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gearing system made by Gleason and comprising six satellite gears (element gears) in mesh, with two central helical side gears. Normally, the torque imbalance
Diagrammatic representation of the Torsen differential
DREXLER
Nine teams out of the top 10 Formula Student cars this year used the Drexler University Special, along with 42 per cent of the rest of the field. This design is extremely small, neatly packaged and has a quick response time. Furthermore, the whole unit only weighs 2.6kg, which is less than the lightest Torsen unit without its housing, the package weighing in at 2.7kg. Its pre-loaded range can be easily adjusted, which can make a big difference to performance, as TU Munich explain: ‘Previously we had a Torsen differential, where if the inner wheel span the outer wheel then the differential began to work. The problem was when we drove into a corner on full throttle, the inner wheel started to spin, and only then would the differential start to block the inner wheel. The advantage of the Drexler is that it feels the moment of the wheel and so can be pre- loaded. When the driver drives in a straight line there is about 20Nm of pre-load, and in a corner, when the differential is pre-loaded with this torque, it stops the inner wheel from spinning immediately.’ Another benefit, according to Brunel Racing, is ‘it offers more adjustability in ramp angles than many of the other differentials available.’ However, this advantage in
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performance comes at a price – about £1700 ($2800), compared
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UK-based Quaife Engineering is the only one of the top three differential manufacturers who do not offer a ‘university special’ for FS / FSAE applications
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TORSEN!012000!University!Special!
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to a Torsen, which costs only £300 (£490). Nevertheless, teams have found several ways to contain the cost. Firstly, it can be used more than once, as Brunel Racing has done: ‘It’s a carry-over part, so it’s an absorbed cost. We’re likely to use it again next year too, so over three years it starts to become more affordable.’ Alternatively, Queen’s Formula Racing from Belfast purchased just the unit and made the rest. ‘We went for just the differential and not the driveshaft package, simply because of cost. We made our own intermediate shafts and CV joints to keep the cost down.’
Other teams that were new
to the competition, with little experience of Formula Student, such as TU Brno Racing from the Czech Republic, simply followed the trend. ‘We knew that almost every team is using this type of differential, so we tried it,’ they said. Drexler seems to offer the majority of teams what they need, even if it is expensive. But for teams on lower budgets who cannot justify the cost, they have had to look for alternative routes.
TORSEN The Torsen unit is made up of 21 components and uses an invex
between the two driven wheels causes the system to ‘try’ and turn the low traction wheel faster than the high traction wheel. However, in the Torsen unit, the gearing structure results in a proportion of the torque that would originally have gone to the low traction wheel going to the high traction wheel instead, where it can be used and optimised. The invex gears increase the total amount of torque that can be transmitted by the driven axles under all traction conditions without restricting differentiation. A major advantage of this design is that it has no effect on the anti-lock brake system because it does not support any ‘wind up’ (appreciable torque) between the drive axles during braking. Furthermore, the gearing and surfaces within the unit have different coefficients of friction so that wear is evenly distributed. Lancaster Racing explain: ‘Because it’s a mechanical unit, it’s very simple and there are other types of differential that are more complicated, like the hydraulic ones. The minute you add complexity to something like this, there’s more chance of something going wrong. We dismissed the Drexler unit on the basis of cost, but the Torsen is also costly because it’s built in America, so the shipping fees were almost as expensive as the differential itself.’
It’s a similar story with the
University of Manchester team, where the Torsen was considered the best option. ‘We went with it because it gave us more favourable characteristics than some of the other differentials out there. We hope it’s going to work better because the wheels have to start half slipping before stabilisation takes place. It’s also got a good torque bias ratio for our car, and we weren’t happy with some of the other systems that we have run previously, so we thought we’d try something different.’
The ultimate difference
between the Drexler and Torsen differentials would appear to
September 2011 •
www.racecar-engineering.com 73
TORSEN!012000!University!Special!–!General!Facts!
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