Differential gear ratios

[TBG]

Rohan that is fascinating stuff.

I have just had a chat with NGT about it and they make both kinds. They have made over 100k sets of gears for mining equipment with the integral tooth count without failure - some running since 1977!!

He says that if the gears are well made and lapped in correctly there is no problem BUT if they are cheapo ones take care whichever you go for.

I am close to going for it................

[661]

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here in Australia that made their version of the original Hewland 5 speed which fits into the 2000e casing, it was originally made as a cheater for a RS1600 Escort which got banned in Historic racing due to it beating it V8 powered competition!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

That is interesting. Perhaps a 5 speed box thought for the future. I must say the Aussie gears I got from SSC via Hollinger Engineering for the Exige were superb. Best on the market.

[redcarandco]

I think that Rohan knows perfectly what he is speaking of and I would like to add a personnal stupidity we made me and the friend with whom I bought my first race car I have rebought 5 years ago(26R14)As we were intended to make some slalom sprint we fitted a jkd (Jack Knight development )gear box with 2.25 first ratio and a 3.5 diff .........We never succeeded to get on the trailer ramp in first gear so long so long it was unusable and if you calculate that 1 st ratio is almost the same that an elan 1 st gear fitted with a 3.0 do you always start rolling in 3rd gear with your every day car ??????very sorry for super poor english
Best regards to all
Roger

[TBG]

A good point Roger! I am still struggling to make head and tail of gear ratios! Going to a 3:1 from a 3.45 is around 15% difference I think - please correct me if I am wrong.

If that is so would a 15% difference make 1st gear unusable? Dunno???

I don't think the difference between 1st gear and 3rd is anywhere near as small as that Roger so I was happy to use a few more RPM on take off.At the moment mine will very happily take off in 2nd.(Probably not on an Alpine slope!)

[rgh0]

Rohan that is fascinating stuff.

I have just had a chat with NGT about it and they make both kinds. They have made over 100k sets of gears for mining equipment with the integral tooth count without failure - some running since 1977!!

He says that if the gears are well made and lapped in correctly there is no problem BUT if they are cheapo ones take care whichever you go for.

I am close to going for it................

Yes I am sure with care and good quality control on manufacture and assembly tolerances integral tooth counts can be eliminated as a potential problem source. Personally for a one off set for an Elan in a Ford diff with all the challenges on tolerances in that process I would go you a non integral tooth count as it is just one of many problems you're less likely to have to worry about.

Getting a low 3:1 diff ratio to work in practice will be about matching the gear ratios and engine torque band to driving use. A wider ratio set of 1st to 3rd gears from a Cortina will help compensate for the low diff ratio in a lot of situations while giving the more relaxed top gear cruising you are looking for.

Another approach is to build a very well balanced and smooth engine using racing build techniques intended for 9000 rpm engines so 5000 rpm with a 3.5 diff on the motorway does not seem so hectic.

Another approach is to go to 14 inch wheels and larger rolling diameter tyre to effectively give a lower diff ratio. Different issues as challenges are around body clearance but another way to get more relaxed top gear cruising with a conventional diff ratio

lots of ways to skin this cat apart from a 5 speed box

cheers
Rohan

[TBG]

Thanks Rohan,

The gears that I am going for are not a one off. The program to set up a CNC machine for a specific job is over £11000 or so!

Thus going for a bespoke ratio is not on unless you have deeper pockets than mine.............

I am grateful for all your other points but bigger wheels will not fit, the clearance is already pretty tight.

[vstibbard]

Whilst cruising is important hence my using the 5 speed box with the stocker I have, don't lose sight of the reasons an Elan is so much fun, for me thats guiding it through the twisty stuff with stone acceleration between, thats where closer intermediate ratios do add significantly to the fun!!!

A standard cortina 2.97 ratio box even with 3:1 will always have a poor set of intermediate ratios, which combined with the long diff I suspect will be rather pedestrian.

Either way, I'll look forward to seeing how you fare with it, and its simpler than finding a good 5 speed and replacing a box.

Cheers

V

[TBG]

V - I agree!

My Guru states that I may have to do more gear changing on country roads - but what fun to have a third gear to help give the thrills!

Gear changing and charging hard is all part of the fun!! I have to say that with this engine my acceleration in top is what third used to be like so I am hoping for BIG grins per mile.

