Swop rear axle

[Paulpw]

Hello, I own a 1980 2.3 L petrol Bedford CF 280 ambulance converted to a camper-van that now has a 2.8L diesel engine from a Diahatsu 4 trak in it. Since the engine speed of the diesel engine is so much slower I need an axle ratio on the back axle to be much less than it is now ( 5.22 to 1).. I seem to think around 3.7 to 1 is about right.

Now the question :- Would I be OK with disc brakes on the back instead of the drums?

My question to myself is :- Since disks are considered to have greater braking efficiency and there are drums on the front, will this cause uneven front and rear braking ?

From what I understand, disc brakes although more efficient and usually have higher hydraulic pressure applied to them via the master cylinder. So does that mean since the pressure applied to the now rear discs from the lesser pressure drum brake master cylinder, that the rear braking won't be so extra harsh. Or am I just confusing myself ?

A further questions might be :-

1. Can you envisage a problem attaching the hand brake cable ?
   
   
2. Because it was an ambulance the rear springs are super soft and maybe weaker now that they were 35 years ago. Could I put and extra spring leaf on top or under the present one ?
   
   
3. Is the width of the donor rear axle critical ( the body is coach built and mounted on the Bedford chassis and it is quite wide . Such that the width could increase a couple of inches before the wheels start to protrude. I was thinking maybe some weird handling concerns. PS I would rarely be going above 60 mph.)
   

Fortunately its not me who has to do the work but my brother who's good at welding and constructing stuff, like his hotrod, but I need want to get any of your ideas first. I know I've loads of questions but answers to any of them would be welcome.

Cheers , Paul

 

[Peter.N.]

Something I used to do years ago when I did a lot of diesel conversions, only I usually used an axle from a similar vehicle with a different ratio.

As most of the braking is done by the front wheels the rears are not so critical but I certainly don't think you would get to much braking on the rear because as you say disc brakes need more hydraulic pressure as they don't have the 'servo' effect of drum brakes, I am assuming you have a servo brake system or you will get very little braking.

If the new axle has handbrake provision you should be able to work out some way of attaching the cable.

The width of the axle shouldn't matter to much as long as its within legal specification.

The greatest problem I can see is getting it insured, I think you will have to pick your company and if you find one to insure it I expect they will want an engineers report.

The engines I used to put in cars came from Bedford vans but they were the smaller ones that used a Perkins 4/108 engine so I used overdrive boxes from Ventora's, Cresta's and VXs, the ratios were to close for the diesel so I fitted the van gears on the overdrive shaft, worked a treat.

The big CF diesel vans used a TK gearbox but they didn't make an overdrive version of that - as far as I know.

Peter

 

[Paulpw]

Thanks, did'nt know servo effect thing but looked it up now and makes sense, preet sure the layout is the same. Learnt something! was hoping could get round handbrake issue. I.ve managed to insure it as is with replacement engine so I'll use same company. Thanks for getting back to me.

Paul

 

[Burnerman]

Are you intending to use the Daihatsu axle? That would help with the prop shaft issue.
We can only throw a few suggestions around here, but the replacement axle should have a hydraulic weight compensator on it, and these are adjustable to some degree.
I'd also go for the Daihatsu handbrake cables, and see how they can connect to the lever inside......you can maybe get some ideas from the Indespension trailer people who have a fair selection of brake parts and cables.
John

 

[Paulpw]

Thanks John, probably going to use the axle thats in place now ,I dont know which one that is ,the Bedford one or the Diahatsu one, I seem to think it is the Diahatsu one, I'm going to leave it to m brother. The cable bit makes sense, there are two parts of the cable, Im sure something an be done there. What got me is this hydraulic weight compensator thing. I've try doing research on it and all I seem to get is references to under water mini subs. What does this thing do please and how would I source it?
cheers Paul

 

[Peter.N.]

