The science behind wheel sizes

[dtype38]

HiRich, Dennis. OK, seems we have a different understanding of what's going on here, so let me run the saw tooth kerb thing past you with my own version of whats happening with the springs and dampers....

For what its worth, my theory goes.... there are basically only two things that limit how fast a wheel can move up and down: 1. its own mass (inertia) ie the heavier it is the slower it will accelerate for the same input force (say a 1" kerb hit at 80mph), and 2. the damper setting (which restricts the oil transfer rate arond the internal piston and hence limits travel speed).

As for the spring.. I honestly can't see any reason why the spring rate would affect the "speed" that the wheel can move in order to follow the contour of the road (pothole or kerb). Surely the speed a bare spring can be compressed is only limited by how quickly the force can be applied?? So wouldn't it be the case that the more powerful the spring, the faster a given weight of wheel will be pushed (accelerated) down into a pothole (I making the gross assumption that at speed the car is effectively a solid platform to push against compared to the weight of the wheel).

So.... if its only the dampers and the weight of the wheel that stops the springs moving up and down fast enough the follow every ripple and bump in the road, then the dampers should be set as light as possible to get maximum grip!

HOWEVER, I'm quite aware that, completely undamped, the spring would tend to rebound and cause the car to jump about, so they do need to be damped, but only to control the spring, NOT as a substitute for the spring doing the supporting in the car. Heavy damping may keep the car flat through a chicane, but only by making the outside suspension effectively "rock hard" and unable to compress the springs. That only leaves the deformation of the tyre walls as the means to ride irregularities in the track. Similarly, in a long fast turn, the dampers will "eventually" allow the body to roll, but then if an anti-roll bar is used to control body roll, its only doing it by effectively stiffening the outer springs, so why not just fit stiffer springs in the first place. Ie if the car can be sprung that stiff half way round a bend and still handle ok, then why can't it in a straight line.

Personally I prefer the idea of having the spring rates on my wheels as constant as possible, giving the tyres an equal chance to stay attached to the road regardless of whether I'm braking, cornering, or flicking through a chicane.....

But like I say, its all only how it works in my head, and maybe that has more to do with my improved lap times than anything to do with physics... who knows!

[HiRich]

On your theory, you must take into account the spring rate. This is why it is important to remember that the input from a bump or hole is a displacement, not a force (whereas cornering is a load transfer/force input). The effects are different.

So you hit this 1" bump. The input to your suspension is a 1" displacement (the bump isn't going anywhere). Let's assume you have a solid tyre for now. So your hub moves up 1". With your 900lb/inch springs, that wheel instantaneously carries an extra 900lb. Firstly, that's 900lb off the other three, particularly the opposing diagonal. So instantaneously, the load and grip on the other three tyres has altered suddenly. Not only are they suddenly working outside their optimum, but the balance of the loads has moved dramatically. The car's going to react, squirm in the horizontal plane, possibly bite you. Not a good thing.
Secondly, you've just put an extra 900lb into one corner of the car body. It's going to buck around vertically.
Thirdly, let's now recognise that your tyre isn't solid, and has a stiffness of 900lb (so your total stiffness is 450lb/inch). The spring may only compress half as much (which I accept reduces the effect of issues 1 & 2, but they still remain singificant), but you have just crushed your outside front tyre by 1/2". Without going into the details, I'm sure you accept that that is not a good thing for either the immediate grip available, or for maintaining performance through the rest of the corner and the rest of the lap).
Fourthly, with both spring and especially tyre compressed, and the car now on the bump, you've got to release that energy back somewhere (the tyre will have to recover its shape, the spring will have to move back to its normal load position), to return to stability. That's a lot of energy to deal with. This is where your hub is now caught between tyre and spring and rattles around, again varying the contact load (an exaggeration I know, but it is happening).

Conversely, with a much softer spring (100lb/inch), the load changes due to the input displacement become almost insignificant (the load transfer around the other corners is dramatically less, the chassis has barely reacted, your tyre has only been crushed a little bit). The rate of recovery to a stable situation may be slower because of the softer spring, but because the start deviation is so much lower, the odds are that recovery will take less time.

