Value of aerodynamics on small RC cars?

[machin]

Synvex,

Autogyro almost got it right... but ignore this comment:

However it does negate the suspension effect of the tyres.

mounting the wing to the chassis or the wheel uprights has no effect on the force exerted on the tyre contact patches; it is the same in either case (try standing on a set of bathroom scales to weigh yourself, and then do it again with a spring between you and the scales - the only difference is the weight of the spring itself - your weight still "gets to" the scales).

If you mount the wing to the chassis the suspension springs will compress as the downforce builds. To avoid the suspension bottoming out (giving you no suspension) you need to make the suspension stiffer... that means traction is made worse as the stiff suspension means it cannot absorb bumps as well as before. By by-passing the suspension (mounting the wings to the wheel uprights) you avoid the downforce acting on the suspension springs and can therefore run softer suspension.

There is one complication; if you fix the wings rigidly to the wheel uprights it would lock the wheels together, meaning that the only allowable independant wheel movement would be by flexibility of the wing itself ; in effect you've created an anti-rollbar out of your wing... the answer of course is to introduce some compliance in the mounting arrangment to give you the necessary independant wheel movement; in effect a second set of springs which act independantly of the car's "ride" springs and only support the wing(s)...

...The Lotus twin chassis car was a good example of this arrangement... but it was banned and we ended up with rules requiring the wings to be mounted to the main chassis...

[autogyro]

You are right Machin. My bad.
However mounting the aero body on the hub carriers does free up a wider range of possible chassis springing and damping.
This allows the tyres to potentialy take less of the suspension forces.
The result should be a better controlled suspension.

Interesting to see how it works in scale.

[synvex]

On the latest cars there are a bar on which the body sit in the centre, which can expand to both sides by telescopic movement and connect to connectors to the uprights of the rear suspension.

[machin]

Can you link to a picture?

[synvex]

That is a bit of a problem because I do not know how! But, Google Serpent Model racing cars and look for the Serpent 966 racing chassis and look at the rear suspension assembly.

[synvex]

I am sorry, I had a quick look at the 966re 1/8the scale racer and the rear assembly now pushes direct onto the uprights. The bar with the telescopic movement seem to have been abolished.

[Blanchimont]

Here's the official manual of the Serpent Car.

http://www.serpent.com/file.php?FileID=5155

Look at page 9 to 14 and 34. Are you refering to part number 903237?

[synvex]

No! That is the rear adjustable anti-roll bar! The is round bars with flats machined on them and as you twist them, they harden or soften. What I am talking abou is part no. 903130 on page 34 with 903260's to connect to the top of the wheel uprights. Look on page 31 for the bar higher than the rear wheels.

[Blanchimont]

Ok, so part 903130 (rear body mount) is connected to 909366, which is not connected rigidly to the chassis.
The slot in 909366 allows the movement (up and down, rotation) around screw 110108.
With this construction the downforce is transferred through 903130 and 903260 directly to the wheel hubs.

The picture shows the parts mentioned, the slot in 909366 is visible under the screw.

[synvex]

Jep, that centre-bolt prevents the bar from swaying to either side so it can move only up and down to make the body move up and down with the suspension, as well as transfer the downforce directly to the wheels. But once again, if the downforce is really effective transferred, this thing becomes a bar that bind the wheels and render the indipendant movement of the rear wheels less effective.

If you mount the wing on the chassis, yes the chassis is pushed down under force and your car need to run higher for it not to drag on the track surface at the back, but for that there is a downstop on the suspension to prevent it moving too far down. At first we tried to drill a hole in the rear of the chassis and fit a titanium screw from the top down so that as it wears off, you can just lower it a bit. It worked wonderful and it protected the chassis from rubbing on the surface, but caused a spray of sparks that was very distracting. Remember, you drive this thing from a driver stand, about 50m away down the straight.

[gato azul]

Jep, that centre-bolt prevents the bar from swaying to either side so it can move only up and down to make the body move up and down with the suspension, as well as transfer the downforce directly to the wheels. But once again, if the downforce is really effective transferred, this thing becomes a bar that bind the wheels and render the indipendant movement of the rear wheels less effective.

