41 replies to this topic

[shaun979]

What are the pros and cons of each of the three droop limiting options - shock spacers, rocker stops, and preload? The only obvious difference with the preload route seems to be that you would not reduce total shock travel as much vs the other two options. Also the geometry changes in running a longer shock and shorter pushrod to keep pre-preload static ride heights, versus shorter shock longer pushrod as with the first two options. Is there anything else?

[Bill Sherwood]

Preload isn't a droop-limiter, it's just the term used to describe how the springs are loaded-up with the wheels at full droop, if any.

I use droop limiter rods on one of my racing cars, and stops on the rockers on the other car. Given the choice, I'd use the rocker stops as it's a neater package.

[shaun979]

But the more you preload, the less droop you have.

[Bill Sherwood]

Originally posted by shaun979
But the more you preload, the less droop you have.

No, they're different things. They just happen to share the same suspension components.;)

[shaun979]

You're not making sense at all, Bill. Preload is in fact a droop limiter because the more you preload, the less droop you have. You can preload to the point where you have zero droop and the tire is the primary spring at lower speeds. Can you list a situation where as you preload, you maintain or even increase droop? Or are you being pedantic for fun?

[Bill Sherwood]

Originally posted by shaun979
You're not making sense at all, Bill. Preload is in fact a droop limiter because the more you preload, the less droop you have. You can preload to the point where you have zero droop and the tire is the primary spring at lower speeds. Can you list a situation where as you preload, you maintain or even increase droop? Or are you being pedantic for fun?

They're still different things.
You can't have preload without the suspension being at full droop anyway, and droop is limited by suspension travel, not spring tension.
Examples- My two sports cars sit at ride height and they both have preload. They are simultaneously both at ride height and at full droop.
My road car has the springs rattling around with the suspension at full droop, hence no preload.
My rally car has a lot of travel in droop from its natural ride height, and even at full droop there's a little load on the springs, so you could call that a touch of preload.

Different things, they just share the same bits of the suspension.

[imaginesix]

The only way you can preload a spring is when the thing it is mounted to has reached it's limit of travel. In the case of suspension, that is at full extension (zero droop).

I think you are confusing terms shaun. When you move the spring perch you are just raising or lowering the car (unless it has reached maximum extension, at which point droop is zero and preloading begins).

So, within the range of motion of the suspension there is no difference between adding shock spacers or moving the spring perch, they both just move the unsprung bits up or down affecting ride height. On the other hand a rocker stop will not affect anything else on the suspension except droop, including ride height or wheel rate.

[RDV]

shaun979-Can you list a situation where as you preload, you maintain or even increase droop?

Dont confuse droop limit with preload.... lets just say if you turn the spring platform so the amount of spring compression is equivalent to the corner unsprung weight (I.E. motion ratio is 1:1,the corner weight is say 450lbs, and unsprung weight is 50 lbs) this means that for a 800 lbs/in spring using 1/2" spring compression will load the spring by 400lbs, equivalent to static weight...corollary when car is dropped on ground there will be no movement of suspension as it will only start to move after load goes over the static weight.... you will have zero droop when you unload the corner...... you can increase preload to say one inch... in this case suspension will not move until you add another 400 lbs of load, be it aero or by transfer (braking , cornering).

But you will still have the same droop(in this case zero from static ride height) when you unload..

Preloading to less than static unsprung weight can give you limited droop..1mm, 2 mm... to whatever your static deflection is... say zero preload= in case above full droop will be 1/2 " or 12.7 mm, if you follow..

Dont forget that there is no physical difference between having a droop stop on rocker or damper...as far as car behaviour is concerned... as Bill says , its more elegant and less damaging to damper to have it on the rocker, as using damper for drop limiter means you are bashing the damper piston against the housing to stop the droop...

edit- oops , see imaginesix and Bill covered the same ground while I was typing this...

[Bill Sherwood]

FWIW, the two droop-limters I use on my cars.

The Saidor, and it's not very good but it works.



