what does spring preload do?

[mousecatcher]

did all the googling and can't find a decent answer. what is the effect of changing spring preload and what problems can i cure? maybe that's one question.

[kea]

What type of vehicle are you inquiring about? Is it drop limited?

[Brian]

spring preload (and droop limiting, I personally feel there is no spring preload without droop limiting) is a really really long conversation...

Brian

[Evl]

I'd love a reference on this too. A quick glance through "tune to win" only turned up that you should have _some_ preload. At the moment, I'm leaving it where people tell me where it should be, but would be nice to know how changing it changes handling.

[mousecatcher]

What type of vehicle are you inquiring about? Is it drop limited?

FE

[RacerDave51]

Oh where is Dave Weitzenhof when you need him?...

[R. Pare]

Try the Search function - it's been discussed a few dozen times already.

[D.T. Benner]

I hear Viagra can help?
Sorry,I couldn't resist. On subject I would like to hear more ideas about the pre-Load guestion?

[Camadella]

The short answer is that it prevents the shock from moving AT ALL until you overcome the preload. So, suppose you have 400# springs, and you wind in 1" (10 turns on a Penske) of preload. That preloads the spring 400 pounds. At that point, it takes 400 pounds of force on the spring before it moves at all. Once you've reached the threshold where it starts moving, it then acts like a normal spring.

That is, of course, SPRING force, not WHEEL force. Wheel force will probably be less, depending on the geometry of your suspension.

If the car has a sufficient amount of preload so that the shock is fully extended when the car is loaded with the driver and is at ride height, the car is said to have "zero droop". This means that the wheels aren't going to go any farther down (in droop) than they are in a static condition.

What the preload causes, at least for a short time in corner transition, is instant load transfer to the outside tire - as you turn into the corner, you are racing a kart, since the suspension is effectively locked until you put a certain amount of load on it. This loads up (and heats up!) the outside tire very quickly on turn-in. This MAY be desireable - if, for example, you have very hard tires in which it's hard to maintain the temperature, and you WANT them to heat up. But - it may also be very undesireable, in a lot of ways.

As I said, it's a short answer...

Cheers,

Chris C.

[DaveW]

The short answer is that it prevents the shock from moving AT ALL until you overcome the preload. So, suppose you have 400# springs, and you wind in 1" (10 turns on a Penske) of preload. That preloads the spring 400 pounds. At that point, it takes 400 pounds of force on the spring before it moves at all. Once you've reached the threshold where it starts moving, it then acts like a normal spring.

That is, of course, SPRING force, not WHEEL force. Wheel force will probably be less, depending on the geometry of your suspension.

If the car has a sufficient amount of preload so that the shock is fully extended when the car is loaded with the driver and is at ride height, the car is said to have "zero droop". This means that the wheels aren't going to go any farther down (in droop) than they are in a static condition.

What the preload causes, at least for a short time in corner transition, is instant load transfer to the outside tire - as you turn into the corner, you are racing a kart, since the suspension is effectively locked until you put a certain amount of load on it. This loads up (and heats up!) the outside tire very quickly on turn-in. This MAY be desireable - if, for example, you have very hard tires in which it's hard to maintain the temperature, and you WANT them to heat up. But - it may also be very undesireable, in a lot of ways.

As I said, it's a short answer...

That is, of course, for ZERO droop (full droop limiting), which is quite extreme. However, even full droop limiting may affect handling either by increasing OR decreasing understeer (see the next paragraph). Partial droop limiting, which allows some downward motion of the inner suspension with respect to the chassis, affects handling somewhat less drastically.

Along with Camadella's description, front droop limiting also forces the front end to move lower once the limiter is contacted on the inside suspension, lowering the roll center. Whether the roll-center effect overcomes the roll stiffness increase that occurs at the same time is dependent on spring stiffness and suspension geometry, etc. So, IMO, droop limiting is something that absolutely needs testing to see if it will work on a particular car.

Having said all that, I have never used or needed it on any of my cars.

[Adam Brouillard]

the advantages of preload are better control of aerodynamic and camber because the car moves less. The disadvantages are rapidly changing wheel rates. So if you are running on a very smooth track it could be an advantage.

