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Learning how to build Lotus Seven replicas...together!
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PostPosted: April 13, 2011, 1:09 pm 
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Location: Whitby, Ontario
We ran something similar in the Minicross and Mini 7's. We were not allowed to run with an LSD so we all got aftermarket 2 pin diffs. They also happened to be built very 'snug' so the friction was quite a bit higher than the standard diffs.

Oh and they didn't destroy themselves when you looked at them, unlike the standard diffs :)

Tom...


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PostPosted: April 15, 2011, 12:37 am 
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Tom17 wrote:
They also happened to be built very 'snug' so the friction was quite a bit higher than the standard diffs.

Tom...


I built up a rally car in Oz for a customer for the Dihatsu Charade challenge (G100) in the 80's and LSD's were not allowed so the diff in that one had 'very snug' clearances indeed. It must have had a 2 second advantage at the start of every stage where others would struggle to get off the line, especially uphill starts.

Zac, yes some diffs are more suited than others for them.

I'm really warming to the 42LE, gunna get me one next month or so and tear it apart and see whats up.


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PostPosted: April 16, 2011, 11:35 pm 
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My one previous effort with a home built mid engined transaxle car was about thirty years ago with a Lotus Europa fitted with a rotary engine and Porsche 911 transaxle. The biggest difficulty with it was the well known problem of routing the gear linkages back around the engine.

Sourcing a transaxle robust enough for a V8 seems to be mainly a problem of money......

Another less neat way to go about all this would be to use a conventional manual gearbox fitted to the front of the V8 engine, and a locked up transfer case to transfer drive back along a prop shaft to an offset rear diff.

The advantages are that the gearbox is right under the gear lever, and don't underestimate the value of that. Plenty of super strong low cost SUV gearboxes that bolt straight up to your V8, and the engine accessories like the dip stick and fan belt are at the back where you can get at them, not buried behind the seats.

Many SUV gearboxes have "cousins" with close ratio gear sets, for example Toyota Supra gears would fit into a Toyota SUV gearbox, which is really the same box with just a different output shaft.

To make the whole thing go in the right direction, you need to have a "front" SUV reverse rotation diff mounted upside down.
The rear half of a Bugatti Veyron works something like this. Like an entire SUV turned back to front.
It could all be very easy to do at reasonable cost, with standard original parts that all bolt straight together.
And if the transmission will not break in a two ton SUV towing a huge boat trailer, your little mid engined sports car is not going to break it with a LOT more power.
It is a bit weird, but I am sure it would work.

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PostPosted: July 21, 2011, 5:26 pm 
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I've seen that Veyron thing done before in some lambo's and some lambo replica's. I've thought about it before for building a midi. The only problem is that it adds weight compared to a front engined setup due to the extra mass of the transfer case and you still end up with a driveshaft that can fly out at you. Would be cool to see someone do it though.

Phil


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PostPosted: July 21, 2011, 6:05 pm 
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For quite some time, the mid engined supercars have been placing the gearbox ahead of the rear engine for a much better quality of gear shift.
Getting the drive back to the rear diff has most often been through the sump rather than around the sump on the outside. But having the gearbox right under the gear lever is such a huge advantage in a two seater sports/racing car, it is well worth the effort.

Single seat race cars still do quite well with the traditional rear mounted transaxle, because the gear lever is always mounted on the outside of the frame off to one side.
The gear linkage can be very direct, and that works well.
It is only two seaters with a central gear lever that create the horrible transaxle gear linkage routing problem.

Putting the gear lever between the drivers seat and door would solve the linkage problem, but there is never enough room to do it, and getting in and out of the car over the gear lever would be a bit problematic.

The only way I would ever consider building another mid engined transaxle two seater, would be using a rear mounted electronic auto transmission, and having paddles on the steering wheel to shift gears manually.


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PostPosted: July 24, 2011, 11:45 pm 
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Location: Cave Creek, AZ
With all the advances in mid-engine- rear transaxle that have occurred over the last 30 years in the offroad world, I just can't imagine anyone, ever having a problem with shifting them. The cable shifter is the way to go in my opinion; adjustable, versatile, and the technology is used in many different disciplines.

The torsion rods down the side in single seaters is fine when they are brand new but as soon as the universals start to wear out, it gets real sloppy fast. In my first two seater, mid engined buggy, the shifter was connected to the now rear facing shift shaft of the VW transaxle ( with a flipped ring gear) with a single curved shaft that passed under the swing axle's drive tube. Work semi well, but was plaged by the universal connection points.

A well laid out cable shifter is the way to change gears.

Tom

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PostPosted: July 25, 2011, 1:42 am 
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Putting the gear lever between the drivers seat and door would solve the linkage problem, but there is never enough room to do it, and getting in and out of the car over the gear lever would be a bit problematic.

Actually it can work quite well. The shifter can be located quite close to the wheel so the knob "falls readily to hand." The right hand steers while the left works the standard H pattern when needed. Short throw shifter is connected via 3 solid rods/rose joints, etc. to (in this case) a Porsche G50/52 transaxle that has been rotated 180 degrees and inverted. No need to flip the ring gear.

