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The Droop Disagreement

The Droop Disagreement

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Old 07-10-2008, 10:24 AM
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After thinking more, I would like you to consider this:

Droop limiting at one end of the car will cause the suspension at the other end to compensate. Imagine if you hit the brakes and the rear suspension hits the droop limiters. It's not like the rear tires come off the ground! What it will do is cause the chassis to act like a lever & put the remaining lifting force through the front springs and down onto the front tires. With this theory, less rear droop should increase dive and less front droop should increase squat...

Perhaps this is why people say it effects weight transfer. Dynamic weight transfer that is...
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Old 07-10-2008, 10:45 AM
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Originally Posted by RCSteve93 View Post
12th scale rocks.
Physics affects 12th scale too! Are you telling me nobody ever messes with the dampers, t-bar and springs?
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Old 07-10-2008, 10:49 AM
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Originally Posted by 94eg! View Post
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...
For any car, your CG is above your roll center (the only thing that I know of off-hand that is the opposite is a motorcyle). And you're wrong about keeping those two as close as possible. While that may beneficial in special circumstances, you need to have your roll center a certain distance away from your CG to facilitate weight transfer. Tires increase their lateral grip as the normal force applied to them is increased (to a point obviously). If your CG and your roll center were at the same height you could only rely on the lateral grip produced by the tires with the corner weights of the car acting as the only normal force.

I have used two vehicle dynamics simulation programs (the better of the two is called OptimumK) and have plotted and measured static roll center and roll center migration. You can do the calculation of roll center within reasonable accuracy for a TC suspension, you just need 8 points (camber link and pin locations in arms) and the location in the center of the contact patch of the tires.
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Old 07-10-2008, 10:56 AM
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Originally Posted by 94eg! View Post
After thinking more, I would like you to consider this:

Droop limiting at one end of the car will cause the suspension at the other end to compensate. Imagine if you hit the brakes and the rear suspension hits the droop limiters. It's not like the rear tires come off the ground! What it will do is cause the chassis to act like a lever & put the remaining lifting force through the front springs and down onto the front tires. With this theory, less rear droop should increase dive and less front droop should increase squat...

Perhaps this is why people say it effects weight transfer. Dynamic weight transfer that is...
Actually, look at a biker doing a "stoppie" (reverse wheelie onto front wheel under front-wheel braking). There is no force from the rear tyre that is stopping it from leaving the ground, no fulcrum at the point of contact of the tyre and the road to provide a lever.

Same happens in a car, just so happens that we don't have the high/forward CG of a bike, and there isn't enough weight transfer to lift the wheels off the ground completely. What happens is they lift from the ground instantaneously, and in a 4wd car this decreases braking force, decreasing deceleration, decreasing weight transfer to the front, increasing weight retained at the rear, and putting the tyres instantaneously back on the ground, from where they will instantaneously become unloaded again if the brakes are still on!

So less droop decreases grip. Although as seaball said quite early on you need to be at very small amounts of droop (he mentioned 1.5mm, I'm inclined to agree), before you start to really see the handling effects. Anything above that and the car rarely hits the droop limiters.
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Old 07-10-2008, 11:00 AM
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All cars I know of today, whether RC or real have roll centers lower than the CG. There have been a few with high roll centers in the past (auto union, Corvair) and they handled poorly. Why? High roll centers creates a jacking force that basically tries to roll the car over on it's roof. Not a good combination. Most cars today have a front roll center below the road surface and a rear roll center at or slightly above the road surface.
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Old 07-10-2008, 11:06 AM
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As for front droop, we often see the exact opposite on 1/8th on road cars and 2wd dirt oval cars. I believe this is mainly aerodynamic. Less front droop keeps the nose down which increase downforce and therefore more steering. More front droop allows the nose to lift and reduces downforce and therefore less steering.
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Old 07-10-2008, 11:06 AM
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For a long, long time I had absolutely no clue how to set droop and was basically running 3 to 6mm droop in a Cadillac type drive that required ‘throwing’ the weight to one side before hitting a corner. I was trying to race a boat. Eventually I actually got it to kind of work like that and figured out how to make it set and hang a line. After a couple friendly wrench beatings, I am down to 1 or 1.5mm. So, I’m no expert, but,,,,,
Droop for rubber asphalt I would guess is much different than foam carpet 13.5 in both setting and performance. Also, unlike full scale where grip is always sought after, carpet foam produces more traction than you’d ever really want and so some of the chassis dynamics operate in reverse…… right?