Are you sure, that this is the case?
I know you race these things, and I'm sure that if you notice a difference in how the car handles/behaves, then there is something to look at - that's not my point.
But I'm not sure that this bar will "bind the wheels" (any more or less).
What I can see with this bar, is that if you have a single wheel bump input, or warp/roll you will twist/warp your whole bodyshell (assuming the bodyshell is mounted rigidly to the chassis at the front? if you have a similar setup at the front too, then the bodyshell will not twist in pure/perfect roll, leaving only warp and single wheel bump with that issue)
So basically you add he torsional stiffness of you Lexan bodyshell to the suspension springs/bars under these conditions.
But I'm not sure, that he bar as such would bind up the rear suspension (left to right) if it is loaded by downforce.
It's a easy enough experiment to do with a set of corner scales (or cheap kitchen/laboratory scales), and some equal weight attached left and right to the bar, to simulate the amount of downforce, you think you see coming from the bodyshell, pushing onto the wheels.
In perfect roll, the bar connecting the bodyshell to the two rear uprights, should just pivot around it's center, not adding anything to this movement. In a single wheel bump situtation ( one rear wheel going up, while the other remains leveled)
you would expect the center of the bar ( the bolt/pin in the slot) by 1/2 the amount of the wheel input.
Yes moving the wheel up against the downforce will be harder, but not harder as doing the same if the downforce would be transferred via the springs/chassis.
Why do you think, that the cross bar would "bind up" the suspension?

[Blanchimont]

The same system on another car.



Imagine the chassis (not the body cover) is pushed down. The wheels and rear body mount stays at the same heigth, but as the center bolt/screw moves down with the chassis it is possible for the bolt to reach the lower end of the slot. At this point no further suspension movement is possible and the suspension is locked. This problem can only occur if the slot isn't designed properly.

Another critical thing could be a large movement of only one wheel. I don't know how to describe it properly, but i'm sure that at a large displacement of only one wheel there would be some kinematic problems with this connection.

[gato azul]

Yes, I can see the first part happening, but then at one point, the chassis would touch the ground anyway, so that being the max. possible movement in this direction. If the slot is long enough, it should not reach it's end before this - IMHO.
And from the photo's/illustrations it (the slot) appears to be long enough.
Yes, if the system is not designed probably, all sorts of strange things can happen, but one would hope, that they put some thought into this before hand.

One thing, which I would not like from looking at the 966 manual for example, is the fact, that when you adjust rear toe-in, you would bend/twist the connection link between the upright and the body support cross bar, as it seems, that they allow only for at rotational movement at the connection points to the upright and the cross bar. While I think, there is enough "give/flexibility" in the material, for that not stopping a toe adjustment, it surely does not help in terms of overall friction in the suspension - IMHO.
But maybe I see it wrong from the illustrations, and in practice they account for it somehow.

Just out of curiosity, is there any specific idea/thinking behind the relative high camber gain they seem to run at the rear?
At the front (seeing the inclination of the top and bottom wishbones) it does not appear to be the case?
Also seeing that the tyres wear quite "conical" ( I now, that this is common with these cars running on foam tyres), It wonder if this is related to the kinematic used. And if this is a "wanted" effect, or just something that happens, but not much thought went into it.

[synvex]

The problem with the 1/8th RC car is getting through corners under power with the purpose of the shortest track time. For this reason the diff has been abandoned for a fixed axle to allow the inside rear wheel to lose some grip for the front wheels to do their job through the corner. Even then you run with excessive camber on the front wheel to have enough under power turn-in to the apex of the corner. The suspension movement is quite a lot larger than what you will find on a F1. The reason being the car need to be able to run on about any surface even though it may be not absolutely smooth and every little bump is magnified on 1/8th scale. So in fact you have a circuit car with a lot more suspension movement than would be expected on a full scale. Also, these cars use their tyres quite a lot longer than you would use a racing slick. A main race could be about 120+ laps on on set of tyres and they are so soft they wear down to the rim if you keep going that long on them.

Funny thing is, when you have raced these cars for a long time, you surely would have felt during the A-main as your tyres start to wear down, your car's handling improve all the time and if you put new larger tyres on the car, it does not really improve. We now use two sets of half worn tyres for a main, making a change mid-way.

[autogyro]

You could always do away with the spring damper units altogether.
Fit a ball halfway between the Cof G and the C of P around the middle of the chassis.
Mount the body shell to it and to the (independent) cross bars at each corner.
That would combine aero with suspension set up using the flex in the body.

Might be interesting to develop torque vectoring diffs at each end.
That would help the tyres and improve cornering.