High speed instability SWB 911's

[redrobin911]

Hi - I have an instability issue with my 65 911 race car at high speed i.e. 100mph +/- 20mph where obviously the lift generated by the (front of) car makes everything go a little light. The front end is particularly nervous and seems to move considerably from a straight-line. I'm fairly sure there isn't much I can do other than lower the front of the car further to increase the rake angle to try to make things better. I also know that I can't fit a spoiler or splitter and neither does anyone else. Just looking for any scrap of info that might help - no matter how little.

[Lightweight_911]

What toe settings are you running ?

[redrobin911]

What toe settings are you running ?

Front - I think the last setting was 15 minutes total toe in, rears 0.5 deg toe in both sides (1 deg in total) - I might just need to check with my mechanic but think this was what we had.

[redrobin911]

What toe settings are you running ?

Front - I think the last setting was 15 minutes total toe in, rears 0.5 deg toe in both sides (1 deg in total) - I might just need to check with my mechanic but think this was what we had.

I checked with my mechanic

Front total toe in 15 minutes
Rear total toe in 45 minutes

[majordad]

Robin as you know I recently started driving my 2.4 S again and am amazed at it's instability with the ducktail removed. On the motorway to join the Wild Atlantic Way at 80mph it was moving all over the place at the back, I felt if was going to crash if I held it there. Maybe it's us and not the cars ?

[210bhp]

Oh no, not these again



Regards
Mike

[MikeB]

Does the lift at the front end, translate into reduced toe in ?

[fetuhoe]

Bumper weights only Increase the Polar Moment of Inertia of the front end and help create understeer. - always a major benefit to turn-in

How much caster do you have and what is the rake angle? I would expect you need around 7 degrees of positive caster and a rake angle of 1.5 degrees nose down.

If you are running Dunlop L or M Section what is your front camber?

Also tyre pressures?

Front end lift isn't a major issue compared to lift at the rear. Ducktails significantly reduce the negative pressure an rear end lift.

[redrobin911]

Thanks to all so far - I will check castor settings and see where they're set - i think there might be something in this, and we're already looking at setting the rake of the car as were fitting an alternative set of dampers (as you know) so will do when they arrive. Bumper weights would assist turn in but just as easy to over fuel the car with an extra 30/40 liters of fuel. The bigger issue is lift at speed esp when approaching a high speed corner - yikes won't turn in. I did notice on the video coverage of the Porsche race at 73rd Goodwood that the cars lacked high speed turn in and also started to drift from the word go - they looked unstable most of the time!

Camber is also very conservatively set from memory less than 1 deg anywhere (more like 0.5 deg) - and tyre pressure on the Avons CR6ZZ at 30 psi +/- 3psi or thereabouts but all equal in pressure.

[fetuhoe]

Robin,

If you increase the Polar Moment around the front axle the car will gain straight line stability and make it less prone to follow camber etc but it won't improve turn in.

The increased weight at the front will make it harder for the car to turn and more likely to understeer.

I would also reduce the front tyre pressures to around 25 psi.

Your LSD won't be helping with high speed turn in and I would think trail braking will help.

[pht9]

the cars are loose - you have to learn to drive it

[neilmcd]

Robin,

I'm not an expert in ride & handling but I doubt your "obviously the lift generated by the (front of) car makes everything go a little light". I don't doubt the front feels light but the feeling might not be coming from where you think...

I once worked with people who knew about this stuff and as I remember it the high speed instability you describe might be due to lift at the rear (makes sense from where I might guess the centre of pressure would be at high speed) and this manifests itself as a light feeling in the steering. We designed a (barely visible) rear spoiler on the top of the tailgate to cure a problem like this on an early 2000's "hot hatch". Didn't do anything for the torque steer, mind you

If I'm right the strategies to fix it are a splitter to prevent flow under the car (but it has to be deep) or a spoiler at the back. Do the rules stop you doing this?

....or you could just get used to driving a "loose" car? (Something I couldn't get the hang of!!) Why do others not suffer? (That's probably your question, too!!)

Probably no help at all.

Neil