald with spirited driving on curvy rural mountain roads. I am tired of them.

In order to finally buy new wider tires I need to know which ones would be a clear improvement, and which ones will fit within my square rear fenders.

So, I decided to take some measurements. Please check my work and let me know what else I should consider.

I want to move to a performance (or as close as possible) 175/70R13 tire. Which wheels should I use? I have two sets to choose from.

Wheel set one: JBW minilite copies. These are popular wheels. 13x5, 4 inch backspace. .79" offset. So they say. I use these, shod with Douglass Xtra Trac II 155/80R13 tires, 22" O.D., section width 6-1/4".

Wheel set two: Freakin' Awesome Libres! 13x5, 3-3/4 backspace (measured). Love these. found them after I bought the JBWs. These are shod with old (unsafe) Futura Euro 165/70R13 tires, 21.5" O.D., section width 6-1/2".

I am ignoring the clearance of both wheel/tire combos because I have plenty of clearance as is. See pix. The Douglass's show no sign of contact with the fender lips.

Rear clearances were measured at the interference points.

Wheel JBW & 155/80 Libre & 165/70
RR Outside 3/4" 1.0"
RR Inside 5/8" 1/2"
LR Outside 3/4" 3/4"
LR Inside 5/8" 1/2"

The effect of the smaller backspacing of the Libres compared to the JBWs can be seen.

If a 175/70 section width is 7" (1/4" wider each side than the Futuras), and those are mounted on the awesome Libres, I might have an inside clearance between spring clamp and side wall of only 1/4" when the care is not moving. Differential shift side to side could close that clearance completely. That would be bad. However, the damage would be on the insides of the tires and out of normal sight and, I assume, probably not structurally significant.

The 175s on the JBW wheels, again assuming a 7.0" section width (3/8" wider each side than the Douglass), would result in the same minimum of 1/4" static.

Huh. No clear winner. Gets worse if the 175 is wider than 7.0, like the nice Pirelli Centurato CN36 (7.1"winking smiley. The smaller OD of the 70 series tire means the suspension travel until possible tire to fender lip contact range is larger than the 80 series tire. I could maybe use spacers to get some clearance for the spring clamp? Are .2" spacers available?

Question: Is it possible to move the spring clamp to gain some clearance?
Question: How much side-to-side differential movement can be expected in spirited driving (rubber bushings)?
Question: How typical/untypical are my measurements compared to yours?













1969 MG Midget with Datsun A15/5 speed

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nonracer Gold Member Steve Codianni
AZ, USA   USA
Tom
I would think adding a panhard bar would keep the axle from shifting side to side if you have leaf springs rubbing issues .
I bought a 65 that had converted bolt on mini lites (WW car ) with 175/70 , they did rub up inside the wheel wells a little. The backspace on these would be completely different since they were designed for the WW axle . I’m sure there are spacers you could get or have made but would probably be a good idea to install longer studs

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Abington, PA, USA   USA
1976 MG Midget MkIV "Rodney"
Hi Tom,

I've been looking for new 145SR13's for some months for my '76 Midget and only found classic Firestone ones at Corker. Rick Fisk just sent a note on my other thread that the Pirelli sport car tires I had put on the car when I bought it new, and have always used, are now back in production. See the link Rick gave me: https://www.lucasclassictires.com/145HR13-PIRELLI-CINTURATO-CA67-145r13L.htm
They are great handling tires, and pretty good for all but snow days (on which the Midget stays in the garage.)

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AN5L8016 Mark Haynes
Nederland, CO, USA   USA
X2 on the panhard rod- Winner's circle has one that only requires one hole in the backside of the axle bracket, the rest goes in under the fuel tank studs.



'58 Bugeye
'05 Mini Cooper S

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58custom Avatar
58custom Tom Crocker
Santa Clarita, SoCal, USA   USA
In reply to # 3906183 by nonracer Tom
I would think adding a panhard bar would keep the axle from shifting side to side if you have leaf springs rubbing issues .
I bought a 65 that had converted bolt on mini lites (WW car ) with 175/70 , they did rub up inside the wheel wells a little. The backspace on these would be completely different since they were designed for the WW axle . I’m sure there are spacers you could get or have made but would probably be a good idea to install longer studs

Steve,

I should research the panhard bar as I assume side to side movement could eat up the clearances I am expecting with 175s.

As for studs, the thing is the shanked nuts have much more thread engagement than the beveled acorns that the JBWs use. Not sure if I can find shanked nuts in 1/4" increments, but I'll check. But if I use the JBWs I won't have to use spacers. Prolly be a good idea to replace those old studs anyway.



1969 MG Midget with Datsun A15/5 speed

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58custom Avatar
58custom Tom Crocker
Santa Clarita, SoCal, USA   USA
In reply to # 3906995 by AN5L8016 X2 on the panhard rod- Winner's circle has one that only requires one hole in the backside of the axle bracket, the rest goes in under the fuel tank studs.

Oh. That's a lot easier than I had assumed. Thanks, I'll check it out.



1969 MG Midget with Datsun A15/5 speed

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GeorgeOhr Nonya Business
Yes, confused, USA   USA
I have 175s on my JBWs on my 79 SWA and they work fine. A panhard would be nice but not necessary. Slight rubbing in front on the inside of wheel well at full lock but nothing to worry about.

I used to love the euro metric tires and it’s a shame we can’t get them anymore.

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200mph Avatar
200mph Platinum Member Mike Joy
Winston-Salem, NC, USA   USA
Hi Tom,

Long standing problem, no easy solution.

One important variable is your wheel offset... I had to source different offset 13x5's for the rear of my 67 due to fender well interference, and I still couldn't use my 175/70 tires, so I went to 155/80s.

Another variable is, how do you drive?

Spirited cornering produces bushing flex as you noted, but also a LOT of tire sidewall flex. A panhard rod will help secure the lateral motion, as will poly bushings, but ride quality will become harsher.

If you like to toss the car around and slide through corners, the 155's are a hoot. Obviously, they break away sooner than a wider tire would. Makes for great fun in controlled settings (track/autocross/etc).



MG drivers outlook:

. . . . . "Anyone driving slower than your MG is a MORON, and anyone who passes you is a FREAKING IDIOT!". . . George Carlin

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75RD350 Silver Member Kent Q
Sacramento, CA, USA   USA
Michelin offers the XAS tire, or tyre, in 155HR13 and 165HR13 sizes and the FF (Formula France) compound should provide a bit more stick than the garden variety 400+ UTQG offerings. Two potential drawbacks are 1) the price, about $200 a pop, and 2) they require running tubes. I have a RWA car but I’m liking the idea of running narrower H rated tires.



Edited 1 time(s). Last edit at 2019-03-12 10:09 AM by 75RD350.

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