In rebound into a pothole dip (like the Bombhole), you are correct that the stiffer spring will make the wheel react quicker, forcing it to track the road surface more effectively. However, since you are really only trying to move the wheel/hub assembly here, there is a lot less mass involved. The performance weakness of the soft spring is limited compared to the benefit when reacting to bump.
Most importantly, the pothole isn't just a rebound dip - there's normally a return bump on the other side. Your stiff pring which reacted so quickly and faithfully to the dip now has to react dramatically to this bump. The soft spring almost ignored the dip, so barely noticed the return. Half a second down the road, the soft spring (and the car) has forgotten it. The hard spring is still bucking about, and you haven't got the wheel straight, right foot hard down. You're slower.

That's the theory, and I hope I've explained it clearly. On a mirror-finish surface, hard springs can/should work. In real life, random dips and bumps can unsettle the car - despite/because of thrashing the wheel about in reaction, you can't hold the car at its optimum. In an imperfect world, predictability can be much more valuable than a larger but unachievable performance window.
Or as a practical example, you pitch your 2CV into a gentle slide, and you can hold the wheel steady with one hand as you drift through the Bombhole, clip the inner kerb, take the rise, ride the exit kerb. In your rigid kart, you're sawing away like an amphetamine-fuelled lumberjack.

Finally, the caveat that every car, circuit and driver are different. Without detailed measurement and analysis of your car's geometry & dynamics, and a couple of days testing with you, we can't tell you what will work for you. Denis and I can only suggest that perhaps you are relying too much on your springs and not exploiting your dampers.

And a final, final point. If you haven't got venturis that need a constant gap to the ground, is roll such a bad thing? Sure, if your geometry is lousy, the camber change (true camber, to ground) can waste your tyres' potential. But roll itself is not the evil - you are controlling roll to manage another problem (normally geometry). Why not eliminate that problem at source?

[greenamex2]

This was my input but HiRich's explanation is a heck of a lot better so listen to him (especially agree about the final comment on roll, subject to your suspension geometry being any good) -

Interesting one, just how much time DO you spend kerb hopping!!!

I suppose the answer is hidden in damper technology. All 'proper' dampers have different low and high speed damping although only the high speed can be adjusted on the cheaper ones. Essentially low for body roll and high for bumps. And even more expensive dampers have a 'pressure blow off' facility just for the purpose of kerb hopping.

Kerbs aside, on a race track you will rarely if ever hit a real 'bump' so the saw tooth scenario never arises, all you are concerned about is the relatively slow effect of cornering, acceleration and braking. As a damper is a velocity sensitive device it isn't simply of a case of if it is hard it will take longer for the spring and hence wheel to return to the ground.

As for what happens when you hit a bump. Not totally sure but in this instance I would guess that the damping had to be correct for the bump, the spring rate and the weight of the suspension otherwise it will 'bounce' or drop too slowly. Given that most kerbs would be on the lightly loaded inside, just how important is this to the typical club driver? I would also guess that the harder the spring the less it is going to compress, leaving some otherwise absorbed energy to be disapated either by the tyre compressing more or by the car rolling.

One point, the weight of the wheel will have neglible effect on the wheel moving up, it is just too light compared to the car, it is going to move no matter what. What it does effect is the mass that has to be controlled by the spring and damper. Hence the heavier the unsprung weight the heavier the spring and the stiffer the damper.

Just as there are many ways to skin a cat (apparently, I prefer them with the skin on, roasted) there are many ways to control both body roll and reactions to bumps. What you use will depend on many factors including budget, tyres, weight, driving style, road surface etc etc etc. The only thing that matters in the end is lap times, and for all the theory and physics this can only be tested on a real circuit. This is where easily adjustable dampers score over slow to replace springs.