“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.”

If I take the static and dynamic forces and place them on an x and y axis, isn’t the motion of the car using both at the same time? As you dive into the sweeper weight is transferred forward and driver side. I would think that the dynamic would act as and exaggerating factor on the static forces?

“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).”

I think/guess it does allow more weight transfer, but running less droop would produce a faster transfer of weight. So it may not be as much weight being thrown into the chassis, but it would get there quicker. So conversely, more droop would allow for more weight transfer, but it would take longer to get there. That’s the experiences of a rook running crazy droop anyways…

“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.

Rod”

For foam, I place a ride height gauge under the chassis plate in the spot right before it bends towards the bulkhead at 4.5mm and then lift the plate to 5.5mm on the ride height gauge and turn the droop screw until the wheel comes up. I do the rest of the corners and then go back to the first and measure again. On the second go, I apply just enough upwards pressure to feel the screws hit the plate while holding down the wheel. It’s kind of finger acrobatics, but with gentle pressure I can usually get a very accurate reading.

This brings up another question though. I have always had issues with the graphite wearing away under the droop screw. I hear that some pro drivers and putting flush screws in from the underside of the arm at big events and just changing out tires to maintain droop setting. I’ve thought about putting a piece of metal onto the chassis, but can’t ‘hack’ the reality of it.

Last edited by The Bear; 07-10-2008 at 11:07 AM. Reason: .
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Old 07-10-2008, 11:14 AM
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Originally Posted by sosidge View Post
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.
That is not the droop removing the grip, but because you'd be pre-tensioning the springs which has an immediate effect on the way the grip is provided to the tyres. I always set my car so that on total uptravel, the springs are not loaded but not floating. Then I adjust by small touches to get the desired handling.

If set this way, there is no sudden unloading of the inside wheel as on total uptravel the spring isn't pushing the wheel anymore so grip is equal to zero on that wheel. I think you have over simplified the effect of droop, as you totally ignore the most important aspect, which is the dynamic aspect of the roll centre position.
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Old 07-10-2008, 11:18 AM
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Originally Posted by 94eg! View Post
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...
I can tell you right now they'll never be any close, if they were the car wouldn't have any grip. Also the roll centre is ALWAYS under the CG otherwise the car would roll the wrong way round.
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Old 07-10-2008, 11:28 AM
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Well that makes good sense now. Although it would look cool if your car leaned into a turn though...
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Old 07-10-2008, 11:33 AM
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Originally Posted by RCSteve93 View Post
12th scale rocks.
I am hearing ya!........
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Old 07-10-2008, 11:35 AM
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Originally Posted by 94eg! View Post
Well that makes good sense now. Although it would look cool if your car leaned into a turn though...
That is sort of what corvair's did. They could "jack up" on the outside rear tire and either spin or in extreme cases flip.
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Old 07-10-2008, 11:38 AM
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Originally Posted by TRF415boy View Post
That is not the droop removing the grip, but because you'd be pre-tensioning the springs
No, in that example I mean the miniumum droop setting possible without pre-tensioning the springs, ie the car rides at 5mm (say) with no pre-tension, but also has no sag when the car is lifted.
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Old 07-10-2008, 11:45 AM
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Originally Posted by THancock View Post
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.



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.
I do feel that you have contradicted yourself here, saying on the one hand that less droop increases grip and on the other hand that less droop decreases it.

I firmly believe that the phenomena of added grip that you are experiencing from your small adjustments are from something other than droop. Perhaps you are stopping the suspension from over extending and compromising the tyre contact patch. Droop adjustments alone will always tend to decrease grip as you approach the limit (which is the zero droop setting I mentioned).

But I don't want to get into a situation of argument and counter argument, if your way with droop "works" for you, go for it.
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Old 07-10-2008, 12:17 PM
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Originally Posted by sosidge View Post
Physics affects 12th scale too! Are you telling me nobody ever messes with the dampers, t-bar and springs?
We do but the amount of droop in a 1/12th car is extremely small if at all and primarily in the rear pod. The front springs tend to be pretty stiff so any droop at the front end completely unloads the spring. In most cases you can get a pretty good setup in a 1/12th without using any droop but adding a little droop to the rear can really fine tune the car nicely.
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