TrackJunke

I was thinking today, on a race car to get the to stop understeering you decrease the spring rate on the front to get more traction and vice versa with oversteer, you decrease the rear spring rate. I know stiffer springs and shocks make the car handle better, but exactly why? I guess there is a point where too stiff will make the car handle worse. I was thinking that they put more load onto the wheel and tire and the more load a tire has the better it grips. I was also thinking that a stiffer suspension should keep the tires in better contact with the road. I used to think that it had to do with load transfer, but now I know that load transfer is only affected by CG, wheelbase and acceleration/decelleration/cornering. Could someone offer up a more technical explanation of how a stiffer spring and shock makes the car handle better? Sorry if this is a dumb question, I just started thinking today about the paradox of reducing spring rates and getting more traction on a race car.

M758

Well it all comes down to bumps.
If you were on an infinitely smooth the no supension is really needed. Just attached the wheels rigidly to the body. You will get minimum body roll and excellent grip due to minumal weight transfer from wheel to wheel.

Now the real world. Even the real race tracks have bumps. If you go too stiff then the car will bounce over the bumps and not stay attached to the track surface. Too soft the car will flop around.

If the spring rate is too high when you hit a bump, either straight or cornering the car will move. This is the entire car moving up and down. This will tend to pull one or more tires off the groud thus losing grip. If you are too soft the car will lean too far in corner and overload one or more tires thus loosing grip. The movement of car is bad since there is relativly large mass associated with the car as compared to a wheel and it is therefore hard to change direction.
It is hard to make the car turn the direction you want if it is moving too far on the suspension.

For an example in the racing world Serbring is very bump track and most teams have to run lower spring rates to keep the wheels on the ground. Usally the key is to run as stiff as possible while still mantain tire contact to the road.

European tracks are much smoother and therefore can those cars typically run very high spring rates.

Note that the anit-sway bar (sway as most call them) are different than springs in the way they funtion so they should not be confused with basic springs for each wheel.

TrackJunke

But does a stiffer suspension affect weight transfer? i was under the impression that a lower center of gravity or widening the wheel base could change the amount of weight transfered to a side of a car under cornering. A stiffer suspension only effects how much that side compresses under that load.

Cupcar

This is such a complicated topic it's hard to answer in a paragraph. Here is my limited knowledge.

As you noted the maximum load transferrs during cornering are related to the CG height and track width and the applied G force. The applied G force is ultimately a function of how well the tires stick to the track, their coefficient of friction. The coefficient of friction is dependent on the tires temperature, load and camber angle.

The amount of chassis deflection, that is to say its rolling response to load transferr, in mid corner, the point of maximum load transferr is a function of the roll stiffness of the chassis. Roll stiffness being the combined effect of spring and anti-roll bar stiffness.

The rate at which load transferrs manifest themselves at the tire patch on corner entry (loading suspension toward the maximum transferr) and exit (unloading the suspension) are related to the roll center height, roll stiffness rate and shock absorber valving and driver technique. Trail braking drivers and drivers with a smooth throttle foot dramatically change load transferr rates on corner exit and entry compared to non trail brakers and un-smooth drivers.

Generally stiffer than street springs is better because the street springs are designed for ride considerations more than handling on a track. It's difficult to drive a car which takes forever to take a "set". The camber angles in a grossly leaning car do not promote grip. But only stiffer to a point not beyond where the tire compliance over the bumps is affected and grip of the tire is diminished by the car leaping over bumps if spring is too stiff.

So it is possible to overshoot and end up with understeer/oversteer if the rates are too high or the difference in rates is too great front to rear. One thing, the wheel rate is what the tire patch sees in relation to the chassis. The wheel rate takes into account how the spring is linked to the chassis, the so called motion ratio.

It takes experimentation to get the right combination of springs, shocks and roll bars to optimize a given situation for car, driver and course.

All this really changes if the car has wings and downforce is affected by chassis attude and ride height. Then understeer/oversteer are affected by more than just management of the vehicle mass by the spring, the attack of the aero elements is affected by chassis motion as well. <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

M758

[quote]Originally posted by Silver1.8T:
<strong>But does a stiffer suspension affect weight transfer? i was under the impression that a lower center of gravity or widening the wheel base could change the amount of weight transfered to a side of a car under cornering. A stiffer suspension only effects how much that side compresses under that load.</strong><hr></blockquote>

All of thing you say impact weight transfer.

Good handling as all about managing grip. To do this you must never exceed the potential of the tire. This a function of the coefficient of friction and loading on the tire. The loading of the tire is in two directions, down to the pavement and side ways (either fwd, back, or side). When the side ways loading exceeds the friction force (down pressure and friction coefficient) the tire slips. This is bad. The actual friction coefficient is quite complicated to figure since it not a smooth surface to smooth surface, but tire to rough surface.

