Massive instability under braking & spin. Oh dear.

[jonrb 0]

Had a bit of an upset in the Corrado on Friday. I was overtaking someone on a two-lane dual carriageway on a slight right-hander in dry sunny conditions and he wandered into my lane and caused me to brake very heavily. The back snapped out to the right and I span about 200 degrees ending up off the road on the nearside. It was a miracle that I didn’t hit anything and ended up in a small lay-by rather than the tree-lined ditch. :shock:

 

Anyway, this isn’t a sympathy thread and I’m not really interested in discussing the driving aspect of it. What I want to know is why the back snapped out so suddenly under heavy braking.

 

The car has Koni Sport top-adjustable oil-filled dampers and H&R sports springs. Both were fitted in December 2000 and have since done about 48k miles. The rear axle bushes, bump stops and top mounts were replaced in May, less than 1k miles ago. The front discs and pads were replaced at the same time. I checked the tyres pressures 2 days before the incident.

 

The car is off the road at the moment as I did suffer slight damage – a split fuel hose that I currently have clamped off either side of the damage. I have a mate coming sometime to help me replace it then I can drive it somewhere to get the car inspected.

 

So what caused the massive instability in braking? :scratch:

[biggerbigneil 0]

Heavy braking on a corner (even if slight) could make that happen, especially if a negative camber road or dodgy surface.

 

If it is more than that might be worth having a full laser alignment and the brake servo/bias checked. Maybe a rear caliper locked up? The original rear calipers will be very corroded if original as I found out last year and changed to MK4 rears

[dinkus 10]

Blimey, lucky escape! The cause of the massive instability was fact that you were cornering at the time...

 

In essence, your tyres can either turn, accelerate or brake, but not all 3 at once. So if you jammed on the brakes, all of your tyre grip is used up with stopping and not enough of it is used up with turning. (Have a google for 'traction circle' if you want to get into the physics).

 

On top of that, when you brake very hard, nearly all of the car's weight moves over the front wheels, which lightens the back up.

 

The Corrado (especially the VR6) chassis is set up to give you good turn-in, which is why it handles so well, but this also means that when you over-cook it, the back is liable to try and find the nearest ditch/tree/pylon...

[Jim 1]

Jesus.. glad you're ok, and that the Corrado escaped relatively unscathed.. that must have been a brown trowser moment and a half :|

[jonrb 0]

I understand the physics of the situation (I actually have a degree in Physics), but what confuses me is why it snapped out to the right on a right-hander.

 

Let's see, the n/s/f would become loaded and the o/s/r would become unloaded. The n/s/r would have more loading than the o/s/r but not as much as the n/s/f. At this point my brain is starting to shut down and I can't work out in which direction you would then expect a resulting loss of traction and spin. Can someone help me out?

 

Incidentally, it was an easy right-hander. It's not like the tyres were really loaded up. Speed was 70mph or thereabouts - nothing hugely excessive.

[dinkus 10]

If you brake hard mid-corner, then you'll spin in the same direction that you were turning, because you effectively just hugely increase your steering input, so it sounds right.

 

You don't have to be going that fast to do it either - I managed to spin my old Corsa years ago by stamping on the brakes mid-corner and I was only doing 40...

[jonrb 0]

If you brake hard mid-corner, then you'll spin in the same direction that you were turning, because you effectively just hugely increase your steering input, so it sounds right.

Exactly. So on a right-hander you would expect oversteer and clockwise spin (ie. the n/s/r steps out). The opposite happened to me - the o/s/r stepped out and I spun anticlockwise.

[biggerbigneil 0]

If it's a right hander, the rear would have snapped out to the left.

My money is on your brake setup and possibly your rear calipers/disks/pads

If it was a front braking issue one wheel would have locked up for a second or so.

[dinkus 10]

Hmm, tres strange then. In that case, it would appear to be a manky rear calliper...

[chrishill 0]

just trying to get my head around this one, you were on a right hander and the back spun anticlockwise when you braked? are you sure you didnt turn the wheel at the same time? I just cant figure out how it spun in the oposite directon to where the load would be placed! in my mind, when turning right it'd be the offside that'd be under load and the near side that'd be light, so for the car to side to the near side the load must have shifted. did you lock up then spin or anything like that?

[jonrb 0]

I'm pretty sure I locked up. I don't recall any ABS pulsing. I do have the famous intermittent ABS warning light issue due to a faulty sensor at the moment, so it could have confused the ABS system I suppose.

