Droop
 

I have searched through the on-road section and could not find a discussion directly about droop. Only regarding specific settings for specific cars.

I wondered if anyone can help or give their experiences, specifically regarding front droop settings on touring cars.

I understand the theory, that more front droop increases weight transfer to the rear on acceleration and more rear droop increases weight transfer to the front under braking and vice versa.

BUT...

I always find that reducing front droop helps to add on power push and calm down an over-steering car, especially on carpet.

Shouldn't the opposite be true? If you increase front droop, thus increasing weight transfer to the rear on power, shouldn't this cause under-steer on power as the rear wheels have more grip?

Or is it a case that more weight on the rear wheels under acceleration in high grip situations actually pushes the car round more and as the nose of the car is in the air and has less grip?

One theory I have is that altering weight transfer with droop also affects the roll at the end of the car that you alter droop.

Is it a case that reducing droop at the front reduces roll at that end and gives a little less lateral grip, thus creating on power push?

I just found this which ties in with my theory. If my car is over-steering on carpet I usually go with stiffer springs in the front, a thicker front roll bar and a small decrease in front droop. This seems to help calm the car by reducing the front grip.

Things reverse themselves past a certain threshold.

Do a test. Zero out droop, drive car.
Then add only front droop, drive car.
Then zero front and add only rear droop drive car.
Then add droop front and rear and drive car.

Record handling traits of each test. Then you have a baseline idea for each chassis.

A sound plan, I just need to set aside a club night and get some testing under my belt. I do find on carpet the track changes each round so you can be fooled into thinking a setup change has given you more grip when it is actually just the track rubbering in.

I have found cranking things to the max and then the min can give false feedback as you mention, over a certain point a setting can have the opposite effect. Best as you say, to try it from 0 and add a bit at one end to observe the effect.

Don't confuse body roll or movement with weight transfer. They are often exactly opposite. Usually softer has more body movement and less transfer of weight. A stiff chassis has effectively infinite transfer of weight and potentially no body movement.
Trophy trucks are a great example. They have huge body movement on soft springs to reduce weight transfer since they race on low traction surfaces. Put them on a high traction surface and they almost roll over.
Look at how anti roll bars work, they stiffen up the vehicle in roll, increasing weight transfer and reducing roll. Usually they result in a net reduction of traction due to the increased weight transfer. That is ok since they are usually used on high traction surfaces to control body movement.

Great point.

To OP, once weight transfer has taken a set in with more droop F, it should push the car on power. But b4 it takes a set in, the rear traction is actually lower compared w less F droop which can make the rear step out. It is only a brief moment tho. If ur car is loose rear with more droop F, it could be due to something else. E.g. rear chassis bottoming, u turn ur car too hard on power,etc.

So if roll increases it doesn't automatically mean the weight transfer does. If I increased front droop and created more body roll towards the rear I may have to increase the rear spring rate to push back against the body roll and thus increase weight transfer at the rear and ADD grip?

Does roll increase grip or is it weight transfer? Or neither or both :)

"a net reduction of traction due to the increased weight transfer" - I always thought an increase in weight transfer was putting more weight over say the rear wheels and giving them more grip. I also thought body roll was the action that added that extra weight. If body roll doesn't add the weight over the wheel and is the opposite to weight transfer why does body roll increase traction such as in the example of the trophy truck?

I am struggling to wrap my head around this but thanks for the replies so far. I just think if I can understand it a little better my setup skills will improve.

If anyone is struggling to get to sleep this site has a VERY in depth explanation of load transfer Weight Transfer: how it works and how to use it in setup

All this has done is confuse me further as to exactly what we mean when we use the term roll centre :)

You have the centre of gravity of the car and an imaginary point called roll centre. Years ago I thought raising the roll centre reduced roll as you were putting the RC point nearer the COG. Then it seemed from reading other guides lowering the RC reduced roll as the "roll point" of the car was lower.

So are we using the wrong term when we talk about roll centre in model cars?

Does lowering or raising the roll centre reduce roll?

According to this website Raising the RC decreases roll and lowering the RC increases roll, https://balancemotorsport.co.uk/suspension-geometry

The Hudy Setup guide says:

Weight transfer is the key to car handling. Consider that a car has a certain amount of
“weight” on various parts of the car, and on each wheel. By transferring weight to one
end of the car (front or rear), to one side (left or right), those tires will be forced onto the
racing surface more, and will have more grip or traction.
Weight transfer is affected by the car’s set-up and by the way that you drive.

