How droop works?

[mkinodu]

Hi, I am not sure if this is a right place to post this question. But since this question is related to automotive engineering, I decided to give it a try.I have been playing RC car (radio control) for few months, there are many tips for tuning your car, one of them is droop setup. Droop is defined as how much down travel your suspension has. I thought it has very little influence on performance and more droop might be better because you want to keep all your tires on the ground when turning corners and on/off throttle. But actually it worked out opposite, I lessened the droop for quite a bit on both front and rear and the car ran much better than before (better turning ability). I asked some people on the track and also checked some articles on the internet, some of them mentioned about weight transfer. I don't think droop setting will have something to do with weight transfer. But how does it affect the performance? I may be wrong, droop setting does affect dynamic performance, but how? Can somebody explain the theory?

 

[hemipanter]

Interesting! How do you arrange the drop stop? Is the springrate altered? Do you have shock absorbers? Are the springs working for the full wheeltravel without drop stops?
Goran

 

[mkinodu]

The drop stop is arranged by a setscrew in the suspension arm, when you screw in the setscrew will push against the chassis so the suspension arm lifts (less droop).

http://rcspecial.com/news/?p=24359

you can check this link for reference. The car has shock absorbers. The spring is normally shorter than the full wheeltravel.

 

[hemipanter]

Did you mean that the inside wheel actually is leaving ground in corners, while using drop stop:s?
Goran

 

[NormPeterson]

I'm finding the RC terminology somewhat confusing, though it sounds like 'droop' and 'drop stops' are related to roll center height and indirectly to how much roll/dive/squat can be tolerated before some suspension corner or other bottoms out (spiking that corner's rate).


Norm

 

[mkinodu]

The inside wheel will leave the ground if the down travel is too short plus the suspension is set too soft. I think I didn't make me question clear. My question is if the magnitude of droop (down travel)has dramatic influence on dynamics performance, and if it affects the weight transfer. I am asking because I thought the front down travel will not affect the performance and the longer the better, but actually after I setup my car has much less front down travel, the car ran much better. Before it didn't have good cornering ability (spin at on/off throttle), after the droop was lessened it just seemed to have better cornering performance. The result worked against my understanding of suspension setup. By the way, the R/C car is not a toy as most guys would think, it actually is a serious engineering piece, most of automotive engineering theories can be applied on this serious hobby.

 

[mkinodu]

oh, the rear droop was lessened at the same time. Lets think the problem this way. When we off the throttle (or brake) before the corner, the weight will be transfered forward so that the front suspension will be compressed which will cause the rear leaves up (assuming no any anti- features added), in this case, we prefer longer rear down travel to keep the rear wheels on the ground. Same scenario will happen when the car accelerates (or on throttle) at the end of the corner, we need longer front down travel to keep the front wheels stay on the ground when the rear suspension is compressed. During the corner, we also want the inner wheels can stay on the ground although they don't take much work as the outer wheels do, so we need longer down travel to avoid the wheels from leaving off the ground. Above is my understanding about how droop works. But the car worked much better after I shortened the droop on both front and rear. Therefore I just can't figure out how shorter droop improve car's performance, I used the same shock absorbers and springs, nothing changed before and after. I am puzzling.

 

[hemipanter]

I am personally of the opinion that these model cars operates much like a big car, the fact that they are smaller is of secondary importance. This is why I think the problem is interesting, for a well built model car you can drive in a manner and to study problems that are difficult with a real car.

The reason I ask about how the problem manifests itself depends on that the actual behavior of the car is important to draw conclusions about where the problem is. I would prefer to see and experience the problem in reality, to get a better vision of how things stand.

I have heard from others who work with racing cars that they experienced better braking performance by using the drop-stop on the back of the car, but I have made no self-tests that can confirm this. In dragraing it was popular to let the front drop out a lot, and we used the 90/10 damper for a slow return. Now days this appears to be used to a lesser degree, one simply let the front wheels leave the ground.

Drop stop or different hard suspension, etc. does not change the mass movement as such, but it changes the speed with which it occurs. When the drop-stop "taking in" so it works like a Panhard rods (one way). You could say that the car rolls around a center at the inner wheel, and therefore only the springs of the outer wheel will compress. It can give an initial addition to the outer wheel grip since the mass of the inner pair of wheels will (after a moment) add to sprung mass. Any such "additional" to the grip has a price in terms of lower grip later (or earlier if it is scheduled). How the overall situation manifests itself depends on the spring hardness and shock absorbers damping factor.

In any case, so far, I do not look at what I write as an explanation, only som sort of inlay.
Goran

 

[mkinodu]

Thank you Goran. I didn't think about the leaving inner wheels will add to sprung mass, that is a good point to think about this problem. You said that the drop stop changes the speed of weight transfer with which it occurs, is it because later one end will add to sprung mass so the total weight that will be moved around will be heavier?

 

[GregLocock]

In some classes of racecar using droop (rebound) limiters is very useful. For instance if you are jumping off a kerb then you can use it on the front to level the body and regain traction very quickly.

On roadcars we use rebound springs in the shock absorbers to provide non linear roll stiffness, it is a quick and relatively easy way of adjusting limit handling under or over steer.

So, yes it is important, but there are no hard and fast rules.

It certainly breaks the simple rule of thumb that you want to maximise suspension travel and use all of it in one lap... but that rule is more of a fond hope based on theory than a proven path to success.

Cheers

Greg Locock

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