[Supercharged 2]

Was there anything on the road John, diesel spillage???

 

..or strange road camber of surface?

[Roger Blassberg 0]

I agree with jonrb that it is unexpected to oversteer so dramatically. You would expect the o/s rear wheel to wave about in the air and the n/s rear to load up, but not as much as the n/s front, and for the ABS to mitigate the tendency to lock either front or rear, with possible additional regulation of the rear braking from the valve. In general I would expect all this to lead to understeer.

 

Do you have standard ARB set up? A softer front/harder rear may promote some oversteer.

 

When you had the rear axle bushes done, perhaps the support brackets were not correctly oriented to give the passive steering (clutching at straws here!!).

 

Maybe you were just unlucky enough to hit a patch of substandard surface (there's a lot of it about...), a manhole cover, a white line or suchlike.

 

Anyway, I'm pleased to hear that you (primarily), and your car survived. Many years ago I was struck from the side in a Mk2 GTi; I did a complete 360 deg spin across the M3 at a rather greater speed than you did in your "moment", and ended up on the hard sholder, facing forwards, 6" from and parallel to the kerb, just as though I had been in control (which I hadn't).

There is someone up there and He seems to be a VW driver; I feel sure that Dom/Dinkus would agree.

 

Best wishes

 

RB

[sciroccotune 0]

Sounds just like what (nearly) happened to me before I fixed my abs... One of the rear callipers locked way before the rest of the wheels and caused my car to be pulled in its direction (was the opposite directions to everything you have said) Once abs was fixed I no longer notice any problems but I have since had a minor clean of the rears and will be fully taking apart and inspecting them next pad change.

 

Or

 

As pointed out road contamination or odd camber. There is a bend round my way (coming out of Jesmond on to the central motorway) that has tried to kill me in both VW coupes the Scirocco was the worst as it just did not suit the camber but the rado is similar…

[dinkus 10]

Any kind of loss of traction (both rear wheels locking up, diesel on the road) would have spun the car in the direction he was turning.

 

If the off-side brake calliper doesn't work then you'll have locked up the near-side wheel and spun the car round in the opposite direction.

[chrishill 0]

There is a bend round my way (coming out of Jesmond on to the central motorway) that has tried to kill me in both VW coupes

 

that wouldnt be the nasty left turn onto the duel carragway towards the tyne bridge would it? if so check the 2nd rail up on the outside of the bend, you should see a stripe of red paint about 4ft long (probably more pink than red now) that was me about 2 years ago!

 

spanked astra

[dinkus 10]

Ouch :?

[Claret Badger 0]

camber

manhole cover

white lines

 

the brakes on the back hardly ever lock up in my experience - if your fronts locked up - physics dictate on a right hander the back would have shifted to your left. If however you weren't turning that much and you "were" going some - you open another debate about the state of your brake setup at the front.

 

 

hth

 

imo

[logicaltuning 0]

weight comes off the back under heavy braking, back end loses traction it previously had - wuhay - car goes sideways its all about weight shift

[dinkus 10]

weight comes off the back under heavy braking, back end loses traction it previously had - wuhay - car goes sideways its all about weight shift

 

Aye, but if the car was braking normally, then it would have spun the other way, so something else must be at work...

[sciroccotune 0]

Chris that’s the one! Nearly had my rocco, ended up letting go of the wheel (last hope) and thankfully it just about ended up going straight!!

 

As for the spinning incident its well worth having all things geometry checked after such an event

[jonrb 0]

Yes, as soon as the fuel line is replaced and the car can be driven again, I'll have a fuel geometry check and have the brakes looked at too.

 

I still can't work out how this happened, to be honest. I really wasn't pressing on; 0.80 leptars at the very most, good surface, dry conditions.

 

When the car is reversed at manoeuvring speeds I do sometimes get a slight groan which could possibly be a binding rear calliper or a wheel bearing, but this is clutching at straws really.

[Roger Blassberg 0]

0.80 leptars

 

you what??

 

Best wishes

RB

[bcstudent 0]

Does the brake bias valve operate correctly on a lowered car?

[Roger Blassberg 0]

Does the brake bias valve operate correctly on a lowered car?

 

Now that makes me think further. If the car was turning right, the rear left would be compressed (effectively lowered);That might have affected the brake bias valve into allowing more braking to the rear, perhaps overbraking that wheel and causing a rear slide.

 

Best wishes

 

RB