limited slip-differential swap and theoretical report big pres [autosaved]

Limited slip-differential swap and theoretical report

Limited slip-differential swap and theoretical report

Adam Costello-DT007/3-Technical Project

What I will be looking at:

Explanation of an Open differential and a Limited-slip differential

The car

Why I chose this as my project

Advantages of an LSD

Options

Differential gear ratios

Finding the LSD

Rebuilding the LSD

Protection of the housing

Removal of open differential and refitting LSD

Insurance implications

Breakdown of Costs

Conclusion

Acknowledgments

The Open differential:

This is the type of differential that originally came fitted to my car and is the simplest type of differential.

When travelling in a straight line both of the wheels will be driven at the same speed.

The pinion is driving the crown-wheel and cage, and the spider gears are not rotating meaning both side gears are locked to the cage.

However when turning, the wheels must spin at different speeds. The spider gears in the cage will start to spin as the car begins to turn which allows the wheels to rotate at different speeds.

The inside wheel spins slower than the cage(less distance to travel) while the outside wheel spins faster (bigger distance to travel).

The Limited slip-differential:

An Lsd for short, is a type of differential that allows a limited amount of slip between the differential cage and sun gears.

Multi-plate clutches are used and are fitted between cage and sun gears.

Inner plates(friction discs) are splined to the sun gear. Outer plates(dog gear plates) are splined to the cage or pressure assembly as its also referred to.

Bevel type gears are used and are put under an axial thrust when being driven by the crown wheel.

The greater the amount of torque being transferred through the differential gears, the greater the axial thrust forcing the gears apart.

Normal driving conditions - low torque - the differential operates normally.

High torque driving conditions - axial thrust on the gears presses and locks the multi-plate clutches to the cage.

What this means is the differential gears will be put out of action which results in both wheels being driven at the exact same speed.

The Car

Daily driver not a track car

1998 BMW 318is

135,000 miles

1.9 litre Dohc

140bhp

Front engine-d

Rear wheel drive

Lowered on coil-overs

17 inch M3 alloys

Why I chose this as my project:

Main reasons :

Wanted to take on a technical project that after completing I can still use on a daily basis.

Thought if I invested so much time and money into the project it would be nice if I were to benefit.

Other reasons:

A limited slip differential was an optional extra of which my car wasnt fitted with.

Found the idea interesting in the sense there were multiple options to chose from.

Challenging hard to find a limited slip differential as they are rare.

Adds improved traction and cornering.

Better performance if I ever decide to bring the car to Mondello Park for a track-day.

Makes the car more sought after if I was to sell it.

Rarity means the differential will hold its value extremely well.

Advantages of a limited slip differential:

Open differential can only deliver the same amount of torque to either sides - a limited slip differential can vary the torque to each wheel by creating inefficiency through friction.

With an open diff, a car with 200nm of torque will have 100nm will be sent to the left and 100nm to the right at all times.

However a limited slip differential can redistribute torque to send 50nm left and 150nm right and vice versa depending on what wheel wants for more traction. Therefore a limited slip differential provides better traction.

With an open diff, during high speed cornering (on a track) the inner wheel will become lighter and lighter until the wheel starts to spin.

However with an Lsd torque can be distributed to the outside wheel to give an oversteering yaw moment which will help the car into the bend.

Options: 1st

Small case. (168mm)

Standard open differential is a small case unit.

All that would be needed is the LSD unit itself.

Same differential housing can be used.

Same differential gear ratio can be kept as same crown wheel and pinion is used.

2nd

Medium case. (188mm)

Medium case differential fits into the same sub-frame as small case.

Shorter two piece prop-shaft needed to cater for extra length of medium case.

Ratio would be 3.15:1 as opposed to 3.38:1.

This would take away from acceleration.

3rd

Large case. (210mm)

Large case differential does not fit into the same sub-frame as small and medium case.

This means the whole back axle would have to be changed to accommodate the diff.