A “roll center” is a theoretical point around which the chassis rolls, and is determined
by the design of the suspension. Front and rear suspensions normally have different roll
centers. The “roll axis” is the imaginary line between the front and rear roll centers.
The amount that a chassis rolls in a corner depends on the position of the roll axis relative
to the car’s center-of-gravity (CG). The closer the roll axis is to the center of gravity, the
less the chassis will roll in a corner. A lower roll center will generally produce more grip
due to the chassis rolling, and the outer wheel “digging in” more.
Roll-centers have an immediate effect on a car’s handling, whereas anti-roll bars, shocks
and springs require the car to roll before they produce an effect.

So it seems my original thought is correct. Raising RC decreases roll and lowering RC increases roll. Roll generally increases grip. But when I reduce front droop it results in a more angled wishbone (higher at outside, wheel edge and lower at inside,pivot edge). This gives a lower RC and thus more roll. This should give more grip to the front in terms of RC left to right and yet it seems to have the effect of less front grip.

So I'm still stuck as to the basic relationship between roll and weight transfer and why reducing front droop reduces over-steer for me? I have tried reducing front droop more and more and with every reduction the car starts to under-steer more and more. It seems to definitely promote under-steer when all the theory says it should increase front grip and make an over-steering car over-steer more?

If I have it straight in my head... (unlikely, but plausible) when there is a greater distance between CG and RC (moment) the larger effect on body roll is seen for the same weight... Like a cheater bar on a wrench, it has more leverage. Where the RC is in relationship to the rest of the car is how that leverage will impact what the car does. If the RC is below the chassis as apposed to above the axles, the effect will be different. This page is one I am digesting now: ROLL CENTER

Changing droop should not effect the angle of the suspension arm while car is it rest.

If it does then you are going to alter ride hight as well. There should always be a difference (1-3mm) between where the chassis sits at rest and if you lift it, how much before the tires come off the ground.

Also if there is no uptravel of the chassis, that means little to no weight transfer.

I believe what you have found is 100% correct — less front droop = less front grip. This is just simply down to the fact your inner front wheel is in the air.

Droop only makes a difference when your suspension arm hits the stop. At that point it will lift a wheel if you keep the load on. Basically the car will roll regardless of your droop setting.

The term "Roll" is usually only for between left and right. Use the term "weight transfer" for front vs rear. This might have confused you when you read some articles.

Weight transfer from F to R or vice versa gives that end more grip like you said.
But weight transfer in general decreases overall grip. Think you can't really drive your car when cornering as fast as when you go straight. e.g. static overall grip is 200lb. when it's rolling, it becomes 180lb. Hence less overall grip.
Body roll increases traction. But what you're missing here is that the statement is talking about "relative" traction. e.g. more roll in F than R. Then more traction in F. Vice versa. Yet again, overall available grip decreases because of weight transfer.
All in all, the less weight transfer the better. Amount of weight transfer is determined by CG height, track width, and weight of the car. Proportional to CG height and car's weight. Inversely proportional to track width. This is why you try to make the car as light as possible, as low as possible and as wide as possible. To add another note on this account, the less droop decreases weight transfer between F and R. This prevents horribly (or unexpectedly) pushing or over rotating car on and off throttle respectively. Generally speaking, because of this, with acceptable grip on road surface, less droop is almost always better. The "less" droop is relative term though :)

overall traction across the front or rear axis does reduce with weight transfer. That's because the grip generated at the tire is not linear, it's a curve. The amount of traction on the laden/ outside tire does not make up for the traction lost on the inside/ unladen tire

Obviously, losing traction is not good. To reduce the amount of weight (and traction loss) that transfers, you'd need to reduce the overall weight at the axle, lower the center of gravity, widen the track, or just slow down

Body roll is a result of weight transfer. You can limit body roll with springs, swaybars, roll center, and droop

So you have two cars going through the corner at the same speed. The car that weighs less, has a lower cg, or a wider track will transfer less weight and therefore roll less.

Swaybars, springs, shocks, droop and roll center don't affect how much weight transfers, just how quickly it transfers and how much the car will roll

Maybe this is the most important aspect we never listen to :lol:

Must go faster!!! :lol::lol:

Great replies by everyone - thanks!