Erm, it works until the lower arm breaks that is!
Gee, that was fun trying to make it stop!


And the Mallock, with the rocker stops.

[shaun979]

Originally posted by imaginesix
So, within the range of motion of the suspension there is no difference between adding shock spacers or moving the spring perch, they both just move the unsprung bits up or down affecting ride height. On the other hand a rocker stop will not affect anything else on the suspension except droop, including ride height or wheel rate.

If a shock has 2" total travel, and I put a 1" spacer in it to preload it 1", then I am left with only 1" of usable travel.

But if I take the same shock and move the spring perch to preload it 1", then I still have 2" travel to use, assuming we are far away from coil bind. The only difference from above is that my ride heights are greater. So it's not just about moving the unsprung bits up or down, once you get into preload, and especially when you get situations where preload force is greater than sprung weight at that corner, as in RDV's example. No?

[shaun979]

Originally posted by RDV
But you will still have the same droop(in this case zero from static ride height) when you unload..

I can see how at the shock droop limit is still the same since extended shock length is still the same. What I was picturing was where you have preload, and you get back to your same original ride height by compensating with the pushrod. When you go into a corner, shock extended length still is the same, but because you started out closer to this limit, your effective droop is reduced - where the inside wheel ends up in a corner relative to where it started out at rest.

Dont forget that there is no physical difference between having a droop stop on rocker or damper...as far as car behaviour is concerned... as Bill says , its more elegant and less damaging to damper to have it on the rocker, as using damper for drop limiter means you are bashing the damper piston against the housing to stop the droop...

Sometimes even the rocker stops fail - even ones designed by big name chassis manufacturers, at tracks with enough elevation change or bumps. Droop isn't even meant to be limited on those types of tracks anyway I suppose.

[rms]

Originally posted by shaun979


I can see how at the shock droop limit is still the same since extended shock length is still the same. What I was picturing was where you have preload, and you get back to your same original ride height by compensating with the pushrod. When you go into a corner, shock extended length still is the same, but because you started out closer to this limit, your effective droop is reduced - where the inside wheel ends up in a corner relative to where it started out at rest.

If you have preload it means the shock is fully extended regardless of where you set ride height via pushrods.
You have zero droop static and unloaded in a corner.

The only case where you may have some droop is if you have enough aero to overcome the preload and therefore have a lower dynamic (compressed) ride height.

Most open wheel race cars use the shock for droop limiting.

[shaun979]

Originally posted by rms
If you have preload it means the shock is fully extended

At the point of setting preload (full droop), but not with the car at rest on the ground. You can have preload with a non fully extended shock.

You have zero droop static and unloaded in a corner.

Zero droop static only if your sprung weight is less than preload force.

[rms]

Originally posted by shaun979


At the point of setting preload (full droop), but not with the car at rest on the ground. You can have preload with a non fully extended shock.



Zero droop static only if your sprung weight is less than preload force.

Sorry shaun979, but it is a physical impossibility.

Every force has an equal and opposite reaction.
Without having a travel limit to the opposite reaction you can not generate a load in the spring greater than that required to support the sprung mass.

[shaun979]

Originally posted by rms
Without having a travel limit to the opposite reaction you can not generate a load in the spring greater than that required to support the sprung mass.

Preload doesn't mean having to have a load in the spring greater than sprung mass. You can preload less than that and not have to run against a travel limit at rest, only up in the air.

You have the car up in the air, and from zero preload, you preload to say 10 lb. You set the car back on the ground, and shock compresses. Your ride height is raised slightly, relative to before this 10 lb preload. You now have preload, but you also have droop as the car sits.

[rms]

OK!
What you are talking about is not regarded as preload in race car terms.

[shaun979]

Alright, what's the definition of preload in race car terms? Thanks

[rms]

The no of turns of the spring abutment or the load in the spring, beyond that to support sprung mass .

[shaun979]

I have to ask where you get that definition because it is the first time I'm seeing the "beyond sprung mass" condition. It would also mean zero droop at rest.