I can't remember what sort of axles they use, if its not a Salisbury type you could possibly change the complete diff assembly over, I remember that on the BMC ones you could change the crownwheel and pinion as a unit.

Peter

 

[Burnerman]

The brake pressure compensator normally sits on top of the axle on light commercials - even heavier cars may have one, I'm not sure......anyway, it senses the depression or weight load on the axle, and provides brake bias as necessary. Without one, the rear brakes could possibly lock up on a lightly loaded vehicle. The load on the sensor is a rather basic system which uses a spring - the length of which determines the pre load on the sensor.
Lets understand this.....you have a Daihatsu engine, maybe an axle - so which transmission are you using?
John

 

[Paulpw]

OK Thanks guys. I shall be doing a bit more research. At moment I have Diahatsu 4 trak engine and gear box using the Diahatsu Axle that my brother has connected to the original Bedford 5.22:1 rear diff and axle. Where as at 4000 rpm on the 4 trak top speed was 91 mph ( axle ratio was 3.36:1) , on my Bedford ( 5.22:1 and 8% smaller wheel diameter than the Diahatsu) my 4000rpm now gives me top speed of around 54 mph. Through calculation and experimenting with the extra low speed gear lever whilst in 3rd, not forgetting that my Bedford camper loaded up is much heavier than the 4 trak. I have come to the conclusion I need an axle ratio between 3.6:1 to 4:1. In the middle preferably. Hence before I started on the search for replacement axles of suitable ratio, I had better find out about the drum v. disc thing.
cheers Paul

 

[Peter.N.]

What I have seen done is the fitting of a gearbox, back to front, in the final drive line, you can select your own final drive ratio that way. Saw this on a Mk10 Jag with a TS3 diesel in it.

Peter

 

[Burnerman]

Blimey Peter that's a new one
I would have thought that Paul would be better off looking for a CF diesel axle......if I recall, they were single and twin wheel versions available (and not all from ice cream vans!)
John

 

[Peter.N.]

First time I had seen anything like it John, I thought I put a lot of work into diesel conversions but I have never seen anything like that before or since. It had an auto box on the engine plus and back to front manual box in the propshaft. He had reworked the inner front wing panels to accommodate the engine and also fitted an XJ rack and pinion because there wasn't room for the steering box.

This was in the days of CB, I was talking to him from one of my Perkins conversions and he said come and look at mine, he was only a few miles away in Honiton.

I got quite adept at shortening propshafts and modifying gear linkages but never attempted anything like that.

Peter

 

[Dave54]

I don't really know that much about fitting non standard stuff like that, but I'd read the thread and wondered about fitting some sort of intermediate box.

The custom guys do all sorts, and they certainly shorten back axles, as well as doing all sorts of "impossible" marriages of parts. I wonder if it wouldn't be possible to graft the CF hubs and brakes onto an axle with suitable ratio, and make the axle the right width at the same time. Need someone with the right kit and skill set though.

 

[Peter.N.]

If you have a tame engineer anything is possible, I could cut and weld but have no turning skills. What you really need is someone with he same enthusiasm as yourself, that was mostly what kept me going but I was younger then I don't think I could be bothered now. When I drove my first CX diesel I came to the conclusion that at last the manufacturers could make better diesels than me so didn't do any more

The most ambitious job I am tackling at the moment is converting an elevated balanced flue Worcester oil boiler into a horizontal one and getting it gas tight.

Peter

 

[Dave54]

I used to know a bloke who had the kit and the know how to do that sort of thing. Biggish lathe and someone to use it is the main thing I'd have thought.
Most decent welders would weld the result up I'd have thought.
Might be a thought for the OP to ask at the local custom car club though. They may know people who do that sort of thing for reasonable cost.

 

[Burnerman]

Gents - is my suggestion of finding a CF diesel axle and fitting it the simplest way for Paul, do you think - or am I missing something?
Just thinking about mountings, handbrake linkage etc.
Maybe the axle ratio is the same as the petrol though...I've really no idea.
John