[Rick Ross]

The November 2008 issue of Racecar Engineering magazine contains a good article on this subject. There are endless opinions out there on this issue, but one way of looking at preload is that it allows the front/rear roll stiffness ratio to change during the corner. On initial turn-in the front supension is "locked", giving instant front weight transfer and rapid response. Once the lateral loads increase in the corner sufficiently to overcome the preload, the front suspension unlocks and the roll stiffness becomes "softer" relative to the rear, causing rear weight transfer to the outside tire. The proper amount of preload will allow the front suspension to "unlock" around mid corner and stay that way until lateral loads reduce.

I have limited experience with my Swift DB4 (only three races), but so far I have found that it performs best with zero droop and significant amounts of preload.

[Rondo]

Thanks for the initial question and all the answers - talk about a firehouse!

To adjust ride height don't you crank the springs up and down - therefore effecting preload. And don"t you adjust corner weights by adjusting preload - at least in a CF type car?

So you are basicaly adjusting weight transfer?

[John Robinson II]

Rondo,
You have a FE. Disregard everything said above as it pertains to a racecar that you can acutally adjust to handle. The FE is droop limited in as much as it is over sprung and the sway bars are backwards. The limited aero adjustments do not allow you to overcome the suspension limitations. What I have already found was that lessening the preload allowed the car to roll so it could be driven into the corner and powered off without chasing it everywhere. The car is basically sprung way to stiff.

To answer your other questions, in an outboard suspension car you adjust ride height by moving the spring perch, on a modern pushrod car you change the length of the pushrod.

John

[Rick Ross]

I think that many of the comments above assume a pushrod-activated inboard suspension with adjustable length pushrods. In this case front static ride height is controlled by pushrod length, and the corner weights can be altered with the rear suspension.

[Charles Warner]

Some cars use (require?) preload. Some can not use it due to suspension design.

Rocker RT-4s do not use preload as the ride height is set with the spring seat. Later push-rod RT-4s use (factory suggestion) 200# preload. Shock units are set with this preload off the car and ride height is set with push rod length. BTW, a tweaked tub or disparity in rocker angles will be readily apparent here. ALL weight adjustments are made with the REAR spring seats only.

[Rondo]

Ok - you guys have forced me out of the closet! I have the dreaded 1985 Reynard FF85. Often hated, never loved. And my back end is numb! So full pushrod and now I understand (I think). The wacky piece is the pivots(bell cranks?) are eccentric, so their relative position must make a difference in the suspension performance. Adjusting pushrod length rotates the eccentric so a different radial position where the response is different. So the shock length must be critical as it determines the eccentric start position? The factory manual ommited the set-up sheet chapter so on my own.
Having fun in-spite of the problems and thanks to all for the input!!!

[aspenripper]

I am happy to hear my FE is not a real race car as this only strengthens my position with the wife to put an Atlantic in the garage. It shouldn't be long now.

[Charles Warner]

The wacky piece is the pivots(bell cranks?) are eccentric, so their relative position must make a difference in the suspension performance. Adjusting pushrod length rotates the eccentric so a different radial position where the response is different. So the shock length must be critical as it determines the eccentric start position?

Yes, they are eccentric. Rising/Falling rate suspension. The critical issue here is that the initial static rocker angles relative the car are equal. Reynard (atlantic) required both rockers to be within .5 degree of each other. The shock length does not determine the rocker angle . . . . the push rod does.

[John Robinson II]

I am happy to hear my FE is not a real race car as this only strengthens my position with the wife to put an Atlantic in the garage. It shouldn't be long now.

Never said it is not a real racecar, just not adjustable. It is what it is. THe Atlantic would be a very adjustable racecar.

John

[bryancohnracing]

I ran one of these for a customer in the mid 90's with some success.
Drop me a note if you want the details on what we did back then to make it work. Not sure if it will be relevant but it can't hurt.

Bryan
bryancohnracing "at" yahoo "dot" com

[Tom Szelag]

I found the Racecar Engineering article on preload to be decent but a bit questionable.

