23 replies to this topic

[Bruce]

A friend of mine e-mailed me and I'd like to appear knowledgable... This is to say that I have no idea and will rely on you boffins out there for this info...

Thanks - Bruce

[nzkarit]

There wouldn't be that much becasue the cars run about half an inch of the ground.

[Bruce]

True - but I expect that it is a lot when compared to the ground effect cars of the early 80s...

[PDA]

About 50 mm front and maybe twice that at the rear.

[desmo]

According to Gary Anderson of Jaguar in issue #30 (p. 24) of RaceTech, the R1 of last year had 25mm front suspension travel and 80mm rear. I doubt that the other team's figures would be a lot different.

[Bruce]

thanks, guys, I'll pass it on...

[kober]

Originally posted by muppet
Most of the "suspension travel" actually takes place as distortion/compression in the tyre sidewall. Less than an inch anyway.

I've heard about 80% of suspension travel is done by tyres and only 20% by suspension itself.

[Wedge]

Captain Cook kind of picked up on it but how much do you think the difference will be in each car coming from the manufacturing process? Sorry for the poor wording, not thinking straight tonight.

[Ross Stonefeld]

Why such a difference front to rear? (on the jag)

[unrepentant lurker]

More weight in the back plus a whole lot more downforce at speed from the rear wing as opposed to the front wing.

[Ben]

The front suspension should generally be stiffer than the rear so any forces applied to it will cause less deflection.

Captain Cook: When you say shock absorbtion do you mean damping as in shock absorbers or shock absorbtion as in what springs actually do?

Whichever the tyres don't have much damping ability, the knodding of the cars at high speed is apparently just as much to do with undamped tyre oscillation as it does with pitch sensitive aerodynamics. This is a fundamental problem for F1 engineers and Gavin Fisher of Williams has said he would like to see lower profile tyres so that 'more of the dynamics are in the suspension as opposed to the tyre'. I think this was probably what he was referring to.

Ben

[Powersteer]

With most teams using some sort of a third spring set-up..suspension travel is varied depending what the wheel on the other side is doing.

[Ben]

Ditto for an anti-roll bar.

Ben

[Yelnats]

The suspension travel of an F1 car is very much dependant on the tracks with curb-hopping tracks and wet-weather setups using a softer setup and high-speed tracks tending towards the stiffer setups.

The softer setups absorb the bumps over the curbs better and spend less time in the air, always a desirable feature when cornering. Wet setups benifit from softer setups because the car is less twitchy due to lower rates of weight transfer and a slower transient response on corner turn-in.

On highspeed tracks like Hockenhiem the car is set as stiffly as possible to reduce dragging of the FIA Plank on the straight while allowing a reasonably low ride height while cornering. This can result in very twitchy handling on high speed corners like the old Tamburella where Senna met his end.

In the heyday of downforce like that generated by the Famous Brabham fan car, high speed corners didn't exist as far as loss of traction was concerned but G forces exceering 4Gs placed heavy demands on nerve, necks and stamina though fortunatly such violent manuvers only lasted for the second or two required to traverse the bend. Movable skirts allowed much greater suspension travels in those days without chassis dragging and one only had to hear comments about the FAN car sucking down on it's suspension while stationary in the pits to realise that development in that direction had to be curtailed before drivers were to require G suits and neck braces on fast tracks!

In summary the suspension travel could vary tremendously and is a closly guarded secret but appears vary to from a low of 25mm on Hockenhiem to 50 mm on a wet day at Monaco on the front with double the values at the rear. The rear of the car is setup higher insure reliable downforce as a positive (nose-up) attitude for an instant will ensure a loss of control in a corner and possibly on a straight when slipstreaming and on crests.

The tires have a considerable effect on suspension travel and some clever chap could calculate the tire deflection assuming 300 sq inches would be required to support a F1 car (1300 lb * 3Gs @ 13 psi) on the straight and somewhat less than 1/3 this at rest. Curbs are another matter and a lot of displacement takes place in the bending of things that shouldn't!

[Ben]

I'll have a stab at this.

I have some reliable data on tyre radial spring rate (Peter Wright, Racecar Engineering V7N3) giving it as 226N/mm.

I have a less reliable but plausible value for wheel rate (Mike McDermott, Race Tech article on the Arrows A22) giving it as 175N/mm or 1000lb/inch in silly units.

If we want to know the relative deflections of a system with these two springs in series we must look at compliances (inverse of spring rate).

The compliances are as follows: Tyre = 0.00442mm/N
Spring = 0.00571mm/N
Total = 0.01013mm/N

If we take percentages we get 56% deflection in the spring and 44% in the tyre.

Although I reckon McDermott's estimate for the wheel rate is a touch low, the deflection in the tyre is not even close to the 80% some people on this thread are talking about.

Nearer to 50:50 as my analysis suggests still explains Gavin Fisher's comments that the tyre plays too great a roll in suspension dynamics but also indicates that the suspension is also crucial.

Ben

[Yelnats]

Ben, the ratio of displacements (spring vs tire) you have quoted sound resonable but there is no indication of what track these were setup for. There are wide variations between setups for fast/slow/curb hopping tracks and the upper limit of spring rates on high speed track must be vastly stiffer to resist downforce when compared with a slow curb hopping track like Monaco. I could see 80% of the displacement taking place in the tire in some radical setups for tracks like Spa or Hockenhiem whereas the reverse being the case at Monaco.

[PAD]

Why are the car manufacturers (McLaren, Minardi etc) happy with 50%-80% of the suspension travel being "out of their hands". I would have thought that they would prefer to have either more solid walls on the tyres, or more likely lower profile tyres, and control the suspension through the shocks/springs? i.e. under their control rather than the tyre manufacturers.

I would have thought that lower profile tyres would reduce the unsprung weight as the weight of the larger wheel rim would be less than the rubber lost from the tyre.

[Marco94]

The FIA rules mandate the small rims as they are used these days. If it were possible, the teams and tire manufacturers would change it to a proper combination in an instant.

[Ben]

Right, I've had another think.

Based on the tyre data given in the previous post, to get 80% deflection in the tyre would require a wheelrate of 904N/mm.

Assuming this and a front sprung corner mass of 114.75Kg (600Kg all up mass, 45% front weight bias and 15% unsprung mass) the natural frequency is 14Hz (33 Hz for wheel hop).

Race Car Vehicle Dynamics suggests that aero race cars have sprung mass natural frequencies of between 3 and 5Hz. McDermott's estimate of wheel rate gives a frequency of 6.2Hz so actually that could be an upper limit.

I accept that different set ups are needed but I'm getting more and more convinced that 80% deflection in the tyre is too much.

Ben

[Ursus]

FWIW, in the May 2000 edition of F1Racing Mike Gascoyne says that 30-50% of total suspension travel.

[Ben]

I assume you mean the tyre is responsible for between 30 and 50% of total travel?

[Ursus]

Yes, that's what I meant.

[GSXRTURBO1]

Ok, the responses prompt me to ask this question: do different tire compounds also affect the way a tire "aids" in suspension duties?; or is sidewall construction the same regardless of compound?

[Lephturn]

Given that the car designers would have to design the chassy around the tire's properties to some degree, I would imagine that only the contact layer compound would change. You wouldn't want one with a varying sidewall stiffness and/or springrate, it would be a nightmare to setup for.

The tire manufacturers would tend towards consistency, and I'm sure the teams would demand it in this regard.