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Old 12-21-2015, 06:07 PM   #1
paulpw
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swop rear axle

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
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Old 01-12-2016, 07:53 PM   #2
Dave B.
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Re: swop rear axle

paulpw wrote:
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.
>> The 5.22 ratio was probably installed because of the relatively small size of the
>> original engine. The engine speed isn't really the deciding factor when choosing a
>> rear axle ratio. Available torque vs. vehicle weight is the important factor. The
>> diameter of the tires on the driven axle is also a consideration. That said, the
>> additional torque of the diesel engine would probably allow you to use a
>> numerically lower ratio.

Now the question: Would I be OK with disc brakes on the back instead of the drums? My question to myself is: Since discs 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?
>> I'm not at all certain that a drum front/disc rear braking system would be that
>> good an idea. Most braking systems are designed to have about 70% (more or
>> less) of the braking power directed to the front wheels. I'm also not convinced
>> that your current master cylinder would be up to the task. You might be able
>> to overcome the problem with an aftermarket adjustable brake proportioning
>> valve, but you would have to sort out the adjustment levels by trial and error.
>> If asked for a yes or no answer, I'd vote 'NO'.

Further questions might be:

(1) Can you envisage a problem attaching the hand brake cable?
>> Yes, I can see this being a real problem. Not all manufacturers use cables with
>> the same type of end pieces, or even the same cable lengths. Again, this would
>> probably require some trial and error to resolve.

(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?
>> Again, probably. Placement of the spring would depend on the results wanted.
>> And, the U-bolts would have to be long enough to accept the added spring
>> height (although longer U-bolts are available in some common sizes...). Another
>> possibility would be to have a spring shop re-arch one or more of your current
>> springs on each side.

(3) Is the width of the donor rear axle critical (the body is coach built and mounted on the Bedford chassis and is quite wide. Such that the width could increase a couple of inches before the wheels start to protude. I was thinking maybe some weird handling concern. PS: I would rarely be going above 60 mph.)
>> I don't think that "a couple of inches" would make any appreciable difference in
>> handling at normal road speeds. If you notice, a lot of trucks now come with
>> dual rear wheels and don't seem to have any other major modifications. Tire
>> width and inflation pressure would probably have greater effects.

>> In any case, I certainly wish you good luck should you decide to tackle this
>> project!!

>> DB
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Old 01-14-2016, 04:21 AM   #3
paulpw
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Re: swop rear axle

Thanks for taking the time to reply, Your answers have given me more confidence in the way forward. I was coming to the conclusion that I would stick with drums on the rear myself. So now I'm looking at donor vehicles as I pass them in the street. Seeing which have Diffs, drums, single axle and 5 studs. I'm going to keep a little note pad with me and then look up their diff. Ratios when I get back

I think my brother and I will be able to come to some solution with the cable ends. I've never heard of having the springs re-arched, I shall have to look this up later and see if there is anybody near me. Again thanks for your help. Paul.
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