Stiffer springs impact weight transfer my lessening the movement of the car body in a certain direction.
For example.

A car with a very low front spring rate is moving at constant velocity. There is very high spring rate at the rear. When you hit the brakes the car will nose dive since there is very little to stop the momentum of the car from going in that direction. This then transfers dynamic load to the front of the car which is a proportion of the mass of the car and the rate of deceleration. Of course mass is proportional to the weight of the car. With very low rate springs more of the weight of the car moved fwd since there is little to resist it. What is important here is the concept of momentum (mass times velocity). The greater the mass (weight) of the and the greater the deceleration rate the more load is pushed to the front wheels vs the rear wheels. If all the load is put on the front then there is no load on the rear and therefore no grip at the rear. This is ok in a straight line. (Dragsters do this under acceleration pushing weight to rear all the time!)

Now here is where the two loads on the tire come to play. If you have unloaded the rear tires under braking a described above (no downward pressure on the tire) and the try to turn the car the rear will not grip and the car will oversteer.

Now the higher the spring rate the less weight transfer you have and therefore the less down pressure you will loose on the rear tire. The combined loading of all 4 tires will always be the same not mater what spring rate you use. If you have too high in the front as compared to the rear your front tires will not see enough down pressure. You cannot ever have too much down pressure on the tires (look at F1 cars and downforce). But without areo devices mechanical down pressure is limited so you must distribute it wisely. When you transfer weight while you gain on one end of the car you loose on the other. The side loading does not change significantly since that is based more strongly on the mass of the car rather than the weight transfer at each wheel.

In a nutshell Higher spring rates equate to less weight transfer. This is good since you ideally want to balance all of the weight and side load equally on all 4 tires.

Weight transfer is bad. Higher spring rates minimize this. Lower CG minimizes this. Wide track effectively lowers the CG in relation to the tires and this minimizes weight transfer.

I hope this is somewhat understandable.

Cupcar

I don't think that stiff springs can eliminate weight transferr. Imagine that the car has the springs replaced with steel tubes so it is now a go cart. The weight transferr is the same, the chassis just does not respond to it with any motion.

Originally posted by Silver1.8T:
But does a stiffer suspension affect weight transfer? i was under the impression that a lower center of gravity or widening the wheel base could change the amount of weight transfered to a side of a car under cornering. A stiffer suspension only effects how much that side compresses under that load.

is a correct statement as I see it.

M758

An stiff spring will not eliminate all weight transfer, but it will tend to reduce dynamic weight transfer. It is the dymanic weight transfer that is the real causes inertia that is the real killer.

Also remember that solid bar has spring rate just like a formal spring. It is just much higher. Also karts while not having springs do experince flex from the chassis and the tires.

TrackJunke

What is the difference between dynamic and static weight transfer? And why is body roll bad? Why does less body roll with stiffer springs cause higher cornering speeds?

M758

[quote]Originally posted by Silver1.8T:
<strong>What is the difference between dynamic and static weight transfer? And why is body roll bad? Why does less body roll with stiffer springs cause higher cornering speeds?</strong><hr></blockquote>

Well, all this is quite complicated, but here is try....

What I mean by dynamic weight transfer is really the inertia associated with a moving mass. Not so much in plane parallel to the ground, but in the one normal or perpendicular to the ground. The inertia of a car under braking in the fwd direction in the plane of ground is independent of spring rate. It is a function of car mass, and velocity. Spring rates effect the vertical inertia. What that means is the a with more nose dive there is more velocity in the vertical plane this more inertia & thus more weight transfer (velocity down in front & up in the back). I consider this to be dynamic since it due to up/down suspension movement (typically called body roll when in corners or squat and dive for acceleration and braking) rather than the movement CG of the car. Some may call this a Moment of Inertia since it strictly describes how the mass car is moved around the CG rather than movement of the entire CG. Now a lower CG will induce less weight transfer due to the relation of the mass about the CG and the location of the CG with respect to the wheels. As a practical example. a mid engine car like the 914 or boxster could have and identical weight distribution as a 944 or 928. However these cars are very different on the track due to the location of the weight and how the car reacts to it motion. With mass at each end of the bar bell it is hard to get the car to rotate, and hard to stop it from rotating due to the larger polar (one end vs other end) moment of inertia about the central CG. The mid engine car s much easier to rotate due to the much small polar moment of inertia due to all the mass being located at the CG.