[Tim Falce]

Dennis, Hirich, dtype, what the hell are you all going on about and can you keep your posts to under three lines please:confused:

[dtype38]

Firstly, Falcemob.... off unless you have something to contribute

Next, HiRich And Dennis... I hear ya.. just don't agree. But here's one or two points to think about :

HiRichc says : "So you hit this 1" bump. The input to your suspension is a 1" displacement (the bump isn't going anywhere). Let's assume you have a solid tyre for now. So your hub moves up 1". With your 900lb/inch springs, that wheel instantaneously carries an extra 900lb. Firstly, that's 900lb off the other three, particularly the opposing diagonal. So instantaneously, the load and grip on the other three tyres has altered suddenly. "

Not true I'm afraid, The 1" displacement to one front does indeed add 900lb to that wheel, but that has very little insantaneous effect on the other wheels. If you take into account the mass and inertia of the car, that corner actually gets instantaneously heavier by 900lb!! (well almost). That's because making the road come up and inch is virtually exactly the same effect as dropping the car on just that corner. It instantaneously sees a higher local G force and gets effectively heavier in reaction. OK.. needs bit of mulling over because we can't both be right.

Denis says : "Given that most kerbs would be on the lightly loaded inside, just how important is this to the typical club driver?"

Well I just raced on the Brands Indy and that kerb on the outside of the exit of Graham Hill is a prime example of being able to open out a corner a bit more if you can ride kerbs without loosing grip. The exit of the Donnington National chicane.. and come to think of it, Redgate and Old Hairpin, both reward a bit of kerb use in the momentum stakes, oh and that outside kerb on the exit of Honda at Pembry is a bit of a tyre chewer. Ok, ok, so I shouldn't be using kerbs as a legitimate part of the track.... but tell me you don't and I'll feel suitably chastised!

Incidentally, since I've stiffened my springs and softened my dampers, running saw tooth kerbs doesn't rattle my teeth any more But more seriously, I think of bumps in the track as being the seams and changes of layer in the tarmac.... not whole corners like the Bombhole at Snet..... now that drain cover that I use as the apex, I like to try and get some grip as I rumble over cast iron

[Tim Falce]

Originally posted by dtype38
Firstly, Falcemob.... off unless you have something to contribute

ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ

[dtype38]

Well...quite! Think that proves my point entirely

[StephenRae]

....surely when confronting this mythical 1" bump it is not the affect it has on the wheel movement but the speed and ammount of force that is transmitted to the rest of the car that matters. If the spring rate is low enough to absorbe all of the force only then can the car can be treated as a solid platform.

[dtype38]

If the damper is set too hard, it doesn't matter what the spring rate is, the impact of the bump will all be transmitted directly to the chassis via the damper itself.

[StephenRae]

Thats true, but isn't that why the shocker rebound is considerably stiffer than the bump...to arrest the bouncing of a softish spring?

[HiRich]

dtype,
A good argument, and there's just two things wrong with it - I'm right and you're wrong!

Seriously, though. I tried to simplify what happens. In reality it's a combination of things. The body is accelerated upwards and will take time to come back down and settle. Loads on all four wheels change. I think we actually agree on the basic mechanics. So think about how that effects the grip at each corner (both the total available, and the balance between corners) and therefore how that affects what the car wants to do.

As an aside, it does also aply to bumps & kerbs on the inside. Try attacking that nipple on the apex of the right hand part of the Cadwell Mountain as you squeeze the throttle. Actually, don't, as you'll be shedding body parts all the way up to the cafe, and the applause from the crowd there isn't really worth it...

[Tim Falce]

So, after a long slumber I am still none the wiser as to what you are all going on about. Can you please put it into laymans terms as to the best way to go. My car still rolls like a pig and I have upped the spring rates to 900lb on a car that weighs 1040kg running yoko 032r 225.70X15 tyres

[Red Dog]

dtype 38 - some thoughts based on my own racing experience...

I tend to agree with your view on spring, damper, roll bar stiffness. I started with soft springs, soft bar, soft single adjustable dampers (all this is relative, they would not have seemed soft in a road car) the car had lots of feel and was easy to get used to and catch, which helped to build my confidence so that I could attack the corners.

After a season (no testing) I changed the spring rates all round - up by 12.5% front and 16% rear and disconnected the front arb (no rarb) I left the dampers as they were. Car was much quicker to react/harder to catch but lap times were instantly better. A few races and it didn't feel twitchy any more so I connected/stiffened the bar. Gave me a bit of understeer on quick corners but got pole and fastest lap at Lydden and damn nearly fastest lap at Cadwell Full. I am not a very experienced driver and I definitely put it down to the car. I don't have more grunt than the others and tyres are control Yokohama slicks on 13" x 8" rims.

