Rostyle offset measurement

After much contemplation I fitted 185/70 14 Avon ZV1 tyres to my 73 roadster which at the time had 1" lowered suspension. I searched the archives on the MG BBS to see if there would be any problems with the tyres rubbing against the rear arches, and concluded that I would probably get away with it.

On leaving the tyre depot and negotiating the first bump in the road, there was a screeeech from the rear LHS wheel !! Arggggggh!! Thoughts turned to £££wasted, body work, slashed tyres etc. 

However a call to Malcolm of Brown & Gammons (Top chap), revealed that later rostyles have a different offset compared with earlier cars. So I measured the offending wheel and the spare, only to find that there was 4.3mm difference !!! 

A quick swap of wheels, front to back, and a thrash round town, driving as fast as I dare over speed humps, and, Robert is you father's brother !! problem solved !

Prior to this I thought that I had an off centre axle but now it is almost plum !!

I replaced a wheel 10 years ago for a recon one which must have been a "Late Rostyle" but with 165/14 tyres, showed no ill effect. This one is now the spare.

The moral: If you have changed to recon rostyles on a chrome bumper B, check the offset before you buy to ensure that you get the right wheels.
The pictures below show a method for measuring the distance between the plane of the rear of the rim and the mounting flange.


Measuring Tools

 

 
Measuring using depth gauge.

 

 Measurement Method

 

 

 

Measurements & Calculations
 

Depth of mounting face from RIM rear plane

 

 

Delta

Early Wheel*

  10.364

cm

 

Late Wheel*

    9.934

cm

0.430

Wheel Width in Inches

5.945

inches

 

Wheel Width in cm*

15.100

cm

 

Distance to center line

7.550

cm

 

Offset

 

   

Early Wheel

2.814

cm

 

Late Wheel

2.384

cm

0.430

 

 

 

 

* Measured dimensions      

A negative value is a negative offset ie: wheels further outboard.
A positive value is a positive offset ie: wheels further inboard.