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Old 07-10-2008, 09:09 AM   #1
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Default The Droop Disagreement

Droop always used to be the most overlooked part of car setups (remember when touring cars didn't have droop screws and we just built the shocks to kit settings?) but I think we have all realised how hugely influential it is on car handling.

My "Spool Shimmy" thread has ended up turning into another "Droop Disagreement" so I thought I may as well write down how I believe droop works.

I am sure that more disagreements will come flooding in!

By droop, I always mean the amount of downtravel, measured by lifting the chassis until the wheels lift. This is the opposite of how most droop gauges read it.

Basically, droop only has an effect when it actually limits the downward travel of the wheels during driving.

If any of the four wheels reaches the droop limit, it will become instantaneously unloaded, reducing grip until the weight transfer caused by acceleration or cornering is reduced and the wheel is loaded again.

Furthermore, a car with ample droop rides the bumps better, if a car is running close to its droop limits a single bump can really unsettle it, more droop will make that margin a lot bigger.

So, limiting rear droop will decrease rear grip in cornering (especially off power) if you hit the limiters. It will also decrease rear grip under braking.

Limiting front droop will decrease front grip in cornering (especially on power), it can also decrease front traction under acceleration.

Any droop limit will decrease the maximum overall traction available to you.

Another thing I want to briefly mention is weight transfer. There are two components, one is dynamic and caused by the acceleration, deceleration and cornering forces on the car, the other is static and is caused by the movement of the CG as the car pitches and rolls. Static weight transfer has much, much, much less effect than dynamic, so much so that it can be safely ignored - however a lot of people mistakenly give it a lot of consideration.

Personally speaking, I always try to control weight transfer with springs and geometry. The only time I would consider running limited droop is a super smooth, super grippy surface where I just cannot get the car to "free up" from the tyre grip, or in extreme circumstances where the car has a handling vice that cannot otherwise be cured. Otherwise I always run ample droop, and the car goes like a train.

Your thoughts?
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Old 07-10-2008, 09:24 AM   #2
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Your thoughts?
nearly identical. afterall, the laws of physics really aren't up for debate. but wait a few hours, we'll get 10+ challengers. dunno what it is about this topic...

i will add, that i don't feel a profound difference with this parameter until i'm inside of 1.5mm. (foam tire)
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Old 07-10-2008, 09:33 AM   #3
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i got one for you:

the popular *opinion* is that more droop tranfers more weight (f/r, pitch direction). how is this possible, when droop itself, is there to allow the unloading tires to maintain their contact pressure (force) on the racing surface?

(essentially, droop reduces the unsprung weight from taking part in the total of that which is transferred).
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Old 07-10-2008, 09:33 AM   #4
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lol, you've opened that can of worms.

I always think of droop as the uptravel of the chassis until the wheels lift off the ground.

So, the higher the value on the droop blocks (at the hub), the less uptravel there is at the chassis.

So, by talking about uptravel, it becomes easier to understand how to use it.

For instance, my car is understeering coming out of a corner and if I go stiffer on the rear springs, the car doesn't generate enough rear grip. What do I do? I reduce the uptravel at the front of the car, thus reducing weight transfer to the rear of the car, thus reducing grip at the rear of car and the car rotates better out of the corner.

If I am not getting enough turn in, but I don't want to go softer on the springs because the car won't respond through quick esses or a chicane, I increase the uptravel at the rear of the car allowing more weight transfer forward thus removing grip from the rear and the car rotates better into the corner.

That's my version of droop/downtravel/uptravel and my car goes pretty well too (sometimes )
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Old 07-10-2008, 09:36 AM   #5
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The main problem I have with measuring droop the way we should with the wheels coming up is that for me its not real exact "until the wheels start to lift" wish there was a better way than to eyeball the moment of lift or at least a way to use the guages we have to give you the same droop effect.

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Old 07-10-2008, 09:39 AM   #6
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I agree almost 100%. You're right about your interpretation of the relative importance of CG migration. For as low as the CG's are for most TC's, there would have to be substantial chassis roll (on the order of 10 degrees or so) for CG migration to come into play in the tuning we can do. One thing to keep in mind with regards to CG, is that when you raise it, you increase the roll moment (a function of the distance between the CG and the roll center), which will increase the amount of weight transfer. Easy way to see this? Light weight vs. Regular Weight bodies. Basically you can think of all the weight of the car as a point on the CG, and it acting as a force perpendicular to a lever-arm (distance between CG and roll center). The higher the CG the bigger the lever-arm and the more weight transfer the car will have for the same weight.

Anyway...

But yes, your droop analysis is spot on.
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Old 07-10-2008, 09:46 AM   #7
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Sosdige please give more info on adjusting for static vs dynamic weight transfer.

Static you stated that use springs and geometry. What do you mean by geometry? (shock angle,camber link, roll center shims etc)

Great stuff! I enjoy the technical side....that seaball guy always has great info!