I have been given some weird advice such as "set your dampers so that when you push down on the car it takes about half a minute for the car to return to normal ride height???" - I have ignored this - I set the dampers so that the car returns to ride height almost immediately i.e. soft.
My dampers are not sophisticated!! I have tried stiffer settings but the loss of grip is dramatic.

I still have too much roll but I am going to fit stiffer springs to see what happens. I have played with tyre pressures and run the softest that does not overheat the tyre at the outer edges.

Spring rates are not very meaningful unless you are comparing with identical suspension designs - work out the wheel rate & frequency - the eibach website has the formulas.

[1200Datto27]

If you run the dampers as stiff as that person said, you would very quickly find yourself out of suspension travel, as the dampers have been bounced down to full compression, and will not come back until you stop the car for a while, and allow them to revert to their resting state.

[dtype38]

Fraid I have very unsophisticated dampers with only a single adjustment... so am I adjusting the compression rate or rebound rated with that knob, or are they the same thing on a single adjustment.

HiRich - agree that situation is far too complex to describe on a forum like this. Of course our mythical 1" bump affects every corner of the car, but guessing by how much doesn't work. It really needs resolving properly in vectors with real world time durations, inertia values and damping figures. I remember a long time ago doing the calculations for the spring dampers on a drop skid for shoving army Land Rovers out the back of Herculese aircraft. It was a surprisingly fine line on the spring and damping rates not to mash the suspension on the car, or bounce it high enough in the air to flip over!!

Looks like Red Dog is going through exactly the same loop as me... going harder on the springs and softer on the damping and getting better and better lap times. Actually also combined with any anti-roll bar giving understeer! What sort of vehicle is it?

Maybe in all of this it very much depends on the weight and distribution on our own particular type of vehicle... and also the level of technology we're running. My car is a replica of a 1950 car using 1960s technology. The front suspension isn't even rose jointed (I'm not sure the chassis could take the shock loading). Then, of course, there's driving style. I remember when Jaques V joined Williams and tried to drive the car as set up by Damon H. Afer a few races of being a second of Damon's times, he told his mechanics he wanted more grip at the front and less at the back. Everyone said that would oversteer like hell and make it slower... but suddenly he started matching Damon's times!

[greenamex2]

An interesting point has been raised by the previous two responses. All things are relative.

I consider I run soft springs (350 front 150 rear, 840Kg RWD coupe, admittedly with many many pounds of gas pressure in the dampers) BUT I notice that Red Dog can bounce his car and it immediately returns to static.

I can't even bounce my car visibly at the front with the dampers on full soft and my dad helping!!!

We also have a number of people contributing based on totally different types of tyres.

I run Toyo road tyres with a single ply side wall and relatively floppy ply tread.

Are the people in 'hard' spring/soft damper camp that have actually tested stiffer damping/softer springs to the detriment of their lap times on a much more racing orientated tyre?

Certainly on the one occassion that I have run my car on admittedly very hard compound slicks it behave in a totally different way to road tyres. Even before I drove it into a tyre wall.

[topwelshman]

Interesting stuff, just had a day at Llandow yesterday and I would agree to spending a day trying all the different variables out

[dtype38]

Denis, when I bought my car as a road car it was a 1200kg front engine rwd roadster and the springing was 375lb/in front and 400lb/in rear (rear lower mounts are more inboard than the front so effectively a lower spring rate at the wheel). It felt absolutely brilliant on the road, but when I got onto a race track and fitted a roll cage it felt like sitting in an arm chair on top of a blamonge! Winding the dampers stiffer just made it a real handfull of understeer-in oversteer-out, but I was still on road tyres then.

In despair I swapped to the Yoko A032R's, added 50% to the spring rates and shed 100kg from the car. This made a dramatic difference and got me up to "class win" territory. Since then I've chucked the front anti-roll bar, shed another 100kg off the car, lowered it c.o.g. by about three inches, upped the spring rates to those quoted earlier (925lb/in front 850lb/in rear). Since then I've entered five races and so far got three front row qualifications. I've managed to convert those to a win and a third (I've had three dnf's - two fuel related and a disintegrated clutch).

