Rear Axle ratio for a TR3a with overdrive

[Ian Vincent]

My TR3a has a 2.2 litre engine (87mm pistons), HS6 carbs and a Phoenix extractor manifold. As I have already mentioned in another post about a sensitive throttle. it pulls very strongly from low revs.

 

After a couple of recent longish drives (circa 200 miles each) on a variety of roads - some motorway and some regular two lane A road - I'm starting to wonder about the rear axle ratio, (currently 3.7:1). Even with overdrive it feels too low. Has anyone had experience of a 3.45:1 CW&P set in a TR3a? Does it work or is it just too low?

 

Note, my days of driving like a hooligan are long over, I tend to stick to the speed limits and cruise on motorways at a maximum of 75 to 80, (circa 3000 rpm in O/D top).

 

Any advice welcome.

 

Rgds Ian

 

 

[Lebro]

Ian. A quick test is drive in non O/D top, the rev counter, & speedo needles should be pretty much at the same place on the dials.

 

Bob.

[Ian Vincent]

Hi Bob,

 

I know the current axle is 3.7:1 because I rebuilt it myself. My question is should I go looking for a 3.45 CWP set?

 

Rgds Ian

[BlueTR3A-5EKT]

Questions:

Which gearbox do you have and what ratios? What wheel and tyre sizes do you run? What is the useable rev band of your engine? Do you drive in 20 or 30 limits much?

 

I have just been through the same thought process for my TR3A. and rebuilt my existing axle with its 3.7 unit.

 

I have a new but scruffy Standard Triumph OE 3.45-1 CWP assy for collapsible spacer use, but chose not to change the ratio to 3.45 from the 3.7 because I could not face having to get the speedo recalibrated - again. I also feared the first gear would be too high with my non standard gearbox first speed, upsetting pull away. Plus living in London would mean I drove about in 2nd or 3rd to be under the limit and keep the engine at a smooth rpm.

 

With a standard 3 synchro box the first gear will be improved over the the 3.7 if the 3.45 is used by giving it 'longer legs'.

 

Look at Minty Lamb axle ratio calculator thingy << http://www.mintylamb.co.uk/gearspeed/ >> You can add in the axle ratio you are choosing.

Peter W

Edited August 27, 2016 by BlueTR3A-5EKT

[timhum]

I have a TR3a with o/d which has been fitted with the 3.45:1 axle ratio during it's rebuild. The engine has been modified to 2.3l with reworked cylinder head, camshaft, 4 branch exhaust and TR4a inlet manifold. This work was done by TRGB who suggested that the 3.45 ratio would be a good change given the extra power and torque being produced. Experience to date has been fine. 1st gear is usable, not needing an immediate change to 2nd whilst the others benefit from being a little 'longer'. Perhaps if outright acceleration is key then the 3.7 ratio might be better but for everyday use this combination works well for me.

Tim

[mike3md]

Ian,

 

Why not change the o/d ratio from 22% to 28%? This will give more relaxed cruising without losing the pep you have in the direct gears.

 

As Peter suggests, you can see the effects on the mintylamb website.

 

Don't forget to factor in a speedo recalibration.

 

Mike

[mjdearing]

Wow Ian 75 - 80 im a 60mph max type, a couple of old palls (UVC166 Paul where are you) did the 3.45 job as i remember you will need a grinder to just nick the casing for clearance of the crown wheel.I think Mike above is bang on though,with the 28%overdrive, i did purchase some 16" wires but never tried em out ?. I too would like to turn my 2 into more of a driving Mrs Daisy cruiser,will be interested way you decide to go and your findings Martin. .

[Ian Vincent]

Thanks for the suggestion Mike but changing the overdrive means gearbox out and would be much more expensive than simply swapping the CW&P, particularly as the latter is something I can do myself. Plus the O/D was rebuilt by ORS when I rebuilt the car only a couple of thousand miles ago.

 

Rgds Ian

 

PS I have already done the minty lamb check and liked the result.

Edited August 28, 2016 by Ian Vincent

[uvc166]

Hi Ian ,I went the 3.45 route years ago ,in top gear O/D engine runs 500 RPM less ,hope that makes sense ,cheers Paul.

[TR 2100]

. . . . . simply swapping the CW&P . . . .

 

I didn't think setting up the factory specified clearances was simple.

Am I wrong?

 

AlanR

[Ian Vincent]

 

I didn't think setting up the factory specified clearances was simple.

Am I wrong?

 

AlanR

It's fairly straightforward if you have the right instruments, the trickiest bit is setting the depth of the pinion without the special Churchill tool. I just take it slowly and then check the mesh with engineers blue to confirm that I have it right.

 

Rgds Ian

[Ian Vincent]

Based upon the collective views above I have decided to move to 3.45 if I can find a suitable CW&P. I currently have a spare 3.7:1 CW&P that is in very good condition - does anyone out there have a 3.45:1 unit that they would like to exchange for it?

 

Rgds Ian

[stuart]

You will possibly struggle to find a good 3.45 as all the 6 cylinder boys have been trashing them for years and good s/hand or even NOS dont turn up very often.

Stuart.

[Ian Vincent]

You will possibly struggle to find a good 3.45 as all the 6 cylinder boys have been trashing them for years and good s/hand or even NOS dont turn up very often.

Stuart.

Rimmer offer them new for £260 ex VAT, are you suggesting that they will be of questionable quality - serious question?

 

Rgds Ian

[ianc]

When my TR4 lost teeth from its pinion last autumn, I removed the axle and gave it to Pete Cox to fit another 3.7 CWP set. Pete advised me that Moss had sold over 600 repro CWP sets with only one problem, so I decided to go with a new set from Moss rather than take a chance with a second-hand set of unknown origin.

Of course, I cannot say whether a CWP set from Rimmer is supplied by Moss.

 

Incidentally, I would opt for 3.7 ratio in a roadgoing TR4 equipped with overdrive - ideal combination for fast cross-country and motorway. In Germany, where autobahn speeds are higher, fitting a 28% overdrive could be a good idea.

 

Ian Cornish