Handful When Wet

[Ian Anderson]

I took the car out for a run today

Dax had an open day and so we went back to her birthplace even although some rain had been predicted.

Well the day was not too bad and I left for home after causing a stir amounst the Seven and Cobra crowd as most were newbies and did not know Dax had made a GT40 in the past.

Anyway on the way home the heavens opened - even a bit of hail so not really GT40 weather but as I was on the motorway not much I could do!

I was in the "slow" lane at about 60mph and she swapped lanes and ended up in the central lane. I dragged her back to the left lane and she wagged her tail a few times through surface water puddles.

At 50-55mph she would walk left and right across the whole lane as the water depth varied.

At 45mph she was controlable so long as I stayed out of the "tramlines" made by the heavy trucks. So slow progress until we cleared the storm.

Now the tyres re not brilliant wet tyres - BF Goodrich 205 front and 255 rear so not major width either which should assist in the wet. Tread depth is good on all tyres (6-8 mm)

But to move so much seems excessive. I have aquaplaned a car before (all 4 wheens) but this feels more like keeping traction on one and aquaplaning the other.

So where to start looking? And how to measure what is not right?

I have thought for a while I have had some bump steer effect on the rear of the car as when going thrugh a sweep (like motorway junction) turn into the corner and then I feel the back "settle" and require me to add some more steering input to keep the desired line.

I am not running any anti roll bars
the diff is open (not LSD) but said to be a 4 spider unit if that makes any difference
The rear end is pretty well all rose jointed (Rubber on front of radius arms and inside of rear suspension mount points)
Front has no adjustment other than Toe
Shocks are SPAX all around - not sure if adjustable - how do I tell?
Car is not lightweight 1150 kg at SVA(neither am I)
Car is a road car with occasional fast blasts. (Too noisy for most UK tracks)

What more input can I give to be given some info on where to start looking?

Ian

 

[Jac Mac]

Ian , when it swapped lanes etc , did it 'tug' at the steering wheel at the same time, or was there no extra loads thru the wheel?

 

[David Morton]

Ian,
The tramlines are there in dry weather on Trmac motorawys as well. They are a handfull in any event in any car and much worse in the wet and will cause your car to attempt to steer toward and out of the nearest one. i.e. if you are close to the left hand side of the tramline it will grab you and move you furthr to the left. Germany and Italy are worse than the UK.
You may also be suffering from Geometry and your car may not be that accurately set up which will accentuate lack of control if you are experiencing any aquaplaning.
The onset of Aquaplaning is 9√Tyre Pressure in PSI, and expressed in MPH.
The speeds you mention seem quite close.
Just a thought as I suffer from these truck tramlines as well, and they really do keep you wide awake.
Dave M

 

[David Briggs]

Sounds like your set up is not good, you are not going fast and bump steer probably not an issue here.(not much suspension travel at low speed on a major road, is my guess)
You want a softer set up with higher tire pressure for the wet, you don't have any sway bars so they aren't a factor, if you had bars, set soft.
I would double check the rear toe, should be toed in about 1/16-1/8" inch total, tires pretty much straight up. 0-1/4 negative camber.
Front pretty similar maybe a little more camber, ride height 1/2-1" lower in front.
What happens under braking?
Under power?
Have you weighed the car? All 4 corners?
Dave

 

[Jim Cowden]

I dont think it will be bump steer.
Not at that speed,the suspension is not moving around enough.
I have seen your video of the hill climb it seems to handle ok.
It sounds like plain old aquaplaning to me.
If it behaves in the dry the only thing different is the water on the road.
You are on a wideish tyre on the rear its getting on top of the water and thats it your barefooting.
Run more air in the wet it will ride in the centre and help push the water out.
Worth a try ,dont kill yourself.
Buy better wet weather tyres, it rains there all the time anyway its not a bad idea.

Jim

 

[Ian Anderson]

Thanks Guys

Mhen I started out it was dry and he sun was adtually shining! I thought I may get some drizzle but this storm was unexpected and hard.
So didn't do any tyre pressure changes - set at 26 rear and 23 front
The engine is a Rover Ali unit so perhaps the 26 is too high compared to the Ford Iron units


Jac
There was some pull at the wheel but not the sort of force I would have needed to input to get the car to do the same manoevers.

Dave M
I've had the tramlines tug at the car before but never to any major effect - she is stable in the dry on the motorways - except once in a sweep (motorway exit) where she starts and then need a bit more extra steering input once the tyres load up.

Dave
Car is set nose down - about 3/4 inch
Under hard acceleration fairly straight until the tyres let go and the lacl of LSD causes sideways moments!
Under really hard brakes from 140mph on Le Mans lap she did squiril a little but that was the right rear locking slightly earlier than the rest - lift of brkes and reapply and all straight line
Never needed to do major braking as it is a road car and not pushed hard

JimC
Yes aquaplaning does sound possible but the rapid direction change does not sound right - aquaplane normally is plough straight on as there is no grip. Again I'd not normally drive in bad weather so not had the need to play with tyre pressures - storm came on so rapid I had not even made it to the next exit ramp from the motorway.
New tryes are on the cards probably the Kumho ones as I cannot justify £300 per corner against £300 for the set for Avons

Reason I was thinking bump is due to the "second steer" needed in the dry - possibly puddle moving suspension enough to cause the bump steer input
Or is rear bump not normally a problem?