[redcarandco]

To answer your question, ratio was around a little bit more but I do not remember how much but just thinking of that now coming back from garage ...it exists a 5 speed gearbox from Kelvedon nothing to do with the one spoken sooner but I think it is a hewland similar and probably easier to fit that the other version but I think price is let say impressive....go to kelvedon lotus site and as long I remember Pat Thomas is a fair man ....
Best regards
Roger

[msd1107]

Some downloadable spreadsheets to help

viewtopic.php?f=42&t=18445&start=

viewtopic.php?f=37&t=16331&start= (at the end)

Any questions, email me. I do not think I have updated these, but if you have trouble I can email you fresh copies.

Cheers,

David
36/7988

[msd1107]

RE: Diff ratios.

the standard 3.55 is a 9/32
The other 3.55 is a 11/39

Mixing the two gives 11/32 or 2.909, a little too high.

The 3.77 is 34/9. Mixing it with an 11 tooth gives 11/34 or 3.091, still a little high
4.11 is 37/9. Mixed with an 11 tooth gives 37/11 or 3.364. This might work.

With 165/80X13 tires, 6,500 gives 130, 7,000 gives 140.

If you have a DHC, these speeds may not be achievable. With a FHC, maybe yes. Depends on your engine tune. Those with a Vehger SAS and 180 BHP could see higher. In any case, using the spreadsheets allows you to easily experiment with possible configurations.

Cheers,

David
1968/7988

[1owner69Elan]

From your original post it seems you were interested in achieving lower rpm cruising around 80/85 mph. As others have noted, It would make sense for you to plug into some of the gear ratio spreadsheets that are out there to better analyze the situation, and factoring in information about your engine (torque band). Otherwise you may be just guessing as to the effects of a very long diff.

One calculator I have found useful:

http://westfield-world.com/gearboxshifts.html

Here is output for my Elan for my 5 speed box (to be installed).

My 3.77 diff with low profile tires is equivalent to a 3.94. The calculator shows around 3800 rpm at 80mph, in 5th. Top speeds at redline are impressive and will never be seen, if even achievable, as I am not racing the car. The fastest I ever went in my car (as stock) was ~120 mph (indicated) on the Autobahn back in 1969 when I picked the car up at Hethel and drove around Europe. The car tended to overheat cruising above 100mph that summer.

[TBG]

1owner69Elan states

My 3.77 diff with low profile tires is equivalent to a 3.94. The calculator shows around 3800 rpm at 80mph, in 5th.

Very helpful. My calculations seem to think that I will be around 3950 at 85 in top with the 3:1 cwp so not much difference.

Gosh there are so many variables. Pulling away on a steep slope in first is the interesting one - but hey I have to slip the clutch a little now - isn't that what clutches are for.....??

[2cams70]

[quote="TBG"

Another approach is to build a very well balanced and smooth engine using racing build techniques intended for 9000 rpm engines so 5000 rpm with a 3.5 diff on the motorway does not seem so hectic.

That usually best happens when the engine retains it's stock 72mm stroke rather being stretched to 1760cc by long stroke tall block conversion. You might gain some low end torque with a conversion but the engine won't be as sweet and smooth even if properly balanced. Careful engine and exhaust mount design can have an enormous effect on the amount of noise, vibration, harshness transmitted to the cabin too

[rgh0]

[quote="TBG"

Another approach is to build a very well balanced and smooth engine using racing build techniques intended for 9000 rpm engines so 5000 rpm with a 3.5 diff on the motorway does not seem so hectic.

That usually best happens when the engine retains it's stock 72mm stroke rather being stretched to 1760cc by long stroke tall block conversion. You might gain some low end torque with a conversion but the engine won't be as sweet and smooth even if properly balanced. Careful engine and exhaust mount design can have an enormous effect on the amount of noise, vibration, harshness transmitted to the cabin too

Using light weight pistons and rods and then carefully balancing everything should get a 1700 CC long stroke twin smoother than a standard one at 5000 rpm. it will not be a smooth as applying the same treatment to a 1600 CC standard stroke engine at 5000 rpm but it will still be better than a typical standard stroke, standard rod and piston weight engine, without precision balancing of all components as is normal for a road car.

However the cost a set of lightweight forged pistons and rods is not trivial, though its come down a lot in recent years and probably now viable for a road engine build these days especially if doing a long stroke block build where you need new rods and pistons

cheers
Rohan