Ratio would be 3.15:1 as opposed to 3.38:1.

This would take away from acceleration.

Small Medium Large

Differential gear ratio:

One important factor when considering what Lsd to obtain was the differential gear ratio.

This is the ratio of the ring/crown wheel and pinion gears in the rear axle.

My car has a ratio of 3.38:1. This means that for every 1 turn of the ring/crown wheel, the pinion will turn 3.38 times.

Its important as it determines the cars acceleration.

Ratio > 3.38:1 better acceleration, revving harder in top gear

Ratio < 3.38:1 slower acceleration, less rpms in top gear

Finding the Limited slip differential

Searched: Donedeal and Adverts 4/5 times daily looking for the LSD.

Started searching October 2015. Visited 2 BMW specialists DL Motorworks and Motor Confidence to no avail.

Decision narrowed to either a small case or a Medium case LSD.

Large case LSD eliminated due to cost factor and being unsuitable.

Phoned and emailed BMW breakers in the Uk such as Quarry Motors, ASWR independent dismantlers and LK spares. Was quoted 595 pounds ex.vat and 95 pound postage for just the Medium case differential. Cost was far too high.

On Thursday February 18th 2016 travelled to CC Automotive BMW in Armagh. No LSDs in stock.

Finally acquired a small case LSD unit on Sunday the 21st of February.

Found add on donedeal. Travelled to Carlow that night at 9pm and bought it.

Donedeal advert:

Acquired:

Rebuilding the LSD

LSD unit rebuilt by ValdasSiriunaitis.

Motor Confidence.

Recommended.

Known for rebuilding LSDs to an extremely high standard.

Rebuilding the unit:

New output flange seals

New seal: Pinion shaft

(Remove Imput flange to gain access by loosening securing nut)

Pinion shaft remained in the differential as the inner and outer bearings did not need to be renewed (low mileage donor differential).

The crown wheel is heat fitted to the unit.

The speedometer ring also has to be heated before it will fit.

Shims and side covers bolted back on.

Lash can be set using shims.

Output flanges can then be pushed back in.

Housing filled with 1Litre of Castrol LSD oil.

Backing plate secured.

Differential standing on the imput flange and all six bolts fitted and tightenedDifferential fully reassembledProtecting the differential casingWhy was the backing plate not sprayed?Designed to disperse heat

Removal and refitting

Helped by a friend: David Cullen works in HalfordsWork carried out on the 29th of March.

Why do you put the car into gear?So the propshaft doesnt rotate when you are trying to loosen the nuts

Photo taken yesterday when the car was up on a lift

Insurance implications:Put it off for so long as I thought it would void my insurance or there would a massive price hike

Rang AXA on Friday 20th May and told Evan

All that was required was a garage report

Breakdown of Costs:Small case LSD unit: 370 Rebuild of Differential by Valdas of Motor Confidence: 250Woodies D.I.Y and Halfords painting items: 44.24 Donor Differential: 30 LSD inspection at Advance Pitstop Ballymount: 0 _______Combined total: 694.24Adding in the cost of the trip to CC Automotive BMW in Armagh and the journey to Carlow to buy the LSD unit pushes the cost of my technical project over the 700 mark. Conclusion

LSDifferential in the car since 29th March.

Loud noises for first 2 days.

Bedding in of the LSD unit.

No other problems since.

Car drives just as before.

Far greater knowledge of the Limited slip differential

Only regret is not attempting to rebuild the LSD unit myself.

Acknowledgements

Dave Lonergan of DL Motorworks.

Unit D5, Ballymount Drive, Walkinstown, D12.Valdas Siriunaitis of Motor Confidence.

Unit 14B, Knockmitten lane, Western industrial estate, Naas road, D12.David Cullen.

Michael G Hall of D.I.T Bolton Street.

Thank you!

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limited slip-differential swap and theoretical report big pres [autosaved]

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