Lotus Elan

Differential gear ratios

PostPost by: TBG » Wed Apr 22, 2020 9:53 am

I am asking for help and experiences.

I am considering fitting a CWP in my Elan with a 3:1 ratio.

My thoughts are that a five speed conversion is very expensive and tortuous and fitting the above will reduce the rpm for a given road speed which in turn will make continental long distance cruising more relaxed. I realise that it may reduce acceleration somewhat but I have more than enough power to cope with that.

Has anyone done this and if so what are your experiences.

Thanks in advance.
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PostPost by: rgh0 » Wed Apr 22, 2020 12:12 pm

TBG wrote:I am asking for help and experiences.

I am considering fitting a CWP in my Elan with a 3:1 ratio.

My thoughts are that a five speed conversion is very expensive and tortuous and fitting the above will reduce the rpm for a given road speed which in turn will make continental long distance cruising more relaxed. I realise that it may reduce acceleration somewhat but I have more than enough power to cope with that.

Has anyone done this and if so what are your experiences.

Thanks in advance.


Finding or getting someone to make a 3.0 to 1 diff ratio set may be an expensive experience and it will certainly give some challenges on the acceleration side unless you have a well developed big capacity engine. A standard set of Cortina ratios may work with a 3.0 diff well if you went that route.

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PostPost by: TBG » Wed Apr 22, 2020 12:35 pm

Thanks Rohan.

I have costed the complete five speed box experience and it comes to £3000 plus fitting and aggravation!!

I have also costed the CWP at £430 plus easy fitting.

What do you think makes sense. I have approx 140/145 Hp

Hope you are well and managing the lockdown polishing your cam covers as I am!! :D
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PostPost by: Foxie » Wed Apr 22, 2020 1:12 pm

TBG wrote:
I am considering fitting a CWP in my Elan with a 3:1 ratio.

My thoughts are that a five speed conversion is very expensive and tortuous and fitting the above will reduce the rpm for a given road speed which in turn will make continental long distance cruising more relaxed. I realise that it may reduce acceleration somewhat but I have more than enough power to cope with that.

Has anyone done this and if so what are your experiences.

Thanks in advance.


What differential have you got at present, and what power do you have ?

I think a one-off custom made 3.0 could cost as much as the Voigts T9 conversion.

I fitted a T9 to my Plus 2 in 2004, and later fitted a 2.89 1st gear to it. I have never had a problem with it ( or the concentric clutch :) ), and think it is very reasonably priced. Really pleased with it , and it is easy to fit :D

Over the years I have developed my engines and now have 169 hp.

I have changed my diff from the original 3.9 to a 3.77 to a 3.54. Last year I fitted a 4.55 (I have been racing hillclimbing for the last 14 years ) but i am now going back to the 3.77, as I had too much revs for normal road driving. I also have a Quaife ATB fitted.

Even with the considerably increased power i have, I would not consider using a 3.0 ratio.

I think 3.54 is the lowest ratio normally available ( Burtonpower), so if you have higher then that at present it would only cost you £232 + VAT to gear up.

:)
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PostPost by: TBG » Wed Apr 22, 2020 1:25 pm

Thanks Foxie.

As stated above I have about 140/150 in a 1700cc and it pulls like a train! Top gear acceleration is about the same as third used to be so I think it could pull higher gearing.

I have 3.54 at present and at around 80/85 am running at about 45/47 rpm . By my calculations this would reduce to about 3950 using a 3:1 CWP.

I am not hillclimbing but whistling about the Continent/Alps and would like a little bit more relaxed rpm.

Your thoughts are most appreciated. A 3:1 CWP custom made by the best is £430 plus fitting. Alan Voights T9 is £3000 plus fitting and much agro!
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PostPost by: mbell » Wed Apr 22, 2020 6:59 pm

What gearing are you currently running?

Have you done any calculations to see what rev/speed you'd have in each gear? (I think there is a spread sheet around here that might be useful but I always struggle to find it.)
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: Gopherit » Wed Apr 22, 2020 7:24 pm

TBG -
Would be interested to know who would make the 3:1 ratio for you, what would be the tooth numbers on the respective gears?

We are currently trialling a 3.3:1 CWP that has been supplied by Retroford (https://www.retroford.co.uk/) who have been very helpful at all stages of buying/assembly. The first assembly was using data from them and did not agree with the Ford Tool settings.

We did however keep to the recommended and found it was quiet on drive and too noisy on overrun. This was fitted into a lightweight +2 hillclimber with approx 170bhp, but run on the road with a straight-cut close ratio gearbox. Benchtesting since then we have reverted to the Ford settings and the comparison indicates this should be quieter - just waiting for lockdown release to roadtest!