Bill and RDV above, both mention preloading to less than static sprung weight to arrive at limited droop..

even at full droop there's a little load on the springs, so you could call that a touch of preload.

Preloading to less than static unsprung weight can give you limited droop..

Do they too have it wrong?

[rms]

Don't know about the others but in open wheel race cars from GP2 to F/Ford, preload is as I described it, and yes you have zero droop at rest.

[LMP900]

Originally posted by rms
The no of turns of the spring abutment or the load in the spring, beyond that to support sprung mass .

That's not a universal definition, and to regard it as such can lead to communication cock-ups between engineers and mechanics. And your definition is too rigid to allow for preload-limited droop where there is no physical stop other than the damper (as mentioned by others), which can be useful.

[imaginesix]

Originally posted by shaun979
Do they too have it wrong?

Those quotes are entirely consistent with what everybody (else) has been saying so far.

Bill: At full droop he has a touch of preload (the only condition in which preload is possible is in full droop).

RDV: When the preload is less than static unsprung weight the coil will compress when the car is lowered to the ground, giving you a touch of droop (you can't have droop AND preload).

EDIT:
Sorry that should have read "the only condition in which preload is possible is with zero droop"

[RDV]

Ok, lets rephrase that....droop can be described as droop from static ride height, your zero datum.

If you turn spring platform compressing the spring with the car in the air, you will reduce the amount of droop availiable FROM static ride height.

If you do this you will have to reset the ride height again, as car will rise up, as the weight applied to spring is being counter acted by turns of compression on the spring, as example above..

this is also why preload on a pushrod or pullrod car is easy to do, as you can reset static rideheight by changing push or pullrod lenght, on a classical coil over damper suspension attached directly to the wishbone you will have to change the damper eye or have an adjustable damper top...

(you can't have droop AND preload).

..once gain we are into definitions... you can preload a spring on a damoper, and if the force generated by preloading the spring is less than the static unsprung weight, as you put car on ground, it will compress spring to the difference in forces... consequently when that corner unloads (be it in cornering or braking/accelerating) it will extend to the value of spring/unloaded force...
...maybe we should call it spring preload and overload to differentiate? :

[shaun979]

Originally posted by rms
Don't know about the others but in open wheel race cars from GP2 to F/Ford, preload is as I described it, and yes you have zero droop at rest.

On the other hand I know that there are people and teams currently work in Daytona Prototypes and Indycars, and have won Indycar and Indy Pro championships as well as worked/driven in CART and F1, designed GTP suspension, etc. who define it as any spring load above 0 lb at full droop, which ranges from anything from limited droop to zero droop at rest on the ground.

====

imaginesix, no they aren't. I think RDV has done a great job differentiating between conditions in his last post which I completely agree with.

[phantom II]

Few other things with pre load. You can use a lower constant rate spring for better curb compliance and the limited droop replaces the ARB or fine tunes roll. Pre loading springs is an art form in oval racing for diagonal loading. Don't know what they do now with the much higher high spring rates on the COT. No more wedging.

[rms]

Originally posted by shaun979
On the other hand I know that there are people and teams currently work in Daytona Prototypes and Indycars, and have won Indycar and Indy Pro championships as well as worked/driven in CART and F1, designed GTP suspension, etc. who define it as any spring load above 0 lb at full droop, which ranges from anything from limited droop to zero droop at rest on the ground.

Wouldn't your initial question have been better directed to the above people/teams ?

Then you could enlighten us as to how preload and droop influences car setup.

[shaun979]

Rms, the question is not just for one group of people. I like to hear what different people have to say and then cross reference it to the rest, considering the different types of vehicles and courses each speak of - why, when and how something is applied. The larger the number of quality reference points and the more unanimous they are on a particular concept, the more I can accept it as industry accepted truth.

