Hydraulic gearbox linkage. 1

[theminiman]

Hi, I hope someone can help me.
I want to build a mid engined car for fast road or sprinting. The engine must be mounted inline not transverse and will sit in the middle of the car. The gearbox will have to sit over the rear wheels and will also have to be a transaxle gearbox from a Renault 25/30 or similar. It would be possible to make a solid gear linkage to change gear but this would be very difficult to use and fit. I have seen in a magazine artical a hydraulic linkage used on a similar car. I have also found on the web an electronic gear changer but this is only for sequential gearboxes. My buget is not very big for this project so any help anyone could pass on to me would be great. Thanks

 

[magnograil]

Probably the easiest way is to use two hydraulic cylinders at each end. One set captures the fore/aft and left/right motion of the shift lever, the second set follows the first. Both cylinders on each circuit have to have the same bore and rod diameter for 1:1 motion. Connect the same cylinder ends (rod/piston) together. You will also need to add a bleed port to both sides of each cylinder.
Have you considered using two Bowden cables? For compactness you would want to use a 90 degree bellcrank at both ends for the lateral motion so both cables can run side by side.

 

[modesign]

Have a look at the Porsche Boxster gear links. Uses two cables with a neat gear change lever mech. The links on the box are simple.The Boxster engine is central.

 

[Warpspeed]

I once owned a Lotus Europa which had a mid engine layout. The gearbox linkages were the worst feature of the car, and I never solved the problem to my satisfaction.

If I was going to build a mid engined car today, I think I would adapt one of the electronic automatic transmissions and replace the torque converter with a clutch. The gear lever or paddles on the steering wheel would with simple electrical contacts painlessly and VERY QUICKLY change gear. Lots of possibilities, but eliminating the clumsy mechanical linkages would sure simplify things a lot.

 

[magnograil]

Warpspeed - I have a Europa with the 4 speed box. The gearshift has always worked fine but a little stiff.
I found after removing the engine/gearbox unit for a rebuild that the universal joint was binding. Since I was able to realign the cross by holding it a vice and tweeking the shaft I suspect the metal has too low a yield point.

 

[Warpspeed]

The fact remains though, that anything with very long flexy or heavy linkages, with bell-cranks, sliding and rotating bearings with several intermediate joints, or very long push/pull cables, will either end up stiff, or eventually wear sloppy, or have too much spring in it to have a really nice "feel". Jumping out of gear can be quite entertaining too.

Getting the gear linkage from the very rear of the gearbox around the engine is never going to be easy in the usually very limited space available. It is the very high penalty you pay for using an engine and gearbox that were never originally designed for the mid engined configuration.

It is easy for Porsche, Ferrari, or Honda (NSX) to make beautiful linkages because the gearbox was originally designed for the mid engined configuration. Building a home made special car that uses a low cost FWD gearbox, or something like a VW, Porsche, or Renault transaxle turned around, you will be in real trouble fabricating something that works really well. It sounds like a very simple mechanical problem, until you try it for yourself.............

Fabricating something that changes gear is easy. Making something that works superbly is far more difficult. A fun little sports car will not be much fun if it has a really horrible gearchange.

 

[magnograil]

The 914 was designed as a mid-engine car and still had the shift link shaft extend to the back of the gearbox like the Europa. They later moved the gearbox shifter shaft to the side, shortening up the linkage a bit. Porsche has always had very long shift throws (this may have changed with the late model cars) which masked the free play. The Europa was a short throw and worked well. Ferrari puts a gate at the lever end to take out a lot of the slop.
An electric shifter will take out all of the feel which is not a problem if you are not worried about the syncros.

 

[theminiman]

Lots of great ideas to get me thinking, I do like the sound of the Modified auto box with paddles. I know that Audi/VW use a cable linkage if this is to much work.
Thanks.

 

[sjrf]

Soleniods might work as well. Add a few relays etc. and you can have a sequential gear selection. You can probably find all the solenoids you need att the lokal junk yard...

 

[dutri]

Hi,
there are several Truck manufacturers in Europe who use a hydraulic gearshift system. It has an hydraulic four-piston manual actuator under the gearlever connected with four hydraulic pipes to two double-acting hydraulic cylinders on the gearbox. This system might well work for your installation if it doesn't need too much space or weight.

 

[cadastrophe]

Neal and CNC make hydraulic throttle systems. They may be adaptable to your gear change.
Wilwood make a hydraulic actuator, originally for converting the old VW Beetle and Bus cable clutches to hydraulic. They would be ideal for your situation.
You might also want to take a look at motorcycle rear brake master cylinders for possible candidates for the actuators. I have used them before in just such a setup. You can replace the feed with a bleeder.

 

[gianniletta]

hello theminiman, I made a road legal sportscar mid engine 2 l. 16v and alfa romeo 33 gearbox: Yes, there are some difficult as the mechanical linkage must be STRONG and ACCURATE but it is possible with success. Then it depends from the quality of gearbox. I owned a mid engin De Tomaso with a nice Hewland and mechanical linkage: gorgeous!

 

[flydeep]

This is great, I’m in the first steps of inserting an International Harvester DT 466 diesel in the middle of a Super Duty F 450 Ford. The engine is from a IH 1750 Load Star. The cab is from a Ford
E 350. It’s a nice 1986 Van body and is stretched for two great bucket seats in the back. I’ll be using a Hyd, linkage or push pull cables. It was great running across this mid engine discussion and has given me a different out look on the problem. My address is flydeep@bellsouth.net and I’d be happy to get more points of view on this subject. The bell cranks and push pull cables are used every where in the air craft industry and have been developed to work great. I know that they use quite a few flat cables that actually run through small bearing surfaces on the inside of the stationary cable component. But the inside cable is very very tight and extremely predictable. Dave .

 

[ddmtrans]

The Chevrolet Cavalier uses a two cable shifter that seems to work fine, and should be inexpensive and easy to find. The Chevrolet Corvair engine rotated counterclockwise, so if the transaxle were to be turned around, and a clockwise engine placed in front of it the vehicle would travel forward. The four speed Corvair transmission is basicaly the GM Saginaw so it should stand up to a reasonable V8. Mid engine V8 kits used to be available for the Corvair, that would allow you to bolt up just about every Chevy rear drive engine, from Chevette to Big Block, they had the same bell housing bold pattern. Except for the 2.8l.