I'm starting to understand the theory better now and I think I have it sussed why reducing front droop takes away front grip. Due to the unladen inside tyre losing grip by lifting and the outside tyre not receiving 100% of the additional grip due to negative side effects of weight transfer.

Roll is a result of weight transfer.

The amount of weight transfer is determined by CG height, track width, and weight of the car.

The only point (or so I believe) I am still stuck on is..... why does the Hudy setup guide and others advise to INCREASE front droop to increase weight transfer to the rear under acceleration, increasing F to R roll to increase rear grip under acceleration thus reducing over-steer. Yet the opposite is in fact true?

Is it because I am simply cornering too fast and cocking a wheel and that aspect of REDUCING front droop takes precedence over the effect of weight transfer to the rear. If I was to corner slower and not lift the inside wheel would, as the setup guides say, increasing front droop give more rear grip?

Years ago I used to struggle with my HB Cyclone on carpet and found the only setup cure to over-steer was to increase rear droop by changing the down stop setting from 6mm to 3mm. Simply so when I was cornering the inside rear wheel would not lift. This went against all setup guides as increasing rear droop should only affect FRONT grip under braking.

It does seem droop settings have more of an effect on the end you alter than the opposite end as many setup guides state.

see above.

People have all these theories about roll and weight transfer etc which may or may not be correct, but the simple fact is, less rubber on the road less grip at that end. This happens when the inside wheel suspension arm hits the stop.

An exagerated example is a motorcyclist doing a stoppie. The rear droop setting had no effect on the front end.

Another example is a top fuel dragster lifting a wheel under acceleration. The front droop setting had no effect on rear traction.

I don't see why you mention droopie and wheelie as they have nothing to do with droop setting just like you said yourself. :confused:

Those are the two most obvious examples that show droop has no real effect on weight transfer front to rear.

The only real argument to go against this, is the fact that the suspension becomes unsprung weight when the droop stop is hit and a wheel is lifted. This is however generally very light in comparison to the car weight, so the effect would be minimal.

back to the subject, a rear wheel lifted can really make a car really hard to drive on a high speed corner. Of course the type of diff/car you have also makes a difference to how it reacts.

You have to remember that droop also affects from side to side as well as front to back. By giving the rear more droopl, you allowed that wheel to stay on the ground and maintain some traction. When you're in the middle of a corner, droop is affecting front-rear roll and side to side as well.

If you don't have it, go buy a copy of "Tune to Win", it will explain a tremendous amount.

Going to look this up... :nod:

Didn't bother to mention it. Well said.

I don't think the dragster example is very good at all, they have huge power to weight ratios and (comparatively) all the weight is at the back to begin with.

Think of an 'average' road car, from standstill if it pulls away hard the front will lift up (more so if it's FWD) and dive when braking. This is the weight transfer in action.
The front will keep trying to lift until there isn't enough downward force on the tyres to produce traction and it either wheel spins or the traction control cuts in.
Sportier versions, sometimes of the same model of car, will come with harder front and rear suspension which help to reduce this transfer.

As they don't have adjustable droop, the dampers and springs are tuned to changed the transfer. We have the benefit of set screws in the wishbones so we can separate some of the this transfer from having to change the springs etc.
Obviously we use a combination to allow us to tune the car for all area's of the track.

Another reason you may loose rear traction with more front droop is that the camber gain at the rear (and/or combined with what the camber was to begin with) is too great and as the rear compresses down to contact patch of the rear tyres is reduced.

Sometimes simply reducing the static rear camber can increase rear traction.

As every setup guide and book keeps telling us, you cannot make a setup change without it, in turn, having an effect on another setting. We're trying to find that ultimate sweet spot for our cars, for each track in each condition.

My 0.02€.

As someone rightly wrote, weight transfer only depends on CG height, track width (or wheelbase if you are calculating braking and acceleration) and acceleration (speed a that moment). Having no suspension doesn't matter to weight transfer, it's the dragster example. That car uses the weight transfer of everything past the engine to help plant the rear tires. That length is tuned so it barely lifts the front wheel (zero weight on the front wheels,everything on the rear) but not so much the car is upset aerodynamically (front pointing up) or has to ride on the wheelie bar adding another point to take load of the rear tires.