There are some extremely accomplished people here (as or more so than the people I know), some of whom have participated on this thread. I come to understand or clarify a concept. What may sound like argument is more an attempt to put out what I (mis)understand so that any confusion or contradiction within it can be quickly pointed out by others so I can correct it and learn. I am in no position to enlighten any of these people. It is always good to hear from them, as well as from just anyone who is inquisitive and discussive.

[McGuire]

Sometimes I would like to take about half the lingo in chassis and suspension science, throw it away and start over from scratch. Too late now I suppose.

[AndrewD]

Originally posted by Bill Sherwood
Preload isn't a droop-limiter, it's just the term used to describe how the springs are loaded-up with the wheels at full droop, if any.

I use droop limiter rods on one of my racing cars, and stops on the rockers on the other car. Given the choice, I'd use the rocker stops as it's a neater package.

After viewing your pictures id have to say that both of these options are crude and quite dangerous.

Why not use bump stops or other means?

And are they aluminium a-arms???

[Bill Sherwood]

Originally posted by AndrewD


After viewing your pictures id have to say that both of these options are crude and quite dangerous.

Why not use bump stops or other means?

And are they aluminium a-arms???

They're two commonly used methods and work just fine.
I can't see how adding bumps stops would be any better, as they could only be placed near the inner pivots for the lower arms and so place huge loads on them when the suspension goes to full droop, and so be far worse.
No, the suspension is all steel, but the guy that built the car had them nickle-plated to make them all shiny. We made the mistake of welding on the droop limiter bracket and that caused the fatigue failuer there. The new arms are simple painted and far stronger, they will not fail.

[RDV]

AndrewD - After viewing your pictures id have to say that both of these options are crude and quite dangerous.

...no, would just say simple... no need to gild the lilly...although the wishbone seems to be plated (not a good idea in any case, and the fitting of a stop braket seems to be the cause of failure, seems to start at end of what can charitably be described as a rather blobby weld (droop loads are quite small , unless humongously preloaded...;)

...and bump stops work in the other direction.....I have used bump rubbers as droop stops to give a less harsh transition, specially on the rear, which is very sensitive to such gizmos...

[rms]

I guess we are getting our 'knickers in a knot' over terminology.

With modern race cars using at or near 1:1 installation ratios and front spring rates of 2000 lb/in or more, the amount of spring travel from unloaded to loaded is minimal (.1" or less).

[Fat Boy]

Originally posted by shaun979


On the other hand I know that there are people and teams currently work in Daytona Prototypes and Indycars, and have won Indycar and Indy Pro championships as well as worked/driven in CART and F1, designed GTP suspension, etc. who define it as any spring load above 0 lb at full droop, which ranges from anything from limited droop to zero droop at rest on the ground.

It all kind of depends on who you're talking to. If you're talking to a mechanic, he may call a suspension preloaded because when he puts the spring on the shock he has to wind the spring platform against the spring a turn as it goes on the car. The engineer doesn't look at it as a preloaded suspension because when you set the car on the ground the spring is compressed 10mm or whatever. RDV (and I) call this dimension droop. Motorcycle guys call it sag. It's all the same thing.

What engineers are interested in is getting the rocker in the same position at static ride height with spring X as it is with spring Y. No real magic. Let's say your static 'droop' measurement is 10mm. With a stiff spring, that may mean your spring platform is backed off 2 turns. With a soft spring your spring platform may engage the spring 2 turns. It's all a means to an end. It controls the position of the rocker at static ride height. It can also control the areas on the track that the damper 'tops out'. This is actually a function of the 'droop' in the car, and not the preload in the spring, although spring preload is a factor in determining droop.

Was that confusing enough for everyone?

[shaun979]

Fatboy, thanks for the info. Actually the group I listed includes engineers. I believe LMP900 is an engineer too and has disagreed with the definition that specifies "beyond sprung weight" as a condition.

What is the engineer definition of preloaded suspension and where does it come from (some SAE standard term or something)? What is the term you would use for 10 lbs of load on a spring at full droop? Has RDV mentioned dimension droop in any of his earlier postings?

