Big twitch accelerating on bumpy dual carriageway

[7oaks]

Hi All

Had a bit of a shock when out in my 420R, accelerating hard in 3rd, in a straight line on an empty dual carriageway. 

Went over some bumps (not big) and the car twitched violently. Don't know if it was the front or the back and it carried on in a straight line, so over in a millisecond but quite unexpected (I'm new to the car).

Just had the car set-up for 50:50 road:track.

Is that normal? Maybe bump steer or LSD?

[aerobod - near CYYC]

If it was a transverse bump or dip that both rear wheels hit at the same time, the deDion rear end can lose grip very quickly. Best to avoid any sudden correction in the steering, throttle or brakes and stay as calm as possible, as the car will spin very rapidly until the rear has gained grip again. Just a bit of a limitation of the deDion setup, but generally not a problem once you know it can happen.

[SM25T]

Easy to find the edge of the road in that situation, especially in the wet. Worth getting bump steer checked and rack height adjusted if necessary. Made my car so much nicer to drive.

[revilla]

Was the road wet? It was pretty wet here yesterday. It can feel pretty twitchy in the wet, I've had the back end step out a bit accelerating in a straight line on a dual carriageway before even without bumps (and mine doesn't have the torque of a 420R).

[Nigel Fox]

I suspect the root cause is the action of the LSD, one wheel losing grip and all the power being fed to the remaining wheel, unsticking that as well.

In my experience this can be dramatically improved by having softly sprung, correctly damped suspension. 

[7oaks]

Thanks all

It wasn't wet but still fairly cold and I had brand new tyres on it, that probably didn't help

[SM25T]

New tyres slippery for 100 miles or so with mould release agents, cold tyres not grippy ... careful with that right foot.

[timb2117]

Totally agree with SM25 - no one tells you that!

We used to do crazy performance stuff on a  large wetted pad (for work believe it or not!) and I lost count of how many people forgot the new tyre = slippery and ended up in the gravel trap, always raised a smile amongst us smart arses.

One solution was to spin the tyres up and abrade the heck out of them for a minute, then they would grip! & Yes we had a horrific tyre replacement bill each year.

[Tony Whitley]

I have to be careful when going out in a track day lunch break to get petrol as driving on normal roads with the dampers stiffened for track use means the car skips off the tarmac at the slightest imperfection.  Also, driving to work on the M3 in light rain one day I accelerated in top gear when the road cleared and became aware that my rear wheels were travelling a little quicker than the fronts! 400 BHP/ton means you have to be careful.

[ScottR400D]

For the time it takes I would check that your LSD is still functioning. 

As I've described elsewhere a massive 'twitch' hitting a bump under acceleration is what lead me to check mine. 

[7oaks]

#10 yes I saw your post and that did make me check the LSD.  Only did the first bit though and took some comfort that turning one wheel does make the other turn in the same direction. Don't have the tools for the torque test but have a mate who owns a garage so will try it soon.

[ScottR400D]

If the wheels aren't counter rotating the LSD is probably functioning as it should, but it's worth a regular check.When one wheel is turned the other should turn the same way. 

[Wrightpayne]

Do you mean counter rotating? This means turning in opposite directions, which is what you'd expect from an open diff.

[ScottR400D]

You're right, edited! 

[rob.mcc45]

I experienced exactly the same issue, which prompted me to check the LSD, it had failed and required a refurb.

 

[Nick T]

Probably not the cause but worth checking the steering rack spacers are the correct way round (they are different lengths) to stop bump steer. When I brought my car they were the wrong way round. I can't remember which it should be and I'm not with the car so can't check, but I'm sure some clever person will let you know. If not I will have a look at the end of the week.

[7oaks]

UPDATE:

Took it out again today, hot and sunny, dry roads and it didn't feel good, skipping all over the road, kept it slow as didn't feel in control.

Checked tyre pressures (new tyres just been fitted) and

FL: 56psi FR: 28psi

BL: 38psi BR: 38 psi

Have put the fronts down to 18 and the backs to 22 and will take it for a drive tomorrow.

 

[revilla]

Crikey, Kwik Fit once ignored my instructions and blew them all of to 30psi and that was awful. Yours must have been dreadful!

[SM25T]

Not just dreadful but life threatening ! Shocking malpractice.

[John Vine]

Re #17:

Good grief!  That's grossly incompetent over-inflation.  The 56psi beggars belief.  Your figures would be bad on a heavy tin-top (my Mondeo estate runs 35 front, 32 rear), but on a 7...!  If it were me, I'd be taking it up with the tyre place.  If they've got a rogue fitter, they need to know.

What tyre make and sizes do you have?  For info, my R400D has 195/50-R15 ZZS at 18psi all round.

JV

[Tazio]

If you are on ZZS tyres. Avon recommend between 18 and 21 for the road, Andy McMillan from McMillan motorsport suggested to me to start at 20psi all round for the road.

[Drumster]

My understanding is that they need to be to inflated to around 30 psi just to get the tyres to bead properly? My garage then asks what pressure I'd like them to be be deflated to.

 

 

 

[ScottR400D]

I'd expect the beads to seat at 30psi or so. If they went much higher without I'd be breaking the bead again and putting more lube on. 

The ZZR/S is a fairly light tyre so it should seat quite easily. 

[ScottR400D]

I'd expect the beads to seat at 30psi or so. If they went much higher without I'd be breaking the bead again and putting more lube on. 

The ZZR/S is a fairly light tyre so it should seat quite easily. 

[7 wonders of the world]

you need about 30 psi to balance them too otherwise the tyre flops around and gives inaccurate readings, once balanced the pressure should be dropped .

Another good reason to always carry a pressure gauge in the car too