Body roll is bad since it is a source for this "dynamic weight transfer inertia" stuff I have been talking about. It is also bad for two other reasons. One is due to the movement that the driver feels. The more a car moves on the suspension the harder it is for the driver to feel how the car is moving relative to the ground. This makes more difficult to drive at the limit since it is more difficult to determine if you are the limit. Note that VERY stiff suspension can be problem for the driver as well if they cannot react as fast as the car can.
The other is due to camber changes the occur when suspensions go through their range of travel. Here we are starting to mix in the importance of suspension geometry. In general the stiffer springs will cause less suspension movement and therefore less camber change. This will allow the proper camber (what ever that is for given car and spring rates) to remain thus provide a more ideal contact patch shape and loading along that patch to ensure proper tire temperature along the patch and thereby optimal grip.

The dynamics of racing are all about physics, both static (constant velocities, constant cornering, constant braking & constant acceleration) and dynamic (transitory braking, corner entry/exit, etc)

I believe the book "How To Make Your Car Handle" by Fred Puhn is good layman's reference to race car dynamics. This book is available from places like Performance Products & Tweaks.

Cupcar

To me the difference between static and dynamic weight transferr between tires is:

1)Static changes are made on the alignment rack using wheel scales to adjust the spring preloads changing to weight each corner shares of the total weight of the car.

2) Dynamic weight transferr occurrs as the car is moving and the wheel weights respond to the forces on the cars center of gravity.

The total lateral load transferr= Lateral acceleration x weight x CG height/track width.

Let's assume our chariot really is a chariot and has only 2 wheels with a spring suspension system. Lets assume it weighs 200 pounds and has a CG height of 20 inches and a track (distance between wheels) of 50 inches. Lets assume a 1 G lateral acceleration.

1 X 200 x 20/50= 80 pounds. Since the chariot weighs 200 pounds, each wheel supports 100 pounds when going straight ahead. So, during this cornering exercise, the inside wheel will lose 80 pounds, only supporting 20 pounds and the outside wheel will gain 80 pounds, now supporting 180 pounds.

This weight transferr is accomplished by transferring load to the outside wheel via BOTH its suspension links connecting the wheel to the chassis and the spring. The proportion of weight transferred through each of these suspension elements is dependent on the relationship of the center of gravity to the roll center of the suspension.

If the roll center is at ground level then the entire load transferr will occurr across the spring. The spring will compress according to it's spring rate by the amount of load placed on it.

If on the other hand the roll center is at the same level as the center of gravity the vehicle will actually not roll at all and all of the cornering load transferr would occurr across the links with none going through the spring.

If the roll center is half way between the CG and ground then 1/2 of load goes through the spring and 1/2 will be transmitted across the links. The car rolls and the spring compresses accordingly.

But the amount of load transferred is the same in each case whether the chassis rolls or not.

The key is to get the whole system tuned to manage the inertial weight transferrs in a controlled fashion in terms of camber control and rate of transferr so the poor contact patch doesn't get beaten to death and give up the ghost. This usually means stiffer springs than the car came with if you're going to the track. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

tonytaylor

Silver1.8T
If you watch some (older?) 911`s they will sometimes lift the inside front wheel under cornering ( and many fwd racers will lift the inside rear). Think what that does to the grip of that wheel !! Since the grip of tyres is finite and in this senario the outside wheel is taking all the weight at a 90 degree vector then that wheel is going to slide earlier than if the inside wheel is taking some of the weight and thus sharing the sidways load

TrackJunke

Cupcar,

So are you saying that the spring rate make NO difference to the amount of load transfered and that stiffer springs and shocks just control the contact patch of the tire so it is optimal in hard cornering.

Cupcar

Yes, my understanding is the TOTAL of front and rear weight transferr is dependent only on CG height and track width factors.

The chariot example works to explain what happens at one end of a car. The problem is the car is a 4 wheeled carriage and the distribution this total weight front to rear and hence oversteer and understeer characteristics can be effected by the spring rates chosen, particularly on corner entry and exit when diagonal transferrs occurr. This is when the forces need to be carefully controlled with the spring rates working with the shock valving to optimize handling by good management of contact patch weights.

The example of the 911 exiting a corner with the inside wheel in the air is a good example of diagonal weight transferr. The relatively soft rear spring allows the weight transferred by the combination of forward acceleration and lateral acceleration as the car exits the corner to squat the outside rear chassis down and actually pick up the inside front tire. This can be controlled with a stiffer rear spring. The weight transferr will not dissappear the chassis just will not respond to it by picking up the front wheel, all 4 tire patches can be used and the car can be tuned to corner faster.

<img src="graemlins/yltype.gif" border="0" alt="[typing]" />