Cheers
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Old 07-10-2008, 09:49 AM   #8
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lFor instance, my car is understeering coming out of a corner and if I go stiffer on the rear springs, the car doesn't generate enough rear grip. What do I do? I reduce the uptravel at the front of the car, thus reducing weight transfer to the rear of the car, thus reducing grip at the rear of car and the car rotates better out of the corner.
All you are doing is reducing traction, the side effect is that the car may "feel" like it has more front grip, in fact it just has less grip all round. Whenever I have run with limited front droop the steering feels inconsistent and the car lacks front grip, so it always surprises me when people say the opposite.

To show this at its extreme, try removing all the droop from the front of the car, or set it to something ridiculous like 0.5mm downtravel. The car will be a total pig.

Last edited by sosidge; 07-10-2008 at 09:56 AM. Reason: Added about extremely limited droop
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Old 07-10-2008, 09:54 AM   #9
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Originally Posted by senna555 View Post
Sosdige please give more info on adjusting for static vs dynamic weight transfer.

Static you stated that use springs and geometry. What do you mean by geometry? (shock angle,camber link, roll center shims etc)

Great stuff! I enjoy the technical side....that seaball guy always has great info!

Cheers
Jamie
Well, like Brian McGreevy said just before you, static CG migration while driving can safely be ignored in TC - better to just assume that the CG stays in the same place.

The position of the CG itself though is VERY important. High CG is always bad, CG towards the rear or towards the front, even in quite small percentages, can make significant differences to the handling.

Geometry is all the non-shock related adjustments you mentioned and more.

Last edited by sosidge; 07-10-2008 at 10:56 AM.
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Old 07-10-2008, 09:59 AM   #10
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it always surprises me when people say the opposite.
does it also surprise you that some 90+% of all changes made seem to go in the "right direction" for people? i mean that's a damn good probability. yet, at the end of the day, hot lap stayed about the same, and they are qualified on their first run.

seriously, how many people ever say "that didn't work right", or "i couldn't tell a difference" or, "i moved in the wrong direction all day"?

and why is every change always worth 0.2/lap?!? we are funny creatures ...
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Old 07-10-2008, 09:59 AM   #11
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how droop effects wt transfer.

when you have more front droop the chassis can lift more when accel before hitting the stops, allowing more pressure to the rear wheels.

having less has opposite effect.

When having more rear droop the chassis can pitch forward when braking more transfering more pressure to the front tires, allowing less wt on the rear tires and creating a loose feeling. Having less has a opposite effect.

these are adjusted to get a balance. the more bite the track has the less overall droop normally used, to help reduce wt transfers and body roll.

Also note caster, camber, and tire selection can influence these items a lot also.

Go to offroad and some other things begin to change as you need to compensate for bumps and jumps also, but the balance part still applies.
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Old 07-10-2008, 10:14 AM   #12
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Originally Posted by sosidge View Post
All you are doing is reducing traction, the side effect is that the car may "feel" like it has more front grip, in fact it just has less grip all round. Whenever I have run with limited front droop the steering feels inconsistent and the car lacks front grip, so it always surprises me when people say the opposite.
I couldn't disagree more

When reducing front uptravel, under acceleration less weight travels rearwards which means weight remains on the front wheels of car, thus providing more grip at the front. Remember though, we're talking millimetres here, going from 3 on a droop block to 4 or 5 can make a big difference in handling.

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To show this at its extreme, try removing all the droop from the front of the car, or set it to something ridiculous like 0.5mm downtravel. The car will be a total pig.
I couldn't agree more. But who in their right mind runs touring cars with no uptravel or no droop screws. It would either be wildly oversteering into a corner and understeering out of a corner, or it would pirouette like it was on ice. I tend to run anything between 2mm and 6mm of uptravel depending on the level of grip available.
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Old 07-10-2008, 10:26 AM   #13
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does it also surprise you that some 90+% of all changes made seem to go in the "right direction" for people? i mean that's a damn good probability. yet, at the end of the day, hot lap stayed about the same, and they are qualified on their first run.

seriously, how many people ever say "that didn't work right", or "i couldn't tell a difference" or, "i moved in the wrong direction all day"?

and why is every change always worth 0.2/lap?!? we are funny creatures ...
you've got it all wrong... on quick adjustments it can go either way, better or worse. But as far as the more time consuming chassis changes (shock oil,piston,different arm sets) the result is ALWAYS, 0.2-0.5 faster.
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Old 07-10-2008, 11:14 AM   #14
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One thing to keep in mind with regards to CG, is that when you raise it, you increase the roll moment (a function of the distance between the CG and the roll center), which will increase the amount of weight transfer.
I believe that is only true if your roll center is lower than your CG. A good chassis design should have the two as close as possible to increase roll stiffness. But considering both are adjustable, there is no good way to know for sure weather or not your adjustments are moving CG & RC closer or further away from one another. Of course you could do the complex measurements & math to be sure...

Here's a mind boggling question: How do you measure roll center if your upper & lower control arms are parallel to eachother???

Sorry for going off subject. I'm just now learning about droop setting. I do know that RWD dragsters use droop limiters on the front suspension to aid traction on the rear tires. That's all I got though...
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Old 07-10-2008, 11:21 AM   #15
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