There were no pulling away problems in first and reverse also no problem, but the final destination for this diff is in a slightly lighter than standard road +2 which will probably run a semi-close ratio gearbox - only because we understand that the close ratio helical cut set available from Quaife/TTR is now classified as "competition use only" aka noisy!

PS: Don't talk to me about the various ideas on marking up the teeth during assembly. Think we've been through a fair few - was Colman's mustard the last one???

Paul
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PostPost by: steveh » Wed Apr 22, 2020 9:03 pm

Gopherit wrote:TBG -
Would be interested to know who would make the 3:1 ratio for you, what would be the tooth numbers on the respective gears?

Paul


I would think this company could , https://www.ntgear.co.uk/buy-gears/
No affiliation to me .
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PostPost by: TBG » Wed Apr 22, 2020 9:44 pm

The teeth are 11 and 33 respectively. I have had talks with NTG who have been most helpful, again no affiliation just googling around the world. You will not believe the number of charlatans there are out there.

What I would love to hear, from your experience, is what do you think running a standard gearbox and around 140hp with a 3:1 CWP.

I think it sounds fun and worth a try..............but what do you think? Am I more potty than I think I am!!
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PostPost by: vstibbard » Wed Apr 22, 2020 10:38 pm

I'd not bother, I expect it would be a pig to get off the line, uphill takeoffs will require lots of clutch slipping and even your 1700 TC will not be making that much torque off idle.

I've a strong 1760 and I'm running it with 3.55 and Lotus 5 speed, granted I'm careful with it, but then I am anyway. I ended up using it as I got bored trying to find a good condition ZF to fit in, as I had sourced bell housing and shorter rear extension housing which moved gear stick forward to effectively the location for an Elan, I figured also that the chassis would need some adjustments.

Another option is Modena Engineering here in Australia that made their version of the original Hewland 5 speed which fits into the 2000e casing, it was originally made as a cheater for a RS1600 Escort which got banned in Historic racing due to it beating it V8 powered competition! Only issue for me, its a dog box and thats not ideal for road use. I started the discussions, ratios were able to be changed, I wanted 2.5:1 1st with its set of ratio's and an over driven 5th somewhere between .85 and .78 overdriven, he needed to make a new gear stick mechanism as his current one was based on the escort extension, and I was after a synchro version, things got in the way.. babies and business.

Cheers

V
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PostPost by: bitsobrits » Wed Apr 22, 2020 10:48 pm

3.00 R&P with a 2.97 1rst gear gives an over all ratio of 8.91.

I ran a 3.55 R&P with a 2.5 1rst gearbox for a year or so: gives 8.88. While I found that just acceptable (with a 1700cc TC), I later changed out the gearbox for the 2.97 1rst and found that much improved for starts on hills.

So my input would be try it. Certainly an improvement for 4th gear cruising, and depending on where and how you drive, the lack of 1rst gear torque may not be an issue for you.
Steve

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Elan S3 1967 FHC pre airflow

Formerly:
Elan S1 1964
Elan S3 1966 FHC pre airflow
Elan S3 1967 FHC airflow
Elan S4 1969 FHC
Europa S2 1970
Esprit S2 1979
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PostPost by: Foxie » Thu Apr 23, 2020 12:47 am

TBG wrote:The teeth are 11 and 33 respectively. I have had talks with NTG who have been most helpful, again no affiliation just googling around the world. You will not believe the number of charlatans there are out there.


Going back a long time to my college days, I seem to recall that a basic rule of good gear design was never to have even multiples of gear teeth unless absolutely necessary. All the available CWP and pinion sets available seem to bear this out.

If you wanted a 3.0 ratio you would use 11/34 ( ~ 3.09 ) or 12/35 (~ 2.92 )

:)
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PostPost by: HCA » Thu Apr 23, 2020 7:34 am

Foxie wrote: I seem to recall that a basic rule of good gear design was never to have even multiples of gear teeth unless absolutely necessary.


Interesting. What is the reason?
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PostPost by: rgh0 » Thu Apr 23, 2020 8:13 am

With an integral tooth count ratio number the same tooth on one gear hits the same tooth on the other gear on every revolution. This can lead to rapid failure as you can get uneven wear on one pair of mating teeth. With an non integral count every tooth on one gear mates with every tooth on the other gear giving more consistent wear and ensuring all wear at the same rate.

You should do the same thing with chains links versus sprocket teeth also where possible so the links in a chain is not an integral number of teeth on the sprocket

cheers
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PostPost by: HCA » Thu Apr 23, 2020 8:51 am

OKaay! Very interesting! I never knew this. I of course see the logic immediately.
Thanks Rohan :D
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