Diff question

[Grumpy21]

As I discover more about my new to me 1098 I’m wondering about the diff.

It seems to be very quiet compared to my previous car and for a given speed the engine certainly seems to be revving less (purely subjective as yet)

Did a quick road speed check against gps at an indicated 60mph is 53 on gps. And 30mph is 26 That’s a fairly constant 15% overread which is a similar difference between 4.2 and 3.7.

Without taking anything apart or fitting a tachometer any thoughts how I could check. ( jack the rear and spin the wheels? Both I presume)

Ta

[geoberni]

Perhaps I've missed something from another post, but what's the actual question about the Diff?

Are you wondering which Diff you have fitted and whether it's the right one for the engine?

You can count turns if you wish, but Diffs are stamped with numbers on the top surface (it'd be too easy to put the stamp where it could be seen from underneath )
The Potteries website had the relevant engine/gearbox/diff details: http://potteries.mmoc.org.uk/ID/EngineBox.htm

[Monty-4]

Yes I suppose if you were to jack up the rear, put the car in 4th (1:1), and turn the engine a few times using the crank handle, you could get a reading on how many times the rear wheels go around for that number of engine turns!

[simmitc]

Except varying resistance (brakes, bearings, tyre balance, etc) could result on the two wheels turning a different amount - that is what the diff is there to do when cornering. You would need to perform that exercise on flat level ground and move the car in a straight line. It can be done with all spark plugs removed and being careful as the car travels towards the person turning the handle, albeit very slowly.

[Grumpy21]

Last line of my post

“ Without taking anything apart or fitting a tachometer any thoughts how I could check. ( jack the rear and spin the wheels? Both I presume)”

I’m thinking the overall ratio is influenced by the gearbox. If I Jack the rear wheel off the ground and rotate them I could count the rotations of the prop shaft. I rotation on the wheels would either be more or less than 4 rotations of the prop shaft and give me an indication.

[Grumpy21]

Yes I suppose if you were to jack up the rear, put the car in 4th (1:1), and turn the engine a few times using the crank handle, you could get a reading on how many times the rear wheels go around for that number of engine turns!

Seems a bit like hard work compared to lifting the rear end. (I have a car lifter so it’s a push button exercise)

[simmitc]

If I Jack the rear wheel off the ground and rotate them

ONE wheel off and rotate, otherwise the opposite wheel will accept some of the input from the turned wheel, so there will not be a direct relationship to the propshaft. As per my post above, it's what diffs are designed to do.

[Bill_qaz]

If you jack up one wheel, gearbox in neutral, you will need to rotate it two full revolutions and count the prop or diff flange revolutions to give ratio..

[Grumpy21]

Surely I’d need to account for the planetary ratios then. Couldn’t reply on propshaft rotations to tell me anything. If I rotate both wheels one rotation in the same direction then only the crown wheel will have spun and hence the pinion turns could be relied on.

[simmitc]

Looking back at some old notes, when getting a speedo calibrated, they wanted the car pushed forwards X rotations of the rear wheels to count the turns on the speedo cable. Pushing your car forwards whilst an assistant counts the turns of the shaft would seem to be the simplest if you don't want to hunt for the numbers. I think that it was two turns of the wheels that they wanted.

[jaekl]

Go for another ride with your GPS but this time check the odometer. It's direct drive so more accurate than a 50+ year old indirect speedo connection.

[geoberni]

Or just look for the numbers on the Diff. See this old Post. viewtopic.php?p=681361#p681361

I added to it last year when I got underneath and checked the numbers on my Diff.
It wasn't hard to do, a bit of a wipe with some cleaning materials (paper towel and solvent spray in my case) and reaching above the Diff with my phone camera.
Numbers are in this area:

Diff No.JPG (30.57 KiB) Viewed 9862 times

[les]

‘look for the numbers’ ——Good idea !

[Bill_qaz]

Surely I’d need to account for the planetary ratios then. Couldn’t reply on propshaft rotations to tell me anything. If I rotate both wheels one rotation in the same direction then only the crown wheel will have spun and hence the pinion turns could be relied on.

You are misunderstanding how the differential works. The Sun wheel and planet wheels are same ratio both sides on standard open differentials, so two revolutions of one wheel is the same number of crownwheel rotations as one revolution of both wheels simultaneously but much easier on your own to turn one wheel than push car with both wheels on the ground. If you try to rotate both when jacked up it's hard to turn both simultaneously as one tries to contra-rotate..
Or as suggested read the stamped casing and assume the Crown wheel and pinion have never been changed to different ratio.

[Grumpy21]

Thanks for the info chaps. I’ll have a play at sometime over the weekend and see what I discover.

[oliver90owner]

Except varying resistance (brakes, bearings, tyre balance, etc) could result on the two wheels turning a different amount - that is what the diff is there to do when cornering. You would need to perform that exercise on flat level ground and move the car in a straight line. It can be done with all spark plugs removed and being careful as the car travels towards the person turning the handle, albeit very slowly.

What is this about engaging any gear or turning the engine or removing spark plugs? Diff ratio is solely within the diff - between the half-shafts and the prop-shaft! Bill has it nailed.

Lift one rear wheel and count the wheel turns and prop-shaft turns with the gear box in neutral!. One needs to take into account that all the drive is to one wheel - but that calculation should be fairly simple!

[simmitc]

If you read the rest of the topic you'll see that it was previously suggested engaging 4th for direct 1-1 rotations of engine to shaft; but that we have already covered the one wheel off the ground.

[Edward1949]

difference between 4.2 and 3.7.

Without taking anything apart or fitting a tachometer any thoughts how I could check. ( jack the rear and spin the wheels? Both I presume)

Ta

Jack up one side only. If your diff is a 4.22, then 2.11 propshaft rotations will give one wheel rotation. If it's a 3.73 you'll need 1.86 propshaft rotations per wheel rotation.

[Jim McCrae]

A higher diff ratio such as the 3.9 or 3.7 increases the road versus engine speed, so if you had one of those fitted you would expect the speedo to under record your speed. eg Indicating 56mph at a road speed of 60mph.

I checked my traveller with the standard diff and found that, like yours, it over recorded speed. Replacing the diff with a 3.9 has cancelled out the error and my speedo is now pretty accurate.

[oliver90owner]

A higher diff ratio such as the 3.9 or 3.7 increases the road versus engine speed, so if you had one of those fitted you would expect the speedo to under record your speed. eg Indicating 56mph at a road speed of 60mph.

I checked my traveller with the standard diff and found that, like yours, it over recorded speed. Replacing the diff with a 3.9 has cancelled out the error and my speedo is now pretty accurate.

The law allows up to 10% error (on the optimistic read-out side) but it must not read lower than the truth.