Significant preload on an axle will bias your TLLTD to that axle until there's enough force to overcome it.

Do not confuse a stiff front and lots of load transfer with "rapid response." If you were to wind a heap of preload on the front springs, at low lateral acceleration you'd have high roll stiffness so the chassis would react quickly to inertial cornering force, BUT you also have a ****load of front roll stiffness. Think of it as a LOT of front spring or bar. It will shift the response to understeer... how much being dependent on your tire rate, compliance rates, and chassis rate. I'm not sure that I buy the RCE theory that it will heat up the outside front tire quickly and help turn-in. I'd have to see more data either way.

Likewise with the rear, if you wound a hell of a lot of rear preload it would be like running a super stiff rear bar until you can overcome it.

Front and rear preload could numb- or liven-up your on-center response, but it's also an easy way to screw your handling if you go overboard.

Same goes for bump-stops, and very progressive (or digressive) rocker geometry.

[mousecatcher]

so until you have enough preload to match the corner weight, are you doing anything other than changing ride height? i guess you are also limiting droop (?) but i don't understand why that would be useful.

[Tom Szelag]

so until you have enough preload to match the corner weight, are you doing anything other than changing ride height? i guess you are also limiting droop (?) but i don't understand why that would be useful.

Static load is trivial. It's all about dynamic loads. Even if you have less preload than static, there will be (or can be) a point during cornering or combined acceleration where you unload that corner enough to hit the "toggle" point for preload.

[formulasuper]

Some cars use (require?) preload. Some can not use it due to suspension design.

Rocker RT-4s do not use preload as the ride height is set with the spring seat. Later push-rod RT-4s use (factory suggestion) 200# preload. Shock units are set with this preload off the car and ride height is set with push rod length. BTW, a tweaked tub or disparity in rocker angles will be readily apparent here. ALL weight adjustments are made with the REAR spring seats only.

Hey Charles, How much better could the newer push-rod RT-4s be made to handle than the earlier rocker arm (RT-4, RT-5) cars? Was there a significant difference?

[HNR Engineer]

2 books that explain some fundementals of suspension etc that i got from JEGS but i am sure you can get them from any book store if you order them for about $15 each

Chassis Engineering by Herb Adams (HP Books 1055)

and

How to make your car handle by Fred Puhn (HP Books 46)

These are two books that are not to scientific and give a good simple explination on most things that the racer should have a good understanding of.

Just a quick thought on PRE-LOAD:

No matter what preload you add to your system the spring rate is always constant so when you mess with preload to correct handling problems you will generally only be hiding the problem and others will appear. Generally preload is used to correct corner weights (as all springs are not made exactly equal for instance) and so a little preload might be required to correct corner weights.

[Charles Warner]

These treads should be combined.

http://www.apexspeed.com/forums/show...922#post205922

[DaveW]

The questions are in black, and the answers in red.

I'm having a difficult time understanding how, specifically, you would use pre-load to remove understeer. I see the following (I think):

1) Using pre-load on the rear would seem to have a similar effect to having stiffer rear springs and a very stiff rear bar, thus breaking the rear loose sooner and perhaps changing the balance of the car in a desirable manner. However, wouldn't this make the car more likely to snap into oversteer? It would seem to be a delicate balance to get this right, particularly if you're running significant caster in the front.

Yes, you have this perfectly correct. However, Steve Lathrop told me about an instance on an Indy lights car where judicious use of rear preload kept the rear ride heigh and the rear roll center under control, making the car much more driveable entering corners on an oval. It was a very critical adjustment, though, IIRC.

2) Using pre-load on the front would load the outside front very rapidly at turn-in, but wouldn't it actually reduce the ultimate front grip by unloading the inside front?

Your statement is nominally correct. However, the inital stiffening, ride height, and roll-center control can and often has been used to correct an otherwise unfixable understeer condition caused by the combination of too much positive camber, and/or ride height increase (jacking?) on the front. The effect of rebound limiting (preload) is always a balance between the stiffening effects and the control of ride ht and camber. That's why, IMO, it must be tested for each track, etc.