There wasn't a 'tween the seats cable shifter option when I built this, there is now but I think I would still go this way.


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PostPosted: July 25, 2011, 2:03 am 
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That is really nice Tom, well done, and pretty original thinking too !


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PostPosted: July 25, 2011, 11:58 pm 
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Running the Porsche box upside down is standard for a race car because it makes the engine sit very low. The input to the transmission and the engine crankshaft sit below the half shafts. Does that work out for you on the street and do you have a dry sump? Just curious. I wasn't sure you could work that out, that what my formula ford does, but it only has 2" ground clearence...

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PostPosted: August 1, 2011, 9:03 pm 
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What about Subaru transmissions? Subaru made FWD versions of some of their transmissions in the 80's, and haven't really changed transmission tech much since then, so gearing and some other things transfer over.

There are also methods of converting the 6 speed Impreza WRX STi trans to FWD mode. This has been known to be a very stout box.

I kinda like the idea of the 'vette T-56 plus a bellhousing. Having the differential at the back of the whole thing moves the weight inboard.


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PostPosted: August 2, 2011, 2:19 pm 
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killer_siller wrote:

I kinda like the idea of the 'vette T-56 plus a bellhousing. Having the differential at the back of the whole thing moves the weight inboard.

I seem to vaguely recall someone else on another Forum (a long time ago) had that very same idea.
The big problem with it turned out to be that the gear linkage at the top front of the gearbox sticks out into right where the clutch needs to go.
And you still have to get the gear linkages back right around to in front of the engine.


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PostPosted: August 2, 2011, 6:36 pm 
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killer_siller wrote:
What about Subaru transmissions? Subaru made FWD versions of some of their transmissions in the 80's, and haven't really changed transmission tech much since then, so gearing and some other things transfer over.

There are also methods of converting the 6 speed Impreza WRX STi trans to FWD mode. This has been known to be a very stout box.

I kinda like the idea of the 'vette T-56 plus a bellhousing. Having the differential at the back of the whole thing moves the weight inboard.


I regularly autocross a 2007 STi, and I can tell you that I have done hundreds of full-on drag launches, grabbed wrong gears, botched gear shifts with clunks and grinds, and done other horrible things to the transmission, and it did not flinch, even with 20% power over stock. When I swap with friends' WRXes at test and tunes, they always ask me to take it easy on the launch, since WRX transmissions have notorious 'glass gears' in them.

That said, a 6MT out of an STi weighs about 100 pounds more than the 5MT out of a WRX, which makes it a poor option for a lightweight car. A portion of that is the trick DCCD that would get pulled out when converting it to a transaxle, but it is a much bigger and heavier gearbox regardless.

I've been researching what it would take to do a Subaru-based transaxle mated to an EG33 for a midi-locost, and I have come to the conclusion that a WRX transmission with something like this: http://www.subarugears.com/index_files/Page497.htm would do just fine. If you want a high power motor (e.g. > 300 ft-lbs of torque) in front of the WRX transmission, a set of PPG or other baller gears can be had for $3500 to $5000 for first through fourth, and will generally take more abuse than an STi gearbox, and you can pick better suited ratios to boot. If you are building a race car, you can go straight cut and/or dog engagement for a marginal increase in cost. A used STi transmission is normally $2500-$3000, while a WRX unit can be had for around $1000.


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PostPosted: September 28, 2011, 11:24 am 
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I found your group hunting old school transaxle info,
The picture posted of the rover engine with the corvair diff then corvair powerglide is a rear engine set up...people run that light engine in the rear.
You can not run the powerglide backwards, if you wanted this to be mid engine you would have to modify and flip the diff upset down...yes, people do that,
There is also the crown set up...while longer engine>trany> then diff you can use a gear box or power glide for mid engine.

My car has the crown style but I was hoping to find the Kelmark style or just move on to the Audi 5000 gear box....I cant afford the high end porsche gear boxes


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PostPosted: September 28, 2011, 9:01 pm 
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Thank you for taking some time out the13bats and welcome.


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PostPosted: October 11, 2011, 12:27 pm 
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Informative thread. The pics on the first page showing the various configurations using a Corvair trans is great. The Crown and Kelmark are the most popular. Crown being the strongest but longest and Kelmark/M.I/D. being weaker but about 9" shorter. There are several reasons for the Kelmark being weaker. First it pushes on the coast side of the ring gear teeth unless you are fortunate enough to have a set of reverse rotation gears. Second is the input shaft, it is .770" or less in diameter and about 2 feet long. Third is the ring gear, it is only 6 something inches in diameter and unless you are fortunate to have found the 4 spider gear diff then it is the weak 2 spider gear set. With all the options in transaxles in the last 15 years or so from companies like Audi or Porsche there are many more options. With all that being said, I am in the middle of building my own Kelmark style SBF to Corvair adapter only because my car is already setup for that arrangement. It is more that a flat plate with two sets of bolt holes like many of the VW adapters from KEP. I hope to have the car on the road this spring for the 2012 Import/Kit Car Show in Carlisle Pa. Wish me luck.

wm


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