[LMP900]

shaun - there is no definition, because how much preload you apply depends on both the effect you want to achieve (RDV's "overload" or just restricted droop) and the package you're applying it to (direct-acting spring or rocker, with or without adjustable droop stop, etc.). Under those circumstances a stone-tablet definition is unnecessary and restrictive, but you do need to make sure your mechanic knows your preloading procedure.

[Ben]

Originally posted by RDV


..once gain we are into definitions... you can preload a spring on a damoper, and if the force generated by preloading the spring is less than the static unsprung weight, as you put car on ground, it will compress spring to the difference in forces... consequently when that corner unloads (be it in cornering or braking/accelerating) it will extend to the value of spring/unloaded force...
...maybe we should call it spring preload and overload to differentiate? :

Adjusting preload in this manner - i.e. to alter the static deflection is the most common way of adjusting ride height on a motorcycle.

Ben

[Greg Locock]

Same on proto cars.

So, what do those who call it preload think is happening when the spring hangs free at max rebound, and they wind the spring platform up by 3mm, with the spring still not in contact? The 'preload' hasn't changed. The spring rate hasn't changed. The ride height most certainly has changed.

It is bad terminology in the general case.

[Fat Boy]

Originally posted by Greg Locock
Same on proto cars.

So, what do those who call it preload think is happening when the spring hangs free at max rebound, and they wind the spring platform up by 3mm, with the spring still not in contact? The 'preload' hasn't changed. The spring rate hasn't changed. The ride height most certainly has changed.

It is bad terminology in the general case.

That is a change in loaded damper length, change in rocker position, and a droop. It could also be a change in ride height, but I'm assuming that the final ride height is set at the pushrod (or pullrod), not the spring platform. There would have to be some sort of ride height compensation done on the pushrod.

Having an outboard suspension changes (simplifies) a lot of this. You move the spring perch until you get the ride height you're looking for. If I were to do an outboard suspension again, I'd do some sort of shim adjustable end eye length to be able to vary the same things you do with a rocker setup.

[phantom II]

I sent my shocks back to Carrera and they shortened the shaft by 2". They said to use their shocks as a droop stop was no problem. http://www.lrbmfg.co...rint_shocks.php Not doing this pounds the hell out of the aluminum platform and ride heights and corner weights have to be checked more often.

Originally posted by Greg Locock
Same on proto cars.

So, what do those who call it preload think is happening when the spring hangs free at max rebound, and they wind the spring platform up by 3mm, with the spring still not in contact? The 'preload' hasn't changed. The spring rate hasn't changed. The ride height most certainly has changed.

It is bad terminology in the general case.

[blkirk]

Originally posted by phantom II
I sent my shocks back to Carrera and they shortened the shaft by 2". They said to use their shocks as a droop stop was no problem. http://www.lrbmfg.co...rint_shocks.php Not doing this pounds the hell out of the aluminum platform and ride heights and corner weights have to be checked more often.

I guess they must make them different now. When I used their shocks back in '93-'94, I specifically asked them if I could shorten the shafts to limit droop. They told me that allowing the shocks to top out was "very bad" and that I should add an external droop limiter to my suspension immediately.

[Fat Boy]

Originally posted by phantom II
I sent my shocks back to Carrera and they shortened the shaft by 2". They said to use their shocks as a droop stop was no problem. http://www.lrbmfg.co...rint_shocks.php Not doing this pounds the hell out of the aluminum platform and ride heights and corner weights have to be checked more often.

Generally, this will also tear up the threads on the shock body because the spring rattles against the thing without being centered on the platforms. Running helper springs can help this considerably, but I've also seen people run very large heat-shrink tubing over the damper body to protect it.

[McGuire]

Originally posted by blkirk


I guess they must make them different now. When I used their shocks back in '93-'94, I specifically asked them if I could shorten the shafts to limit droop. They told me that allowing the shocks to top out was "very bad" and that I should add an external droop limiter to my suspension immediately.

Some dampers now have internal bump and/or rebound stops; some don't.