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 Tallest back axles and how to tell.
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Mr A.the Hun

United Kingdom
38 Posts

Posted - 20 Oct 2020 :  12:25:50  Show Profile  Send Mr A.the Hun a Yahoo! Message  Reply with Quote
Hello,
Regarding Series Rapiers.
What are the 'tallest' (highest geared) ((lowest revs. for the speed)), back axles and how do you tell which is which externally?

I am asking because my Series 3, alpine engined, 1725cc, (+40 thou so actually 1770 I'm told), Rapier is in 4th by the time I'm doing 30 MPH and I'm only left with O/Drive for all other speeds. Also it has more than enough torque to accelerate from 30mph in 4th. Just seems a bit under geared.

Looking for a more relaxed cruise.

I appreciate any change would require the Speedo to be sorted

Thank you for any appropriate info..

pruyter

Netherlands
300 Posts

Posted - 20 Oct 2020 :  13:59:02  Show Profile  Reply with Quote
The following rations are available:

4.86:1 with wheel rotation 296 degrees
4.78:1 with wheel rotation 301 degrees
4.55:1 with wheel rotation 316 degrees
4.44:1 with wheel rotation 324 degrees
4.22:1 with wheel rotation 341 degrees
3.89:1 with wheel rotation 370 degrees
3.70:1 with wheel rotation 389 degrees

Non overdrive Rapiers as the series III had standard 3.89:1 while the overdrive version had 4.22:1
For instance in my Rapier series IV with standard overdrive I changed the 4.22 version for a 3.89 version.
All diffs are interchangeable also the Arrow diffs. In my Series Rapier is fitted a diff from a Rapier H120.

To check which diff your Rapier has, jack up one rear wheel (chock the other one well), put the car in top gear and use the starting handle to bring the engine to top dead centre (watch the timing marks). Now mark the bottom centre of the tyre whilst holding the wheel back against transmission backlash. Turn the engine over two complete turns to exactly TDC again and, holding the wheel back against the backlash once more, see how far the wheel has rotated.
If you have a 3.89 diff the wheel should have done just over one complete turn and if you have a 4.22 diff it should have done less than one turn.

The 3.70 diffs were fitted in automatic cars from the Arrow range.

Regards,

Peter

Edited by - pruyter on 21 Oct 2020 10:40:48
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Mr A.the Hun

United Kingdom
38 Posts

Posted - 20 Oct 2020 :  18:50:00  Show Profile  Send Mr A.the Hun a Yahoo! Message  Reply with Quote
Thank you for a brilliant comprehensive answer, I shall do that as soon as I can. Spookily I have just ordered a crank handle from the spares secretary, which will make it easier. Thanks again.
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