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Lots of Single Seater racing cars are now featuring motorbike engines driving via a chain to a differential. Chain drive differentials are also put to good use in bike powered trikes and off road buggies.
Fig.1 - Mini differential in modified Formula Ford
There are commercial drive systems such as those used on Radical sports racers but they are a bit expensive.
A cheaper alternative is to use a front wheel drive car differential converted to chain drive.
Here is how a couple of my chain drive diffs were manufactured:
1. Standard (open) BMC, BL, Rover Mini typeThe crown wheel is unbolted and an adapter plate and chain sprocket is bolted in its place.
(The standard Mini crownwheel is also one end of the diff therefore a new adapter plate is required).
The adapter plate includes the left hand side bearing spiggot (The plate can be fabricated or machined from the solid).
Fig.2 - Sprocket Adapter Plate
To keep lubricant around the cross-pin and planet gears, a can has been manufactured from a large oil filter cannister.
One end of the can butts against the sprocket and the other end has a hole which fits over the RH output shaft,both ends are sealed with RTV and a couple of small brackets hold the can to the sprocket.
I use a mix of grease and Hypoid 80 oil to lubricate the diff, a small tapped boss has been welded to the can to allow filling.
The diff uses the standard Mini output shafts and these are supported by sealed bearings housed in aluminium alloy bearing blocks bolted to the chassis.
The standard Mini output shaft covers are bolted to the outside of the bearing blocks and provide further support and sealing for the output shafts.
Bearing blocks are usually split, with long bolts clamping the two halves to the bearing and securing the blocks to the chassis.
Fig.3 - Split Bearing Block
I have seen bearing blocks made by cutting up the rear of a Mini gearbox and machining a flat face on the resulting blocks.
Fig.4 - This diff uses mounting plates bolted through the output flanges to bearing blocks cut from gearbox.
Because the chain is offset to the left compared with the Mini crown wheel, it is necessary to adjust the drive shaft lengths to suit.
This can be done in a number of ways (some cheap, others expensive!)
I have one diff which uses the early Mini rubber inboard drive couplings.
The RH coupling was lengthened by cutting & welding a length of tube to move the splined portion further out.
Fig.5 - Right hand inner coupling has been lengthened to move diff to left.
I have seen similar things done with the later pot-joint type couplings.
If you have the Cooper S type Hardy Spicer couplings, it is easy to make up spacers and use longer bolts.
Another method is to cut , sleeve and weld the drive shafts or get longer shafts made.
On the Royale, Formula Ford hubs and shafts were used, suitably shortened and special flanges were made to weld to the Mini inboard couplings.
The standard Mini differential is not very strong (it was designed for about 30 BHP!) - With wide slicks and 150 BHP from a 997cc Kawasaki engine, I was destroying standard diffs in about 30 miles.
The sun and planet wheels would break up - result no drive.
I also managed to shear a pot joint type output shaft at the splines whilst doing a standing start.
Fig.6 - Typical Mini diff components
One way to get a stronger Mini diff is to use an aftermarket Mini twin cross pin diff.
These are a lot stronger and are a quarter the price of a new LSD.
2. Quaife ATB LSD (Mini)A bit more expensive than the previous version but is very easy to modify to chain drive.
You don't even need to manufacture an adapter plate for the sprocket.
The Quaife ATB diff has an end plate onto which the crownwheel would normally bolt.
It is just a case of getting a sprocket made to match the bolt pattern.
(The Quaife end plate is tough stuff so use the standard size bolts 'cos you can't drill the plate!)
Fig.7 - Quaife LSD end plate with plugged holes.
There are a few small holes in the end plate which can be plugged (I used interference fit aluminium alloy plugs and some Loctite).
I also made up an alloy ring to bolt through the end plate and sprocket (saves fitting nuts & washers).
This alloy ring was also grooved to provide a seating for the can which keeps the oil in the diff.
The can was fabricated from aluminium alloy with a tapped hole for a plug for filling with oil.
Once again the can was sealed to the ring and diff housing using RTV and was secured using two brackets and bolts.
Fig.8 - Sprocket backing ring and oil can.
The Quaife ATB was again supported in split bearing blocks with sealed bearings and Mini output covers.
Output shafts for the Quaife are floating and are kept in place with the Mini output covers.
The Quaife outputs are very strong but very expensive! (and are available to suit 'S' Type couplings or Pot joints).
3. Other Quaife diffsQuaife produce many other ATB diffs for front wheel drive cars which could be adapted in a similar fashion.
They also produce a special diff based on a Ford Fiesta ATB which is designed with the same output flange spacing as a Hewland racing box (This would be useful when converting Formula Fords to bike power).
4. Other diffsA few competitors are using the lsd diffs from the rear of the Ford Sierra Cosworth.
These are taken out of the diff housing and adapted in a similar manner to the fwd diffs.
This is probably the cheapest way to get an LSD and has the advantage of suiting strong Ford drive shafts.
5. Sprockets, Chains & AdjustmentsI use aluminium alloy (530 chain size) rear sprockets. These are custom made but are very reasonably priced.
Blank sprockets are drilled for the desired mounting PCD bolt pattern and centre clearance hole, they then have lightening holes added.
Some people have their sprockets split to ease quick ratio changes without having to split the chain.
As I said, I use 530 sized chain, O-ring type (there are lots of good strong chains about).
The chain is lubricated with the usual spray on bike chain lubricant.
Most chains are now riveted with a 'soft link' which makes the joint. A special chain splitter and rivet tool is required.
You can get removable split links which make taking off the chain easier (but are not generally recommended).
The easiest way to adjust the chain is to shim out the bearing blocks from the chassis but this can be a bit fiddly.
Other solutions include eccentric bearing blocks (which rotate to adjust), pivots to pull the diff further out, chain tensioners or adjustable engine mounts.
To align the front sprocket with the rear sprocket shims could be used.
On my Hawke single seater, I can shim my entire engine transversely to align the two sprockets (only by about 5mm either way due to the proximity of the chassis).
There are now cheap laser alignment devices which can be used to check and set up the sprocket alignment.
6. Reverse GearIn the UK, kit cars undergoing IVA and road and modified hillclimb and sprint cars and all race cars must now have an operable reverse gear.
Most common are electric reverse mechanisms acting on a ring gear on the diff.
The gear on the electric motor is pulled into mesh using a cable in the cockpit.
Fig.9 - Radical Clubsport Electric Reverse
The early Radical Clubsport used a geared electric motor which engaged on a special front sprocket retainer.
When the electric motor was powered, two balls engaged on the drive bar on the sprocket retainer and the car reversed (only problem was trying to disengage the balls afterwards - they tended to stick and drive the electric motor backwards resulting in the balls flying out of their housing or damaging the electric motor).
Westgarage Engineering electric reverse fitted to a Sierra LSD chain drive differential.