More droop means more chassis motion which can be good up to a point, different droop front-rear means one end will provide more weight to the other, the one with more droop of course.

Droop is good that doesn't make the suspension stiff during most of the travel. Have you imagined your 1:1 car ridding on the bump stops every turn?

Thanks for replying snuvet75 I really appreciate the input.

"May or may not be true like I said above. If you overdid it, then yes.
If you did it just right, not true and you will have the desired effect that the book says."

I started with JJ Wang's carpet setup which is not a million miles from the standard TCXX carpet setup. This had 8.75mm front down stop setting. I only increased this to 9.5mm and saw a reduction in over-steer, 9.5mm is often used by Team drivers according to online setup sheets so I don't think it is overdoing it.

I just wonder if the setup theory is fine for full size cars but perhaps with model cars due to the much faster cornering speeds it doesn't always apply 100% of the time?

I will have a play and try increasing front droop but I'm pretty sure it will make the over-steer worse or not reduce it.

When watching Renault Clio racing they often lift the inside rear wheel when cornering fast. They obviously don't have set screws to adjust droop but maybe shock length. You would think this was an undesired trait?

Thanks Skiddins, yet more great input. To give further background on my setup, I was running 2 degrees of camber all round with little to no camber gain. After the first run I felt that the car was oversteering on corner exit when applying the power coming out of the bend. My hunch was too much front grip rather than not enough rear grip. For the second run I changed from 2.6 front spring to 2.8, went from 1.3mm to 1.4mm front roll bar. The car was better but still a little wayward at the rear so for the third run I increased the front downstop from 8.75mm to 9.5mm. This small change balanced the car, reduced the over-steer and the car felt much more balanced and gave much quicker times. I know the grip was coming up each round due to the track rubbering in and with more runs I was getting used to the track, but the setup changes definitely improved the rear of the car.

As it was the last run I went with reducing front droop as this is what I have known to add a bit of push which is what I needed. I didn't feel increasing front droop as I've tried in the past would have helped but I am definitely going to try 8mm on the second run tonight to see what effect that has. According to theory that should add more rear grip. According to my experience it won't. Tonight will tel!! :D

That happens for a couple reasons:
-shedding rear grip (one less tire on the road keeps balance for a FWD car);
-trying to keep the front end to dip and roll too much.

You are bringing FWD cars into the mix, hope you're not getting confused because they have to do some tricks to behave well. 4WD cars like our TC sure benefit from 50:50 weight bias and less pitch motion.

I know this complicates this thread a little more but I tend to set our cars up with more rear weight bias by a few %. Car just feels to rotate better

And you are not wrong. Setting the car with more rear weight bias keeps the front tires from getting overloaded, or at least making them overload later than rear tires. It also addresses some of the corner weight balancing issues all* cars have, that the rear springs are too stiff compared to the front springs (or the front springs too soft compared to the rears depending on your take of the current situation).

*all measured cars by me. I calculate wheel rate and all current and old cars by extension derive from the same design (rear shocks closer to wheel). I have some cars that spring balance was achieved by reversing the recommended spring setup front to rear.

So I started round 1 with the same settings as the previous Friday nights racing. The rear was a little loose on power again so this time I increased the front droop without making any other changes. I went from 8.8mm to 7mm on the front downstop setting. For round 2 the car felt better on power but I noticed the rear was still struggling for grip going through the chicane after the main straight.

After more careful attention and really focusing on when it felt loose I felt the rear was braking loose mid corner rather than corner exit and it felt like lazy over-steer. I shortened the wheelbase for round 3 and the car felt much better but still had a tendency to loose the rear through the chicane.

For the Final I added a little more rear droop to avoid lifting a rear wheel during the fast direction changes and it seemed to do the trick. I went from 8.5mm to 8.0mm and the car felt the best it had all night.

During the final I also increased the -VE steering expo and that further helped, so much so I could turn up the dual rate a little to gain some slow corner steering lock.

I think overall droop behaves the way theory explains but when a wheel lifts due to the downstops it reduces grip at that end so can sometimes reduce grip at the end you decreased the droop, the opposite of the theory.

Like a lot of people have mentioned, by changing one setting you impact many more and it is finding that balance that works.

At least now I think I can see how droop affects the handling and droop is better used to fine tune over-steer/under-steer, relying more on spring rate and roll bars in the first instance.