Avoiding rear wheel lock up

stefan einz · Joined: 25 Sep 2006 Posts: 146

We've chatted before on this forum about the benefits of matching revs on downchanges, particularly in the context of powerful rear wheel drive cars where, in extremis, one can experience rear wheel lock up and a consequent loss of control.

On other forums (not ones as enlightened as this, naturally!), people have been known to state that surely this whole risk of rear wheel lock up is overdone.

Well, I came across an interesting section from the Porsche Carrera GT workshop briefing manual. It reads as follows:

"Engine drag torque control (MSR)

Within the TC function there exists also engine drag torque control. This may be required for instance after changing down to a lower gear on slippery ground. On changing down to a lower gear the resulting engine braking effect can be so severe as to fracture the rear axle. MSR prevents this by calling for power (giving gas) through the engine management system. There is always the possibility that the Carrera GT driver may switch off the traction control functions by pressing the button labelled “TC”. He is then reminded of this by the display “TC off” in the multi-function instrument.

It is however inadvisable to switch off the TC functions. Doing so will in any case not lead to better lap times on the race track."

Extract, Porsche Carrera GT workshop manual

I have marked in bold the sentence which makes the point quite well!

(I also rather like the last sentence.)

This system does not, in fact, blip the throttle (as with F1 type systems; this is a traditional manual gearbox car), but clearly the software is clever enough to detect when the torque through the drive train due to a poor downshift is too high, and compensate accordingly. (Such systems are common on powerful motorbikes.)

So, I think this provides good evidence to rebut the doubters!

Kind regards

Steve

StressedDave · Joined: 24 Sep 2005 Posts: 532 Location: Cambridgeshire

My former Fabia vRS also had MSR - it's about the only thing it did have in common with the CGT...

TripleS · Posted: Sat Mar 22, 2008 12:26 pm Post subject:

I think I may have been guilty of dismissing this problem to some extent. My view was that anybody driving a powerful car has no business being clumsy enough to provoke this sort of problem anyhow.

Sorry for any upset I may have caused, but I accept it could be a serious problem.

Best wishes all,
Dave.

stefan einz · Joined: 25 Sep 2006 Posts: 146

TripleS · Posted: Sat Mar 22, 2008 4:48 pm Post subject:

Hello Steve,

The loss of grip is easy to envisage if a low gear is suddenly engaged at a brisk road speed, but I would have said an actual component breakage ought not to happen.

Best wishes all,
Dave.

martine · Joined: 17 Oct 2005 Posts: 1267 Location: Bristol, UK

stefan einz · Joined: 25 Sep 2006 Posts: 146

stefan einz · Joined: 25 Sep 2006 Posts: 146

hardboiled · Joined: 16 Jul 2006 Posts: 199

I know it's not a car but I was interested to spot that the 2008 model Honda Fireblade now has a slipper-clutch fitted as standard to assist with rear wheel lockups. It's a common device on racing machines but this is the first time that Honda has fitted one to it's standard production line bikes.