MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Rear axle ratios

Hello All,

I've been reading the threads on this excellent bbs now for some time and need some advice. I'm part way through a conversion, SD1 lump and box into a rubber bumper shell.

Tomorrow I'm going to see a converted chrome bumper shell to see what bits I can re-use - in particular I'm interested in the back axle. Unfortunately there is no history with the car , so would i be right in thinking I could tell what the ratio was by lifting the car, then turning the propshaft by hand (there is no engine or box in it) and counting how mant times the rear whees went round to see if it has the 3:1 ratio? Is it that simple or am I being a bit thick?

Anthing else i should look out for rear axle-wise?

Thanks for any advice.
Liam

Determine the ratio by turning both wheels -simultaneously- one turn and count the turns the axle or flange makes. Probably 3.9 turns, which I have behind my SD1 setup. I would prefer 3.3.
Edd Weninger

I have an SD1 and 5 speed box going into mine. How do you find that ratio ?

I have read that 3.9 to 1 is supposed to be very difficult to drive and that going for something nearer 3 to 1 makes for a much more drivable car.

What are the practical implications?

I'm hoping that because this car has already been converted in the past they will have changed the crown wheel and pinion. The guy who owns it now has no real idea.
Liam

The gear ratios of the transmission have as much impact as the rear. My 3.9 uses a 2.65:1 first and a 0.63:1 fifth. Quick first that I can still use but low enough on the interstate.
Search the web for a variety of rpm calculators. You can play around with tire diameter, rear and trans ratios to see your rpm and speed.
Scott

I put a 3.07:1 from an MGC into my SD1-powered roadster. Turns about 2800 rpm at 70 mph. Lots of grunt in the lower gears and cruises nicely on the highway.
Simon Austin

Liam
Mine is (was) a rubber bumper and I stuck with the orrigional back axle (3.9:1) , reasoning that if I didn't like it I could always change later. I like it alot. I can take off with heaps of accelleration in second gear and not have to change until well over the speed limit, then I usually change to 4th (or 5th. This makes for very easy driving in traffic because i can leave the mob (accumulated at traffic lights) behind and have a little more road to myself. I doubt that I could do this with a 3:1 diff'.The low first gear is necessary for peak hour because you don't really want to be jumping on and off the clutch all the time.). I'm using a 3.9 fuel,injected motor with a supra gear box. This gives me a fith gear of 0.78, which is just fine.
The rubber bumper axle is more than strong enough,Leyland used to use it on a small truck/Van.
Peter

I don’t use my v8 3.5 rover conversion to the highway, only to the smalls roads with many corners in my area, or track days, I use LT77 gearbox with close ratio gears 2.33/1, 1.61/1, 1.21/1, 1.00/1, 0.83/1, I put a 3.7/1 (no 3.07), used by mgb automatic, with Quaife ATB differential. I don’t know the maxi speed, nor the consumption, but the output is perfect.
Regards.
michel

I had a look at this car this morning, it is a previously converted 1973 model.

Oddly, the propshaft turned one complete revolution for every one and a half turns of the road wheels.

What ratio does that give me 1:1.5 ? how has that come about, what does it mean!
Liam

It means that one of your rear wheels was not turning, which doubles the effect to the other wheel. That would make your ratio 3:1, or more likely 3.07:1

Wayne
Wayne Pearson

It means you have to go back and check it again. You need to turn both wheels simultaneously, in the driven direction, exactly one full turn and count the drive shaft rotations.

Good luck.
Edd Weninger

This thread was discussed between 09/11/2005 and 10/11/2005

MG MGB GT V8 Factory Originals Technical index

This thread is from the archives. Join the live MG MGB GT V8 Factory Originals Technical BBS now