3) My car has a severe understeer tendency; the team I am working with has gone to rear pre-load and a LOT of rake. This is resulting in instability under braking that I'm not happy with, so I'm looking for ideas to try at our next test session. Any thoughts you are willing to provide would be greatly appreciated.

High rake helps power understeer, but at the cost of trailing-throttle/braking instability. Rear preload, as mentioned above, is a very touchy adjustment,and, IMO, generally not very good on road courses. IMO, the best approach would be to increase spring rates all the way around to control chassis attitude and reduce both trailing-throttle oversteer and power understeer.

Check out this handout that I made up for a comp clinic:
http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

[HNR Engineer]

Just a simple rule of thumb related to the above answers:

A tire grip does not increase as the load applied to it increases, in fact the grip increases with load up to a point and then actually decreases with any further load applied above this point.

Load in the above definition refers to anything that adds stress to the tire ie, acceleration, braking, throttle braking, lateral cornering loads etc and the total load is a combination of these stress factors as any or all are occuring at the same time.

So if we look at understeer for example it means for that particular combinations of loads there is too much load on the front tires (or outside front) as they now are loosing grip compared to the rear tires which in effect can still take some loading.

The solution in the understeer case above (and as pointed out in the answers above in red) is to increase the rear loading.

This could be for instance be achieved increasing rear spring preload, increasing the stiffness of the rear sway bar etc

[DaveW]

Just a simple rule of thumb related to the above answers:

A tire grip does not increase as the load applied to it increases, in fact the grip increases with load up to a point and then actually decreases with any further load applied above this point.

Load in the above definition refers to anything that adds stress to the tire ie, acceleration, braking, throttle braking, lateral cornering loads etc and the total load is a combination of these stress factors as any or all are occuring at the same time.

So if we look at understeer for example it means for that particular combinations of loads there is too much load on the front tires (or outside front) as they now are loosing grip compared to the rear tires which in effect can still take some loading.

The solution in the understeer case above (and as pointed out in the answers above in red) is to increase the rear loading.

This could be for instance be achieved increasing rear spring preload, increasing the stiffness of the rear sway bar etc

See this link to a handout I wrote - it pretty much states those things: http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Adv%20car%20prep%20Handout%202005a.pdf
So, you can see that I do understand what you said.

While you are mostly correct, your analysis is too simple, and does not take into account the camber and roll center height control effects of preloading. THOSE are the reasons that preloading the front can sometimes get rid of understeer. Also, as I mentioned, rear preloading can be a very touchy proposition.

[Marshall Mauney]

So if we look at understeer for example it means for that particular combinations of loads there is too much load on the front tires (or outside front) as they now are loosing grip compared to the rear tires which in effect can still take some loading.

The solution in the understeer case above (and as pointed out in the answers above in red) is to increase the rear loading.

This could be for instance be achieved increasing rear spring preload, increasing the stiffness of the rear sway bar etc

What about the opposite situation - where there's not ENOUGH loading on the front, and the rules package won't allow you to put more weight in the nose? In this case, the front is certainly losing grip, but it is doing so because there's not enough normal force on the tire to generate lateral acceleration. In this case, can preload help create more load on the front? How would it work? Clearly, stiffening the rear to get it to overload and brek loose can help get the car balanced, but you have arrived there by reducing the total grip avaialble, not by solving the core problem (not enough loading at the front)......?

[DaveW]

What about the opposite situation - where there's not ENOUGH loading on the front, and the rules package won't allow you to put more weight in the nose? In this case, the front is certainly losing grip, but it is doing so because there's not enough normal force on the tire to generate lateral acceleration. In this case, can preload help create more load on the front? How would it work? Clearly, stiffening the rear to get it to overload and brek loose can help get the car balanced, but you have arrived there by reducing the total grip avaialble, not by solving the core problem (not enough loading at the front)......?

Actually, the front and rear grip level is always a compromise. Loading the rear more is not wrong, if it helps the front and in the process makes the car faster. However, you should try not to sacrifice driveability in the process.

