Lotus Elan

Elan tunnel size

PostPost by: h20hamelan » Thu Feb 07, 2019 3:09 am

I am still contemplating an overdrive unit.
Suppose because its winter, and every right minded Lotus is still hibernating (here in the North).
However,
http://www.gearvendors.com (no affiliation)
and others make thin, narrow, smallish overdrive units.
Can anyoneconfirm tunnel size please, as mine are nestled snuggly in their mountain cave.

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PostPost by: rgh0 » Thu Feb 07, 2019 11:59 am

Dont know if what you posted fits but the shape of it got me thinking that whats needed is an over drive diff nose for the Ford English axle. More room at that end in an Elan and a much bigger market with all the Ford axle cars and easy to package into an integrated diff nose and overdrive with simple installation to a standard diff housing bolt circle.

Maybe one day

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PostPost by: prezoom » Thu Feb 07, 2019 4:42 pm

Might be simpler to design a new tail shaft housing that incorporates the shift mechanism, sort of like a Voight setup, but for the Ford gearbox. The overdrive unit would then bolt to that adapter. The shift lever would remain in the same place as the original housing. May even be possible to utilize the original shift mechanism, which would allow the extension shifter to be used as well. Gear Vendors manufacturing unit is about 30 miles from me, hmmmmmm........
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PostPost by: h20hamelan » Thu Feb 07, 2019 10:06 pm

I asked gear vendors (GV) if they would consider looking into English Ford diff mount, based on Rohan’s experience etc (I did not give names to GV).
Not saying a trans mount would be better or worse, but rather costs and quality I would consider GV to be of top quality.
I suspect if GV would/could supply 1000 units for UK Austrailia and NA, the costs and quality would be better than most one off attempts.
I suspect we could have a go at a supplier direct group buy. And most likely other clubs would want to do the same. Giving GV some idea of production numbers.

Over/under drives can split all gears. So one might be tempted to use 1 low or 1 high, 2 low or 2 high etc. I would personally be tempted to shift from 1 high, to 3ed low. Eliminating the most used gear, thus saving it for less wear and tear.
They have a ratio calculator on their site, and depending on final drive, tyres etc, some good options for about $2k, with no major modifications.

I would prefer to cruise my Twinks at lower RPM.
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PostPost by: 1owner69Elan » Thu Feb 07, 2019 11:10 pm

Back in the 60’s, before my Elan, I had a 58 TR3A with an overdrive. As a teenager I enjoyed going through the gears 1, 2, 2od, 3, 3od, 4, 4 od. 7 gears. Whether this was optimum for performance I don’t know, but it was fun.

The TR with an uprated 2.2 liter engine was pretty quick. But, compared to the Elan it was a veritable truck (Lorry). The TR is long gone, sadly ended up in a scrap yard due to an irresponsible later owner. The Elan is still with me.

Be interesting to see if this o/d is a viable option for the Elan.
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PostPost by: Mazzini » Thu Feb 07, 2019 11:34 pm

By coincidence I was talking to Alan Voigt about od's last Saturday.

In about 1985/6 there was an advert in Motorsport for an Elan with an overdrive, intrigued I called up! The guy had used a unit from a Spitfire. He said it all worked fine. I've often wondered what happened to the car.
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PostPost by: prezoom » Fri Feb 08, 2019 2:18 am

xx
Last edited by prezoom on Fri Feb 08, 2019 2:49 am, edited 1 time in total.
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PostPost by: prezoom » Fri Feb 08, 2019 2:47 am

A quick measurement on my spare mid close, as compared to the measurements earlier posted, and the Gear Vendors unit is no taller than the top plate of the gearbox. Another rough measurement shows that the overall length should be close to equal to the original gearbox. It appears the speedo gear would move to the OD unit, simplifying the input shaft to the overdrive.

Back in the 70's, I installed an Advance Adapters 2-speed box that installed between the original transmission and the clutch housing on my Chevy Dually with a 4-speed manual. It offered a 17% split that was absolutely perfect, splitting the gears evenly for the 2-3 and 3-4 gears, over driving the top gear. Only trouble was the units self destructed. After thee of them, I threw in the towel.
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PostPost by: prezoom » Fri Feb 08, 2019 2:47 am

double post?????
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PostPost by: HAIRY » Fri Feb 08, 2019 5:41 pm

I have a vague memory that in the 60s I borrowed a Jim Clark biography and read his Elan had an overdrive and he could 'cruise all day' at high speed, and 120 mph comes to mind. However, I cannot confirm the quote as I do not remember the book title.
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PostPost by: baileyman » Fri Feb 08, 2019 6:15 pm

My new frame has 5" gross width available, net somewhat less, which seems to rule out this above dimensioned box.


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PostPost by: Esprit2 » Fri Feb 08, 2019 6:34 pm

Is installing an overdrive unit the best way to achieve the goal? Or is this simply the case of an idea grabbing your interest, then being worked to it's limits just, "Because it's there"?

Wouldn't a 5-speed be an easier and less complicated approach? And more in keeping with a Chapmanesque, "Simplicate" approach?

And if a 5-speed is a good way to go, then for about the same amount of work, why not consider a modern 6-speed... like the MX5/ Miata's?

