rv1890
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| posted on 8/4/06 at 07:08 AM |
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rear diff
I have a general question that may have been discussed before, but I am unable to find a post on it. What are the advantages and disadvantages in
having a BEC w/ a chain drive and one with a shaft drive??
I have a 1200cc bike engine and availability to salvage yards. I was thinking about having a mid engine car with a bike engine. The car will be driven
by a very short shaft (U joints with a short shaft on each end) connected to a a differential out of a IRS car.
My last post someone told me they used a bmw 318 rear end, and that all the information he provided. What determines what ratio rear end I should go
with. 2.42, 3.10???
Any tried and true designs??
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JoelP
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| posted on 8/4/06 at 08:08 AM |
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normally the acceptable range is 3 to 3.6. For track work, 3.6 is often fine, motorways are easier with a lower number (longer gears). To work out for
your situation, you can compare it to the bike. To avoid messing about with individual gear ratios, you can compare top speed, final drive (rear teeth
divided by front teeth) and circumference of the rear wheel. For example (and i must stress, im about to make this up completely!) if the bike had a
top geared speed of 170mph, a 2.5 final drive and say 1m rear wheel diameter, and your car has a 3.6 rear diff and a 1.2m rw diameter, your top speed
will be circa 141mph (170x2.5/3.6/1x1.2). If you redo this sum with the correct figures, you can then decide if top speed is suitable. Most tracks ive
been on wouldnt need more than 120mph i reckon, for a 130bhp car.
Thats the ratio bit covered 
Now if its rear engine, i think chain is probably better. I may be wrong but i think there are less transmission losses in a chain, plus you can
easily change the final drive ratio by changing cogs. Even the shortest shaft/prop drive is going to add 6" to the length of the engine bay,
unless you could just use a rubber ring to link it all together.
[Edited on 8/4/06 by JoelP]
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ReMan
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| posted on 8/4/06 at 09:41 AM |
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Though normally we are talking about bike engines which normally use a chain drive to the rear wheel and the range of ratios used front/ rear sprocket
is fairly narrow I would guess.
hoiwever a bike engine designed for shaft drive with the diff in the rear hub so to speak MAY be different, you would need to check.....
Am I reading this correctly in that you have an FJ1200 or similar with shaft drive?
www.plusnine.co.uk
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rv1890
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| posted on 8/4/06 at 09:54 AM |
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appreciated
Appreciate it my friends. That clears things up a bit. Im currently reading "engineer to win" by caroll smith, and will be working out the
details later.
Just to clarify I plan on building this for autocross and for fun. I plan on pushing the drivers seat more foward and mounting the shocks inboard in a
horizontal direction to make room for drivers feet and space in the engine bay for a shaft.
Im still debating this idea because I want something durable and strong and low maintence. Also, the chain drive differentials Ive noticed are too
expensive. Any less expensive chain drive diffs available?? Id break my back and my wallet if I had to get one of these Quaife deals.
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JoelP
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| posted on 8/4/06 at 12:58 PM |
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if its a shaft drive bike, it might be tricky putting a sprocket on it to use with a chain drive.
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rv1890
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| posted on 8/4/06 at 02:10 PM |
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options
Im still looking at my options. If I can find a relatively inexpensive chain drive diff then I will sell my shaft motor and find something more
suitable. Like I said Im still in the planning stages.
Is the chain drive that much more beneficial? Im know it will be lighter and consume less space, but is there a performance difference, or certain
characteristics that a shaft drive wil not deliver??
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JoelP
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| posted on 8/4/06 at 02:16 PM |
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ive never driven a middy myself, so cant comment on practical sides of chain vs prop. However, if you are changing engine, you can pick up zx9s for a
few hundred, probably blades too. If i was wanting to make a cheap middy, id use one of these longtitudinally with a short prop and a sierra diff.
Thats got to be the cheapest IMHO. Maybe research a rubber coupling that might avoid the need for two universal joints etc.
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