Snet Shunt post mortem...

[ss_collins]

Well those of you who follow the thread in other forums will know that I had a very spooky shunt at Snetterton a couple of weeks ago in the vee... an explanation of its cause has yet to be worked out though there are some thoerys.

First what occured...

Coming up to the end of Revett Straight (longest straight in the UK) I started braking for the chicane at the end just before the bridge as I was taking it very easy (track was a bit wet and very greasy) the instant I touched the brake pedal the car veered sharp left and hit the barrier hard...

What caused the shunt...? discounted thoeries - left hand side brake problem (functioning well after the shunt), something in the suspension broke - again nothing apart from a damper was broken even after the shunt (the damper was broken by the impact), a marshal who witnessed the shunt said he thought a half shaft had snapped, he was wrong its still intact. The only tyre that could have punctured is the right rear all the others are still fully inflated - the RR was bashed in the impact so hard to judge.

One possible clue is that the car was wandering loads coming up to that point on previous laps. Also the car was at the fastest speed it had ever achieved.

Ideas? I'm concerned about rebuilding it and the same thing occuring...

[Revracing]

Sam, this may sound daft but it is one thing that should be eliminated, how much if any bump steer have you got?

I only ask because as you said it dived off to one side under braking...

[DaveStyles]

The halfshaft comment made me think... How about a diff problem ?

[Tim Falce]

Do you know if the front veered to the left or the rear stepped out to the right? I doubt it would be a half shaft or diff problem if the front went first.

[ss_collins]

Hard to tell I think the front went but it was very high speed, the back sure came out (but cause or effect) - diff - could be though it is a control part so unlikely (will check it though). Rev not sure - I'll check that - though it hadn't pulled or done similar any of the previous laps.

[StephenRae]

Perhaps you were generating lift...you said it had felt odd the lap before...you braked...front left locked up, then when you slowed a couple of mph and lost the 'lift', it bit...exit stage left.

[chezza]

Thats a possibility Stephen...Sam you did say that you thought it might have been lifting!

As for the diff...even though it is a control part...it doesn't stop it going wrong or something!

[imull]

no bearing failure or any such?

did the brake balance adjustment get altered somehow. I have seen a balance bar gradually screw itself off due to a nut not being properly tightened.....

you say the track was damp/ greasy. Did you touch the white line or get on a spot where one side of the car was damp hence lower braking force available?

[Tim Falce]

Could you have hit some oil as you braked, I have done that under the bridge but luckily the car just under-steered until I lifted off the brakes. Very frightening as the speed down Revitt is sooooooo high.
Other than that could you have been aquaplaning or have got a bit of lift under the car if you were going faster than normal.

[ss_collins]

no bearing failure or similar, brake balance is not adjustable on that car. No miles from any whilte lines or similar, I find the lift thoery the strongest so far... and the most worrying, oil unlikely to make it dive left under 5 - 10% braking (I'm used to running through oil - its vee!)

Generating lift... my helmet certainly was!

[dtype38]

Might be worth a careful think about the handling on the laps before the accident. Had it always felt odd at that point on the track, or only on the previous lap. Something like lift should happen every time, but something getting worse indicates something going wrong with the car. You could have picked up a slow right rear puncture a couple of laps earlier which could be a simple explanation, but you would probably have noticed the car getting a bit squirmy at more than just that place on the track. :-)

[chezza]

When the car came past me for the last time before the accident it didn't sound right to me...something was flapping or ir the engine sounded like it was chugging a bit.

Thats why I didn't worry when he didnt come round again, I thought he had had a problem and gone in the pits.

[Tim Falce]

Out of interest and sorry to go a bit off topic, how hard did you hit the barrier and was it head on or at an angle? Just interested as you are still walking around OK and you don't wear a HANS, do you use a neck support?

[chezza]

I don't know much about the angle at which it hit as I didn't see it. Sam had the sense to spin the car so that the back end of the car hit the barrier, otherwise we would be in a different situation as his legs would have taken all of the force...which was significant, the back of the gear box was wedged in the armco and had been ripped away from the rest of the box.

Sam doesn't wear a HANS or a neck support.

[ss_collins]

I only heard the impact - didn't feel a thing. Was busy trying to spin the car some more.

Dtype - the only thing that was different was that I was gettting hitting speeds that the car had never achieved before. It was wandering very badly every lap from halfway down the straight onwards

[chezza]

No sense no feeling then Sam! You didn't even have any bruising, whiplash or anything!

[dtype38]

Hmmmm... half way down the straight I wouldn't expect you to be going any quicker than at the end of the start/finish straight, or on the main straight of quite a few other tracks. If you'd never gone that fast before you may have been picking up a tail-wind, but then I would have thought that should have reduced "lift". Chezza thought your engine sounded a bit odd... were you getting to "valve bounce" sort of revs? If you were then your engine braking when you got to the end of the straight might have been more than you would usually get, upsetting the brake balance considerably.

[Frog Lover]

I'd check the nut behind the wheel first

Vive La France!

[ss_collins]

Nah valve bounce not really possible in a vee in top - it was wandering on the main straight too but it got really bad approaching the bridge every lap - put it down. reason I know it was faster is that the car has never had the amount of power its got now and it was totally flatters down the back there. Also since it last ran it has had a major rework. So really it was the first time the car had run at high speed for over ten years and back then it was at least 10 bhp down on power (and not as many torques lived in it then either).

[Anuauto]

If it was instant on hitting brakes that has to suggest a sticking right side calliper piston? Or, as has been suggested, a slow deflation in right rear (which would cause right rear to lock on braking, having held up on straight at speed). If you have an entire night free, read the extended debate on Atlas F1 on Jim Clark's accident, which includes the thoughts of well known drivers about how slow punctures first show up. It doesnt take much to make it let go if the track is greasy?

[MikeBz]

Strong gusty crosswinds? There's a refernce to a Formula Renault being caught out by that in a recent Snetterton test here: http://tentenths.com/forum/showthrea...14#post1281314

Mike

[avsfan733]

the wandering leads me towards somethign suspension related as opposed to aero. it would probably be kinda hard now but what is your alignment settings usually at?

I think based on his comments that it would be more of a front end thign as he says the car veered as opposed to the rear end coming round which would usually be a pretty distinctly different phenomenon creating a different style of car movements before the impact

[chezza]

Originally Posted by Frog Lover

I'd check the nut behind the wheel first

Vive La France!

I can assure you that on this occasion "the nut behind the wheel" does not need checking, as strangely enough he was being sensible on this occasion. I think that is a little bit on an inappropiate comment, given the tone and nature of the thread.

Where i heard the car sounding odd was at riches, the first corner, when he wasn't at full revs and something didn't sound right...whether it was the engine, or a deflating tyre flapping or what I do not know...the angle at which I was at meant that I couldn't see it properly.

[Anuauto]

Cancel Frog Lover's Visa.
The debate over Jim Clark seemed to conclude that even he could have been caught out by a deflated tyre holding up on the straight. The correlation with SC first touching the brakes seems important. Was there any real remaining control involved in "attempting to spin" or was the reality being a complete passenger? As said before, it doesn't take much on a wet/greasy surface - even something as simple as marginally more water sprayed on to one brake causing minute difference in bite- and I'm not convinced in such conditions anyone is really capable of analysing which end let go first.

[Eric Falce]

Don't know the car but it would seem that if the car was reaching higher speeds than before, that somewhere you have lost downforce,braking and veering off would also point to this by not having downforce on the front end,sudden braking would bring the nose down and sods law would work as to which front wheel bit first!