Ian

 

[Cliff Beer]

Ian, the wet/puddles will act to dramatically accentuate any slop that may happen to be in the steering - this is because you're likely hitting different water depths on L v. R at different points in time and the roadway surface. I'd reco an initial check of the rack mounting and then ball joints and steering arms for any movement/slop. Next, I'd look at the suspension arm bushings - rose joints can have wear/play just like rubber bushings.

Assuming you don't have excessive negative camber for the street (>1degree) you car shouldn't pull like that. The BFG's aren't great in the wet (or the dry) but I don't think it's your tires provided you have 6-8mm treat depth and even wear. I have even wider BFGs on my gt40 and they're quite fine in the wet as far as resistence to aquaplaning.

A very careful inspection just might reveal something.

 

[Tony Hunt]

Ian,
I agree that aquaplaning usually manifests itself as ploughing straight on under braking, but you say that the water depth was variable, depending where you were in the tramlines.
I believe you had a bit of wheelspin on the 'deeper' side. Loss of traction on that side will then cause yaw towards the spinning wheel.
I'm not talking wild wheelspin here, a little is enough when the front wheel grip is pretty marginal too. I have had the back step out on a wet bend when I thought I had very little throttle on.
Tony

 

[Frank Catt]

Ian, on this type of ( replica ) built cars it comes down to looking very closely at the front suspension design, particularly the relationship between KPI and caster, . It is very easy to build up a car with individual bits that are not compatable, and the easiest mistake of all is sometime in the selection of wheels and offsets that are not considered in relation to the rest of the suspension as a whole. I have seen, and driven, cars that act as your description in the wet, and it usually comes down to some error in the selection and design of parts.

 

[Jac Mac]

Ian, my thoughts are that when the car gets into the tramlines with either front wheel the existing front camber setting is such that the effective tyre contact patch swaps from the inner half to outer on one side ( the one in the tramline ), as the side not in the tramline is now slightly higher( and presumably not in water as deep ) that the car adopts a temporary status where the effective scrub radius is increased on the low ( tramline) side while the high side remains the same or decreases slightly. The water just adds to the effect.
As Cliff & frank have pointed out its the combo of parts that may be the major cause, assuming that you find no major play or faults in the system a bit more negative camber might help, but this might be detrimental to tyre wear & braking. If you have enough outboard offset in your front tyre/rim combo then the change in effective contact point when the outer tyre shoulder rides on the tramline could cause a rapid direction change.
I find a lot of this in road cars where wider tyres/rims are fitted at front end and old positive camber settings retained.

 

[Mike Pass]

Can you borrow a set of different tyres to try? The BFGs are very hard/crap. If the hard tyre forces itself through the water to the road it will still effectively not be pressing with the full weight of the car and this combined with the hard rubber not deforming to the surface contours will dramatically reduce grip. This is why very soft tyres can work well in the rain as the soft rubber can still deform to the road surface even when cold and hence still give grip in the wet.
Try a set of Avons. The historic racing GT40s use them as wets on the track. They do not have a particularly good water clearing tread but are very soft. I would also get a four wheel alignment set up done as well.
As to cost of tyres - they are the only bit that touches the road and everthing else works through them and I am of the opinion that you buy the best and change them when the tread gets too low. Your peace of mind and safety (andfun) is worth spending money on especially compared to the cost of your car and what it would cost to repair; not to mention repairing yourself!
Any tyre will aquaplane if the speed is too great relative to water depth/speed/tyre tread clearing capability. The best answer for aquaplaning is to go to bigger wheels with the best wet tyres you can buy, but they don't look period.
Cheers
Mike

 

[Julian West]

Ian,

Just my 2c worth, but those BFG tyres are a really old & outdated tyre. they won't perform anything like as well as a modern tyre like the Avon.

I agree with what has been said here regarding your geometry, setup etc which could well be a contributing factor. A *better* tyre won't cure your geo problems but it will make a big improvement to the way the car drives, stops, etc.

Re tyres it may be worth considering this Car Bibles : The Wheel and Tire Bible Page 1 of 2.

Your rear end should be fairly easy to set up as you say it is rose jointed. Setting the bump steer isn't that hard to do on either end of the car & has been covered in detail elsewhere on the forum. as long as you get a little toe-in in bump you'll be on the safe side.

Appreciate what you say re the costs of the tyres, but I genuinely believe a decent set of tyres will make an improvement to any car.

Hope this helps & good luck getting it sorted, with the state of the British weather it's well worth having a car that handles well in the rain!!!!!

Regards,

 

[Mike]

Check your toe settings. If you are toed out in either the front or the rear, it will cause the car to wander. As far as 60 MPH in the rain?! That is DEFINITELY fast enough given the 1) size tire you are running, and 2) the weight of the vahicle to hydorplane. Especially since the weight distribution is some what light in the front compared to most cars. I have hydroplaned a semi at 60 when there was enough rain. It's really a function of how much water there is, the weight of the vehicle, tread pattern of the tires, and how fast you are going.