Lateral grip when travelling forwards

[Buda]

I am playing with idea of getting some wider wheels - i.e. the nice five spokes from Caterham with not that sticky tyres (I do about 20.000 miles a year and can't afford to replace the sticky ones that regularily) - the question is will wider wheels/tyres prevent the car being deflected by crosswinds to any great extent, this morning It got rather hairy over about 60mph.

[Jam Mad]

err.. i suspect not. that would be down to windage on the side of the vehicle and a number of things like tracking/ camber set up and the steering rack that would affect how easily the car was deflected off any given course.

 

no 7 would ever be pushed sideways by wind to the extent that the wheels were losing traction.

 

i'm sure there'll be someone along soon with technical babble to pad out my 'educated guess' theory..

[Jam Mad]

AAAAAAAAAAAAAAAAAGH !!!!!!!!!!

 

i'm a windbag !!!!!!!!!!!!!!

 

wink.gif

[mav]

Al,

 

I have the 16" wheels. I still get blown around all over the shop, but that may be due to the sv having a larger side profile

[Graham Perry]

If anything I would have thought taller wheels and tyres might have improved things due to gyroscopic effects (I am expecting to be corrected here by PC) . I must admit a cross wind problem is not something I have ever noticed with my car, but then I don't tend to use it at this time of year. Does anyone know if removing the screen would make a difference as obviously not all cross-winds hit the car perpendicular to the direction of travel.

[Buda]

I know that it doesn't do it as much when the hoods down, I havn't taken the screen off to check yet, my thoughts where that as there was a larger contact patch at 90 degrees (assuming the winds at 90 degrees to the direction of travel) then there would be more friction to alleviate the force caused by the wind direction, obviously this would have to be offset against the smaller point load on the contact patch making the car effectivley lighter per area of contact. there is of course the option of adding some big hairy spoilers to increase downforce,, aggggghhh nooooo I'll be buying an evo next.

[Peter Carmichael]

The key issue here is not the amount of grip, but the stability of the dynamic conditions of grip.

 

Grip is developed by slip. The tyre pointing in a different direction to the one in which it is traversing the ground. This is usually a very small angle.

 

When you get hit by a cross wind, the wind's force on the car has an effective centre of pressure. Remember you have to consider the apparent wind, when your forward speed is added into the equation. This centre of pressure pushes the car sideways but it might also try and yaw the car into understeer if it is forward of the centre of gravity or it might try to yaw the car into oversteer if it is behind the centre of gravity. Take my word that 90% of the time the force is forward of the centre of gravity and is trying to make the car understeer.

 

Now if the car is being hit by a crosswind, it is going to have a net force away from the crosswind. What is important is the balance of how the tyre forces build up to resist this force. What you want is for the tyre forces to largely sort it out without a steering input.

 

If the car is pushed sideways and the tyre forces don't balance up to resist the understeer, then when the force is removed, the car is pointing away from its original line - this is plainly not stable. If the sidewind is sustained then a slight understeering attitude presents more cross-sectional area to the apparent wind and the sideways force becomes greater.

 

The more you allow the car to track off line in an understeering manner, the more it will get thrown around by crosswinds.

 

This is similar to turn-in. A sharp steering response when the wheels start to turn is similar to a sharp response when the car is pushed sideways into slip. The classic way to sharpen up the turn-in is to have a smidgen more negative camber at the front.

[Buda]

So thats a no then, I need to play with the front camber. Why can't thing just be simple, how much is a smigen? - roughly?, I'll put money on the answer being something like "It depends on the amount of wind you experience" fnarr fnarr

[Jam Mad]

hooray ! i was hoping pc would get in on this one...

 

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[old captain slow]

I have a set of 5 spokes for sale if you are interested. Proceeds to NTL.

 

C7 CDW

 

 

[James Arnold]

Peter,

I don't understand the

then a slight understeering attitude presents more cross-sectional area to the apparent wind

---

I would have thought this effect would be trivial - as you say, the slip angle would be very small.

I have always assumed that the reason relatively small side winds (when compared to forward apparent wind) have such an effect is due to a low pressure zone down one side of the car. When the resultant apparent wind is not directly head on, the car becomes like a very crude wing on end. The apparent angle of the resultant wind is the angle of attack of the wing. Thus a low pressure zone will appear on one side of the car, which I thought the main contributor to the whole car being deflected. Given the air flow down the side of a caterham must be so turbulent, I would expect a 7 to be more stable than another car. However the lightness of a seven will make it more susceptible.

I remember the original sierras were meant to have poor M'way stability, partly was a result of the wind-cheating jelly-mold shape.

I have never thought this through sufficiently to see if the maths backs it up, does it make sense?

J

 

 

[Peter Carmichael]

That is exactly my view. The Seven is a wing, side on. The wing's coefficient of lift is related to angle of attack. Angle of attack is based on apparent wind. You have gone as far as observing how the forces build up on the car, but not how they get resisted. If the slip angle develops to understeer, no stable condition is reached. The car slips to a greater and greater angle, essentially steering on a curve.

 

The important thing here is that some Sevens get blown about and some don't. I hadn't picked up that the problem being described was particularly encountered with the hood up, but I suspect the front camber is all that needs to be changed to correct this.

 

Remember that changing the camber will change the toe setting and toe should be reset afterwards. I usually reset to 0 degrees and then see how the car tracks on a motorway (no crosswind) before re-considering the toe.

[Buda]

Thanks PC,JA, I have to admit to being slightly less in the dark than I was. Principaly though, (a) Changing the width of the tyres has little or no effect on stability in crosswinds, (b) changing the front camber will provide sharper turn in, this having the effect of reducing understeer caused by crosswinds, or, this enabling one to react quicker and more effectivley to the deflection caused by the wind therefore preventing the understeer in the first place - sort of, I think, maybe, perhaps?

 

The point being I can't use the excuse of "They will stop us being blown around as much and enable us to progress in a faster manner my dear wife" (she being particularily critical of the movement on motorways or the like in the wind) as an excuse to buy some shiney new (second hand) wheels.

 

David W, how much will you be wanting, have you got and pics you can send me re condition etc, assume they come without tyres?

[old captain slow]

I'll try and send you some JPEGS this weekend. No time at the moment.

 

Thought about £250 ????

 

Condition not bad at all. Some early signs of corrosion on the inside rims but nothing to write home about. They are not more than 5 years old, but have been garaged for the last 2 years approx as I went the other route and now run on 13". They have Michelin Pilots 205/45. 4 part worn and one new.

 

David

 

C7 CDW

 

 

[Asklepios]

Buda,

You may find your castor has been set very short and makes the car very twitchy in side winds. This would typically create a sphincter testing moment when passing a bus or artic. on the motorway. It is a matter of taste and individual style of driving,but shimming the lower wishbone forewards will give you more stability in these situations.

[James Arnold]

A few years ago, when I sometimes traveled by two wheels, I was advised on this topic by a Goldwing owner. He advised me the best policy when suffering from side winds was 'nail it mate' and assured me that when he got 'well over the ton', it all got better.

I suspect that with his not insignificant form onboard, his kerb weight may have been similar to a 7, although I have yet to see a caterham with a tow bar....

J

[Tim Sture]

Buda - bear in mind that many people, having originally bought the 15 inchers want to change to 13 inchers. you could speak to Stuart Forshaw (who's a local to you MAD lad) about his opinion on 15 inchers - he had some temporarily last year on his car for one of the Lancs runs and he did not seem entirely keen..... speak to a few people (who know what they're talking about - not me then!) about the effect on the driving experience of 15v13 before you buy