Now I feel my car corners as well as anything of comparable weight and size. The next challenge is to go faster in a straight line. My current engine puts out 230hp at the flywheel, but in most of my races I'm up against cars in the 300hp+ range and in some that can be 600hp+ (It's really quite depressing to see how much difference that makes down the back straight at Snetterton)

Ohhhh for more horsepower !!!!!

[greenamex2]

dtype38 - Can I assume that your car has coil overs front and rear? What, roughly, are the 'motion ratios' of the dampers (ie how far in/out are they mounted compared to the pivot point and wheel centre)?

Interesting about your first experience. Did you check for dampers bottoming out or whether you had enough/any droop travel?

Does anyone actually like stiff anti-roll bars???

[dtype38]

Denis.. correct on the coil overs both ends.

Front pivot:damper/pivot:wheel centre = 25/45cm = 0.55
Rear pivot:damper/pivot:wheel centre = 30/57cm = 0.525

Multipying these up with the spring rates gives me an effective front end stiffness about 14% higher than the rear. (I was aiming at a bit higher but DT didn't have any 950lb springs in stock so I took the 925s). Anyway, best as I can estimate (haven't got any corner weight scales) my car is distributed about 535kg front/460kg rear which is about 16% front heavy... hence my choice of springing. Just seemed a logical way to do it for a car with no anti-roll bars.

Rear dampers could have been running on the extra large bump stops stops I used to have fitted (to prevent hub/chassis contact) but I was never really sure about that. I've since remounted all the dampers to make sure they're about mid travel in rest position.

As for big anti-roll bars... I currently race against a cobra which is set up the opposite way to mine... to get his 670hp on the ground he has chosen to go with very soft springs for traction, and to keep it flat in the corners he has anti-roll bars that make scaffold poles look weedy

[greenamex2]

What sort of angle are your coilovers at?

[dtype38]

About 15deg off vertical front, around 20deg off vertical rear.

You calculating stuff, or modifying your car?

[greenamex2]

Trying to calculate what my spring rates would be if my car weighed the same as yours, just to work out how more stiffly spring yours is.

My wheel rates, weight and spring angle adjusted, would come out at about 400lb/in front and 200lb/in rear.

I can understand my the front being 25% softer than yours, due to me having a front anti roll bar and running less sticky tyres. However my rear is less than half your wheel rate and my car is perfectly balanced. My damping is more or less equally hard front to rear. My tyre temperatures are very similar front to rear (about 53 degrees C).

It is quite possible that I could stiffen the rear springs and, if it started oversteering, regain the lost grip by softening the dampers. It may also cure my cars tendency to lift the inside front wheel.

If it makes the car faster then it proves, in my instance, that it is better to have stiffer springs and softer dampers. If it makes the car slower then it proves the opposite. If it make no difference then it proves that there are many ways to achieve the same ends.

[dtype38]

Aha, that good old skinned cat again.

Driving style might also have a bit to do with it, as I like my car VERY level and I very much go in for the "smooth" line and "smooth" use of controls. My car bits me back quite hard if I do any sudden changes in throttle, steering angle etc... but I like it like that - makes me concentrate more.

Also my car is very pitch sensitive on the brakes. I find that the stiffer front springs I run to make up for no anti-roll bar also dramatically reduce "dive" under brakes, making it easier to control the braking and turn in. The only down side of that is I find it a bit keen on locking the inside front wheel if I begin turning before I've come quite a bit off the brakes.

Ho hum

[greenamex2]

Tried stiffer rear springs at Thruxton. Once I backed off the bump on the rear dampers I regained the cars balance and it understeered less.

Unfortunately I then got chronic graining on the tyres and completely trashed them.

My conclusion is stiff springs are more harsh on tyres than stiff dampers for the same balance. However, stiff dampers will still allow what can become unacceptable body roll in a medium to long corner.

Running on genuine road tyres means that I am probably more concerned about tyre overheating than ultimate grip.

The search, and expenditure, continues.