Just a thought not in keeping with the original post.

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PostPost by: h20hamelan » Fri Feb 08, 2019 9:03 pm

I had asked the same question back in 2012, not that it matters.

lotus-gearbox-f37/4th-gear-adding-overdrive-driveshaft-t25659.html#p167776

There are many gearbox threads. MX5 6 speed etc. I would prefer a Nissan CD009 over any. But that’s me, unfortunately it is almost twice as heavy at 148 lbs.
And there have been many who are happy with their 5 speed. As I read it, a 5 speed without changing rear diff only increases top cruising speed minimally.
Of course a 6 speed mx5 would be the choice (as you can only get an 8 speed in a auto at this time). If I or someone had taken the steps to make adaptor plates. But there is a reason SJS (i could be wrong again, might have been TTR or Sue) put a 6 speed mx5 box in an elan with the intentions of going into production.
So I waited, as I dont live near a city, and do no. And dont have time or money to make a plate myself. Though i suspect there are other hurdles. Might have been angles of shaft, or spline or something. Again, that is not the only 6 speed box, but the mx5 6 is the lightest i believe.

Many people have their opinions on why the original close ratio box is best. I tend to side with them. Not saying I would not go with a 6, 7 or 8 speed.

I believe the reason for a Over/Under drive is cost, fitment, weight distribution (if one considers diff mounting). Of course I could be completely wrong on all of those points.

A used MX5 6 speed is about $600. One would consider a rebuild as the ones listed on car-part.com (no affiliation) have high mileage. So that gets the costs to about what a GV unit should be. Then another $400 for adaptor plate as long as that is it. Close to the same costs.
Six speed transmission from MX5 including clutch fork, but not throwout bearing or shift lever: 87.5 lbs
Maybe someone knows the weight of the 4 speed. I would guess they are about the same, and someone might know the length of a mx5 box. Which I would suspect is longer. So possibly adding more weight to the front. Which is probably the opposite of what the baby elan needs if memory serves, it needs another 50lbs in the rear. Again, i could be very wrong. Merely asking.

Yes, a 5 speed would be quite capable of cruising at 120mph. Not sure about tyre size and gear ratio, at what rpm... I bet someone knows. So if the costs for a proper built 5 speed is $3500, and new diff ratio at $700 it is a bit more costly. And arguably lower top gear. I could/should/might have gone through the ratio calculator and figured it all out.

So I read it like this. 5 speed $4500ish vs 8 speed $3000ish. Do correct me if my math is off. I suspect a rear diff mounted over/under balances the car. Further if there is good reason the 4 speed is rubbish. I enjoy both of mine! And ultimately, dont want to change, but I do, and have for a long time wanted overdrive units. Might be because my Volvo estates have electric 5th. Though I have a CD009 adaptor plate for one of mine.
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PostPost by: stugilmour » Sat Feb 09, 2019 6:50 am

Does the circle of eight studs attach directly to the rear of the transmission case, lengthening the unit by about 12”? That setup could make lowering the engine/transmission/overdrive unit in to the car difficult. Even with my Spyder frame it may be pretty tight to attach the unit with the transmission in place.

Rohan’s rear placement idea sounds interesting, but installation would have challenges. I managed to get the diff in and out a few times with my Spyder frame, but darned if I could describe the exact angle of attack that worked. Adding a 12” component on the nose of the diff, get the kids out of the garage before we try that out. :D Some frame mod probably required to fit things.

I think your assumption about weight distribution is correct. Can’t find my numbers right now, but I think my Plus 2 weighed in at about 52/48 front/rear with me in the driver’s seat, full of fuel, and a modest amount of stuff in the trunk. The roughly 40 extra lbs seems a modest add.

I think the 0.78 overdrive ratio with a 3.78 diff should end up pretty close to my MT75 0.82 fifth and 3.54 diff combo. There is a really good spreadsheet available on the forum to refine ratio details, but I think you would land close to my overdrive 3500 rpm at 80 mph, which is perfect for four lane distance cruising. My modestly tuned Twin Cam would never hit redline in fifth though. Without more than stock power fourth-high may not add to top speed much. Being able to slip into fourth-low to pass would be cool; fourth-high might feel pretty flat trying to accelerate, even in a much lighter Elan.

Just some random thoughts, not discouragement.

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PostPost by: rgh0 » Sat Feb 09, 2019 8:34 am

stugilmour wrote:Rohan’s rear placement idea sounds interesting, but installation would have challenges. I managed to get the diff in and out a few times with my Spyder frame, but darned if I could describe the exact angle of attack that worked. Adding a 12” component on the nose of the diff, get the kids out of the garage before we try that out. :D Some frame mod probably required to fit things.

Stu


What i was thinking was to replace the whole nose of the diff with an integrated overdrive and pinion / crown wheel assembly that the Lotus Alloy diff pumpkin bolted to. The length and size would be similar to the existing diff nose so no chassis mods. Weight would be similar if made with an alloy casting or CNC machined from aluminium billet.

The diameter of the new diff nose just forward of pinion bearing would need to be made big enough to accommodate a suitable planetary gear set and electrically actuated overdrive clutch

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