DIY Kit Car Prop Shaft Options

[montyjohn]

Hello, I'm building a Jag E-Type kit car (actually I'm re-building an old kit car).
Original kit car is Cortina Based.
I'm fitting BMW engine and gearbox

I'm scratching my head with what to do with the propshaft.
I know a custom propshaft is the correct answer here, but I wanted to explore some DIY options first.

I have a single piece cortina proposhaft and a two piece BMW propshaft in my possession.
The axle end of the Cortina shaft has a worn staked UJ so might not use this shaft and hunt around for a cir-clipped version.
The length of the Cortina Prpshaft is roughly what I need.

I have three options in my head, but unsure what the pitfalls might be.

Option 1:
Simply join the BMW rubber donut end to the cortina prop.
On Land Rovers these donuts can take a huge amount of articulation, on the BMW they take almost no articulation so unsure if all donuts are created equal.
This option also requires replacement of the worn staked UJ which wont be fun and I'd like to avoid.

Option 2:
Shorten the upper shaft of the BMW
Join the Cortina shaft to the second shaft of the BMW
Again, UJ would need sorting.

Option 3
Buy a 2 piece cir-clipped Cortina propshaft. I assume these are much longer than my single piece Cortina shaft, so unsure if it will need shortening or not.
Join the BMW rubber end to the two piece Cortina shaft.

Under all options, I would jig, sleeve and tig weld, then send it off to be balanced (if they'll touch it?).

Picture below should explain all.

Any thought or suggestions here?
Many thanks

[Auntieroll]

Option 1 looks the best to me , this retains the Giubo joint at the front , a circlipped Ford rear section joined to the BMW front using a turned section to snugly fit the different IDs will ensure a true running shaft , the welding needs to be done by someone competent , why not talk to your local propshaft specialist ,? he may be able to advise you regards available parts/ options etc;

[DrDeAtH]

Whatever you do will need dynamic balancing... its probably quicker and easier in the long run to get a custom shaft made. They are not that expensive.

[stevieturbo]

Whilst there are many factors...ie speed, gearing etc...they all boil down to shaft speed.

There are limitations with a 1 piece shaft when it gets too long. A shaft supplier should be aware of the term critical speed, you must stay below this.

Obviously for a 2 piece this becomes much higher as it is 2 small shafts. As to whether your vehicle/gearing might be at risk of this, is up to you.

But if the vehicle had a 2 piece to start with, you're probably better retaining that, and it can help with install angles too, shaft clearances etc
And any shaft must be welded together properly and balanced ( although many have got away without it, but it is a bad idea to do it DIY as such )

[Hashtaggggg]

Why can you not replace the UJ. We used to do this In a boggy field with very basic tools.

That was a few years ago, but the principal is the same

[montyjohn]

Thanks for all the responses.
I am leaning towards just getting this done professionally.
I'm confident I can weld it just fine in terms of strength, but I can see me being in a situation where it can't be balanced (or they wont balance it as it's too out of tolerance).

A quick email to JW Engineering near London and they believe they can sort something out (unsure what their plan is as yet or price).

@Auntieroll, yep, Option 1 seems like the most straight forward. With an RPM of 7000, I guess it comes down to whether the length is appropriate or not.

@DrDeAtH I think I've arrived at your conclusion now so thanks for the input.

@stevieturbo I guess it's the engine speed that dictates the max length. So if the BMW engine had a two piece shaft, then my kit car should also. Now, it's worth pointing out that the propshaft that came with he car was shorter and one piece, so I might just let the professionals decide what's best here.

@Hashtaggggg I have no concerns changing a cir-clipped UJ. These staked UJ however are not designed to be maintained, It's a new propshaft job. However, with some effort, they can be removed and swapped for internal circlip types. However, I believe there is a high risk of damaging the yoke given the amount of force required to remove them. I'd be surprised if you swapped a staked UJ in the field, but then again anything is possible.

Thanks, whilst it wont be anytime soon, I'll feedback what the solution end up being.

[OnTheEdge]

Presumably you have a Cortina (live) rear axle, so whatever choice you make the propshaft will require a sliding splined section to cope with length changes during axle articulation. I don't see anything like that in your suggestions.

[Lotobear]

The Cortina prop has a sliding yoke to cope with rear axle movement but would need the Ford gearbox/tailhousing to make use of it.

For what it may be worth this firm in Gateshead will shorten and re balance your prop for £58 + VAT or make you a custom one. I would call them (very friendly/helpful):

http://www.cpsdrivelink.co.uk/

[montyjohn]

OnTheEdge said:

Presumably you have a Cortina (live) rear axle, so whatever choice you make the propshaft will require a sliding splined section to cope with length changes during axle articulation. I don't see anything like that in your suggestions.

Haha, yes. That was a dumb omission from all my options.
This just reinforces the the fact that this custom shaft is a little trickier than first thought.

[stevieturbo]

montyjohn said:

@stevieturbo I guess it's the engine speed that dictates the max length. So if the BMW engine had a two piece shaft, then my kit car should also. Now, it's worth pointing out that the propshaft that came with he car was shorter and one piece, so I might just let the professionals decide what's best here.

No, as already explained it is nothing to do with engine speed...it is everything to do with shaft speed, which is basically vehicle speed/gearing/tyres

[montyjohn]

Fair enough,
My logic was 5th gear is 1:1 ratio, so you'd base it on the maximum possible propshaft RPM which is equal to maximum engine RPM.
Appreciate this is playing it safe as you'd never reach max engine RPM in 5th so working the other way from max (expected/likely/possible?) speed makes more sense.

[stevieturbo]

montyjohn said:

Fair enough,
My logic was 5th gear is 1:1 ratio, so you'd base it on the maximum possible propshaft RPM which is equal to maximum engine RPM.
Appreciate this is playing it safe as you'd never reach max engine RPM in 5th so working the other way from max (expected/likely/possible?) speed makes more sense.

IF....IF your 5th gear is 1:1, then yes.

As for reaching max rpm in 5th....again, that depends on overall gearing, and of course whatever power is available ( and engine rpm )

[InitialDave]

Once you're having to pay someone to sort out the balancing etc, I'd say it's really not worth it over just telling someone what you need and letting them get on with it. It's not hugely expensive to have it done.

[TallPaul]

I had a home made (by the previous owner) propshaft on one of my cars years ago, it'd had a different engine & gearbox fitted so they basically did a cut & shut of 2 props. It had been done fairly well, it was welded nicely and looked to be pretty straight but was very slightly out of true and not perfectly balanced. Due to my mechanical naivety at the time, I knew it wasn't quite right but didn't know why. It kept breaking gearboxes and cracking bellhousings, I think I replaced 3 gearboxes before I finally twigged it was the propshaft. I got a custom one made for about £90 back in the '80's and is was perfect after that.
My advice is get one properly made, thats without even thinking about what would happen if it snapped...