In the case of too low load on the front (tires running cold, but hotter in the center), resulting in power-on understeer, things to try, IMO, sort of in order, should be:

1. Add more weight to front (but you say you are not allowed to do that)
2. use softer-compound, smaller tires on the front (do you have spec tires?)
3. Increase rear and decrease front roll stiffness (springs and/or swaybars) - however, that will make the front tires run even cooler
4. add more front rebound and rear bump damping to keep the front end down and the rear end up during acceleration (this will reduce understeer, but may also make the fronts run even cooler)
5. significantly decrease front tire pressure and increase front (negative) camber and toe-in to heat the fronts more and to get more front grip - however if the compound is too hard, this may not work
6. If #5 didn't help, try increasing front pressures significantly to get smaller contact patch to heat the part of the tire that is being used more (again use more negative camber and toe-in as in #5)
7. use more front downforce if that does not unbalance the high-speed corners
8. and, finally, you could try front preload to stiffen the front to heat the tires - however, as when trying to correct understeer even in a fully adjustable car, this is a very situational-dependent setting, and it will take much experimentation to get it to work. It may even have the opposite of the desired effect, causing the front tires to run even cooler.

Check the effect of damper changes in the handout I referenced to make other possible changes.

These are all I could think of at the moment - maybe there are more I forgot.

[supertiga2]

Dave
Have tried several times to read the handout in the link you posted.
It is a PDF and for some reason this current incarnation of my computer will not do a pdf.

Did a Google and found this HTML version of what I think is the correct paper.

http://74.125.47.132/search?q=cache:...&ct=clnk&gl=us

Is this the correct one?


Thanks
David

[Charles Warner]

Dave,

I assume you deliberately skipped the easiest way to add grip to the front, figuring it would have already been tried. You can, in fact, add "weight" to the front by playing with ride heights. A 10 mil overall difference F-R can make a world of difference.

[ffrgtm]

Dave,

I assume you deliberately skipped the easiest way to add grip to the front, figuring it would have already been tried. You can, in fact, add "weight" to the front by playing with ride heights. A 10 mil overall difference F-R can make a world of difference.

This has more to do with the relative roll centers from to rear correct?

[Charles Warner]

IMO, it has more to do with changing your weight distribution. I don't think it is a simple statement to say how the roll centers will act as this depends on the suspension design. That's way above my pay grade. Dave, Steve and Richard are the gurus here.

[R. Pare]

Lowering the front won't change weight distribution enough to notice, or probably even measure on the scales.

It does, however, change (decrease) the jacking effect, and very slightly lower the CG - if the weight distibution is say 40/60, it will lower the CG 40/60 of the change in front ride height - ie - not very much. The change in jacking effect is the one you will notice most.

[DaveW]

Dave,

I assume you deliberately skipped the easiest way to add grip to the front, figuring it would have already been tried. You can, in fact, add "weight" to the front by playing with ride heights. A 10 mil overall difference F-R can make a world of difference.

Lowering the front won't change weight distribution enough to notice, or probably even measure on the scales.

It does, however, change (decrease) the jacking effect, and very slightly lower the CG - if the weight distibution is say 40/60, it will lower the CG 40/60 of the change in front ride height - ie - not very much. The change in jacking effect is the one you will notice most.

Yup - Richard has it right. Lowering the front even a whole inch, which is not usually possible, would change the rear to front weight distribution by less than 1/5 lb, even if the CG was 1.5 ft off the ground.

[DaveW]

Dave
Have tried several times to read the handout in the link you posted.
It is a PDF and for some reason this current incarnation of my computer will not do a pdf.

Did a Google and found this HTML version of what I think is the correct paper.

http://74.125.47.132/search?q=cache:...&ct=clnk&gl=us

Is this the correct one?


Thanks
David

Yes, but it's somewhat screwed up. Can you not download Adobe reader? It's free for the taking...

I sent you an email with the files.

[Charles Warner]

Not sure of the effect, but if I raise the rear ride height on the Lola by .250" the front weights change quite a bit, and I can notice a definite difference in point and turn in with three turns up on the rear spring perches as a quick fix at the track.