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Good day.

Bearing in mind that I am a sidescreen "virgin" with no experience of driving or owning such so forgive my ignorance in the following:

Whilst trying to calibrate my speedometer I became puzzled by anomalies. Jacked up the car on one side so the wheels on the other side are grounded and set about determining the axle ratio. Expecting to find the prop shaft rotating 3.7 times for one rear wheel revolution I was surprised to find it rotated only 1 and 7/8th times WTF!

I drove the car with my dash-cam recording on MPH units and set the car at 3000RPM in top (4th) gear and recorded 48MPH. 

What is going on?

I had the unit overhauled back in 2014 by transmission people, the only strange thing then reported was that there was a 9" drum on one side and an 8" on the other.

The number on the diff is: TS56502, my car is TS25208L

 

 

Axle as found.JPG

Casting stamp.jpg

Diff:Final Drive Number.jpg

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It's the diff at work John. Because you are only rotating one wheel, the crownwheel only makes half a revolution due to the action of the differential. You need to make two rotations of the wheel.  As it is, 1 7/8 = 1.875 which x 2= 3.75 so you probably do have a 3.7 crown wheel and pinion. 

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Assuming the clutch isn't slipping because that would be obvious in this situation.

Check your tacho. You have a 3.7 diff so in 4th you should do very close to 20mph per 1,000rpm. It can depend a bit on your tyre profile but only by up to 1mph per thousand.

3,000rpm doing 48mph the engine would have been howling.

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12 hours ago, John Reilly said:

Thanks Iain,

Were all Diff's stamped like that?

I must examine mine more closely to see if it is marked 10/37 but I don not recall seeing that.

John

 

 

Just bear in mind that whatever is stamped on the casing ( if at all) is the ratio that left the factory - over at least half a century, there is no guarantee that the ratio hasn't been changed in the meantime - different ratios all fit within the same casing so the only way to be sure is to jack up one wheel and check it as described earlier, but get your formula/maths right!

Cheers Rich

Edited by rcreweread
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48mph is low even for a 4.1 diff, should be more like 54mph at 3000rpm in top.  

Ralph

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51 minutes ago, rcreweread said:

Just bear in mind that whatever is stamped on the casing ( if at all) is the ratio that left the factory - over at least half a century, there is no guarantee that the ratio has been changed in the meantime - different ratios all fit within the same casing so the only way to be sure is to jack up one wheel and check it as described earlier, but get your formula/maths right!

Cheers Rich

Agreed Rich.

The works cars for 58 all had 4.1/1 diffs, having owned this car for 40 plus years I was surprised to find this marking in 2020 when I completed a chassis refresh. (When I rebuilt it in 1979/80 I had no idea of it’s history or such detail.)
I met a PO some 5 years ago and he said “ I never understood why this car was so much quicker than his previous Tr2? “ after 55 years I could give him the answer! :-)

When this axle ratio is fitted the Speedo gearing was 1300 as marked on the face of the dial.

I have not seen markings on casings for the 3.7/1 axle, but then I’ve not studied that many.

Iain

 

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Before asking the question here, I was thinking of taking the diff cover off to count the teeth on the crown-wheel and pinion but baulked over the mess etc and decided to instead humbly expose my misunderstandings. In my career working I used say "never be afraid to ask the dumb question" 

I'm happy to say that here on this forum it is generally a pleasure to trade info and to further my education, thank you to all who respond

Regards

John

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This is a pretty good site 

you need to put in the parameters at the top that suits your car and set up. 
 

48 to 50 mph in top 3000 is about what I get with a 4.1 diff and my 55 profile tyres. 
 

http://www.mintylamb.co.uk/gearspeed/

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1 hour ago, ken foster said:

Hi, could the speedo readings be affected by a non standard gear box?

Just a thought 

Ken

yYes. Different gearboxes have different gearing for the speedo drive.

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7 hours ago, Lebro said:

Indeed, a saloon box speedo drive ratio is 3.5:1, TR box is 3.5:1

Bob

BOB ??? must be a typo as you have shown they are both the same, but they are not  - is one 2.5?

cheers Rich

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A saloon box must be the 3.5 :1 as they make the speedo read about 30% lower as I discovered. I managed to get the speedo reading slightly high at 30mph, but reads progressively lower as the speed increases, although the error on mine is canceled out somewhat by also having a 4.1 diff. As far as I can tell using the sat nav for comparison 65 on my speedo is actually 70.

Ralph

Edited by Ralph Whitaker
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16 hours ago, rcreweread said:

BOB ??? must be a typo as you have shown they are both the same, but they are not  - is one 2.5?

cheers Rich

That what come of using a mobile phone without having glasses on !  yes, TR is 2.5:1.  so if a saloon box was fitted, then speedo would read slow, not fast.

What is the TPM (turns per mile) number printed on the speedo dial under the odometer.

Bob

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On 6/22/2022 at 9:01 AM, John Reilly said:

I was thinking of taking the diff cover off to count the teeth on the crown-wheel and pinion but baulked over the mess etc

Hello John,

Well, it couldn't make any more of a mess than is already there on the diff case could it.:D

You have a serious leak somewhere. The prop flange looks shiney so it's a good guess you need a new front seal and/or the breather is blocked.

Easy and cheap enough to do, the hardest part I found was getting the old seal out.

Maybe worth taking the cover off anyway to check the state of the gears. It might have been run without oil for a while.

Charlie

 

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