MT75 gearbox + DOHC engine mounting

[Matthew]

Hi

I've just fitted the gearbox mount for my MT75 gearbox, and the output shaft has ended up about 25mm higher than the input to the diff. Will much difference be OK?

If I really had to I could drop the mount down a bit, but the bottom of the main part of the box already sticks out about 15mm below the chassis rail and I'm not keen on putting it any closer to the ground.

Matt

[twinturbo]

What joints are you using?

Idealy you want it in line if possible.

TT

[Matthew]

Hi TT

I don't have the propshaft yet so the joints are still TBD, but I was looking at the Westfield type with the rubber doughnut at the front - not sure if that has a UJ at both ends or not. The other possibility is to go for something like Dax use in the Rush which I don't think uses the doughnut.

Just worked out the angle of misalignment - about 1.8deg

Matt

[silvertail]

Hi Matt,

I'm working on a DOHC/MT 75 build as well, just haven't got as far as you yet.

If you're having the propshaft made up by a specialist, I'd contact them and ask about it - no harm in letting the professionals explain it! Alternatively, I think "axle" on this forum has already completed his DOHC car - maybe he can explain how he did it?

Regards, Mike.


For a bit more help/info though:

1) This question has been raised on Locostbuilders a few times, with no real conclusions! A lot of the threads refer to this webpage.

Scroll to the bottom of the page, and you'll see what limits the page's author puts on the propshaft angles. However the working angle limit of 4 degrees only covers a max. of 3500rpm; the DOHC engine at full whack in 5th gear can be doing almost twice that!

3) Don't forget to consider lateral offset of the propshaft as well. If you're looking directly down on the car, you want the engine and diff to be (almost) perfectly in line (if that's possible). Otherwise, the propshaft angle will be greater than you've calculated.

[twinturbo]

Could you move the diff and angle it so it's in line?

TT

[teun]

Hi i use a 2.3L dohc engine with a MT75 gearbox. I do have the propshaft in line. The problem is i do not know how to place a picture

[Matthew]

Hi

I did a bit more measuring and ended up dropping the gearbox to line up with the diff. Apart from anything else, I found that if you don't do that then the front of the engine sticks above the natural line from the nosecone to the scuttle.

As I haven't finalised the engine mounts the only change I had to make was to drop the gearbox mount to the bottom of the transmission tunnel. The difference in height is now so small that I can't believe it will make a difference. Downside is the bottom of the gearbox is below the bottom of the chassis, and the bellhousing/sump sticks down quite a long way so will need a fairly major chop.

Mike - I've been in contact with axle, and he's been very helpful and sent me some pictures that didn't really make complete sense until I tried it myself. I do have the engine in line and horizontal so that's one less thing to check.

TT - From looking at the website Mike posted, one important consideration is to have the input and output flanges parallel so that's what I've tried to do. Angling the diff would have meant they were at different angles, and I'm not sure what effect that would have.

Thanks for the help

Matt

[Matthew]

Originally Posted by teun

Hi i use a 2.3L dohc engine with a MT75 gearbox. I do have the propshaft in line. The problem is i do not know how to place a picture

Teun

If you want to place a picture then you first need to host it on a web site. Then you can put the URL between IMG tags by clicking on the "Insert Image" icon at the top of the reply box and pasting in the URL.

Matt

[Davey]

In an ideal world the gearbox and diff flanges would be perfectly in line but unless you're using a live axle which is going to give a large variation in axle/gearbox alignment then its not going to be an issue. As long as there is sufficient fore/aft movement (which will be minimal) of the propshaft I don't see a 25mm difference as a problem. My camper van is based on an Iveco 4.1 Tonne van and the gearbox to axle vertical distance at rest is about four inches (100mm) and this can vary by probably a similar distance, that's why we use flexible couplings.

D.

[Enoch]

if you think back to the days of live axles the angle of the prop shaft changed continuously as the diff was attached to the rear springs. If you loaded the boot up the angle could change by several degrees - more when you went over a bump. The moving joints in the propshaft are there for that reason - to allow for misalignment. You want to get it as close as possible to give the maximum movement in all directions but it does not have to be perfect.