WHEELS & TIRES

The car is fitted with 13x5" K&N minilite replicas. I like the style very much and had the same on my MGB. Unfortunately the rim is too wide for the original "narrow" rear wings, so the tire is protruding approx. 12mm from the wing edge - even with 155 tires. To prevent the tire from rubbing the wing, the former owner has fitted small diameter tires and the raised the ride height. Leaving - in my opinion - too much gab between tire and the wing arch.

The only solution is to fit slightly wider rear wings. Not the big ugly 11" ones from Caterham, but close to 9" as on Series 3. The wider wings will allow me to fit a decent set of tires with the right diameter. See rear-wings.

Tires:

The tires was obviously too small in diameter when I brought the car. They are labeled "Le Mans" 155R13 and the diameter is measured to approx. 555mm giving a "profile" close to exotic 72%. The overall width is 164mm.
The original wheels on the Series 2 were 3 1/2" x 13" well-based bolt-on rims by Dunlop with chromium-plated hubcaps (4x3.75” PCD). The tires was 520 x 13 (cross ply).
The nominal inflated diameter and width for such a tire is 579 x 140 mm - very close in diameter to the "modern" 155R13 (radial tire). Standard fitting on Series 3 was 165/13 on 5 1/2" x 13 rims with Ford PCD. It is quoted that the tires and wheels are a part of the suspension design, so it not recommended to fit very wide low profile tires.


Tony Weale recommend 185/70R13 on 5 1/2" or 6" wide rims for a Seven with standard suspension and live-axle. My rims is only 5" wide so I will go for 175/70R13 which is more than adequate for such a light car.
This size will raise my car approx. 10mm compared to the fitted tires. And I hope this will allow me to lower the car accordingly without compromising the free height under the sump.

Gearing:

A secondary reason for changing tyres is the possibility to alter the cars overall gearing by changing the rolling circumference of the tyre. The Km/h per rpm can be found with the following calculation;

Engine rpm multiplied by 60 = Engine revs per hour.

Divided by gear ratio divided by axle ratio = Number of wheel revolutions per hour.

Wheel revs per hour multiplied by rolling circumference = mm travelled per hour.

mm per hour divided by mm in a km (1.000.000) = Km/h

The information needed is engine rpm, axle ratio, gearbox ratios and tyre rolling circumference. For my car that is:

Max engine rpm = 6.000
Axle ratio = 4.11 : 1
Tyre rolling circumference for 155/72-R13 = 1744 mm
Tyre rolling circumference for 175/70-R13 = 1806 mm
Tyre rolling circumference for 185/70-R13 = 1850 mm

Giving the following gear speeds for my car with 4 speed Sprite-ribcase box:


These are off course all theoretical values, but choosing 175 tyres from 185 can give a more realistic maximum top speed with an associated improvement in acceleration. Negatives include a reduced road clearance.

Wheels:

Other cars with the same 4 x 3.75” PCD (Pitch Circle Diameter).
Austin: Marina
Lotus: Elan, Europa, Seven
Triumph: Herald, Spitfire, GT6, Vitesse
MG: Metro, Montego, Maestro, MGF
Formula Ford Racers

Be aware of differences in the way the wheel is located on the hub, some rims use tapered lug bolts (standard Lotus) others use the center hole in the rim for locating, and have clearence around the lug bolts.

Offset (ET):

Wheels have a mounting offset from centre known as ET (German = Einpress Tiefe, literally meaning "push in depth"). If the wheel rear mounting flange is in a centre line to the wheel rim it has zero offset. If the flange is further forward from rim centre line then it's a positive ET, if it's back it's a negative ET.

The standard offset on Triumph Spitfire wheels is +0.5" (13 mm).


I have measured the offset (OS) on my KN Minator "Minilites", to approx. 19mm compared to 13mm. This will "move" the wheel 19 - 13 = 6mm inwards. The wheel is 1.5" (38 mm) wider than standard. So the edge of the wheel will extend (38 / 2) - 6 = 13 mm, very close to my measurements.
The easist way to measure the offset (OS) is to measure the total width of the rim divide by 2 and subtract the backspace (BS). The overall width of the rim is approx. 151.5 mm and the overall diameter is 362mm.

I found these measurements on "Blatchat":
"I have measured one of my 5 1/2J steel wheels (1969 s3) and one of my Dunlop alloy wheels. The ET measurement of the steel wheel was 18mm, for the alloy it was 11mm. The weights were very similar (just under 6Kg on our kitchen scales)."

I managed to find a small error in Tony Weales excellent book ("bible") regarding the offset (p. 112):


it should have been correct positive offset.

Here is a very good source of information: CarBibles - wheels & tyres