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Old 04-25-2002, 04:24 PM   #1
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Default Roll centre question

Now, I understand that a car with a lower roll centre will roll more into it's suspension, giving more grip into a corner for the same spring rate.

But, a car with the lowest possible roll centre, i.e. parallel arms with a long upper link, will make the outside tyre acquire positive camber as it goes through the corner, actually causing a loss in grip at the tyre.

So, what's a better way to set the car up - with a roll centre that keeps the outside wheel at a fairly consistent angle as the car rolls, or with a roll centre tuned to give the car more theoretical grip, by letting the suspension compress?

I only ask this because the Pro3 doesn't have a great range of high roll-centre mounting positions, and I'm starting to experiment with a high-roll centre to see if it makes the car work more consistently on the slippy surface I race on.

Thanks
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Old 04-25-2002, 05:10 PM   #2
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The length of the upper link isn't as important as the angle of the upper link in determining roll center, at according to what I've read on the subject.

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Old 04-25-2002, 06:06 PM   #3
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Dave

Don't know whether you've seen this, a big guide to TC car tuning including a quite compehensive section on roll centre :

http://gallery.uunet.be/heremanss/c2/index.htm
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Old 04-25-2002, 07:06 PM   #4
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Sosidge:

First of all lateral weight transfer is independant of height of roll axis and amount of roll. It is a function of height of centre of gravity , track width and cornering speed and radius.
What is significant about a lower roll centre or softer springs is that the rate of lateral weight transfer is reduced. A pair of tyres on the same axle produce maximum grip when they have even weight on them. As you turn a corner weight transfer cause the lateral grip to reduce from this maximum. Therefore a low roll centre increases grip on turn in to a corner as it takes longer for the grip to drop off. On corner exit it takes longer for the lateral grip to come back so grip is reduced. Roll centre doesnt affect lateral weight transfer mid corner.

Unfortunately we have to consider weight transfer in the longtitudinal direction as well. If you have softer springs on the car this weight transfer will increase. In this case more weight on the front or rear axle does mean more grip for that axle. If you have a lower roll centre at one end of the car than the other weight will be transferred on to the end of the car with the lower roll centre. So a lower roll centre at the rear would give more rear end grip.

Basically to answer your question wheel angle is more critical mid corner but roll centre is more critical on corner exit.

I'm not sure you fully understand how to calculate roll centre position from your comments. For instance upper and lower arms that converged on the outboard end of the suspension as opposed to the inboard end as on most setups would give you a lower roll centre than parrallel arms. If you don't understand. there are plenty of text books or websites about that explain it.
Associated or Orions sites being possibilities.
It really is impossible to explain without a diagram though.

If you are running on a polished wooden floor as I suspect you might be what I would do personally is go for quite low roll centres with very soft springs to allow for lots of front to rear weight transfer. Remove any antisquat as this will kill lateral traction. Do not use antiroll bars under any circumstances on wooden floors.

I apologise if this is long and overly complicated. This is the simplified version of the theory!
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Old 04-25-2002, 08:52 PM   #5
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Apart from the position of the upper arms in relation to the lower A arm, another factor to consider is the center point of the base of the tire.

The lower the tire's center base point compared to the A arm the lower the roll centre. Assuming upper links no change. The TA04's steering knuckles (front uprights) are assymetrical and allow very fine tuning of the roll centre by swapping the knuckles.

One point of note: I have been searching around 1:1 scale websites and noticed that the trend is to use a low rc in the front and a higher rc in the rear.

front low rear high will give moderate turn in capability, but has very good grip out of the corner.

front high rear low will give very good turn in due to the large mount of initial camber change but tends to understeer out of the corner (no more roll out of corner).

I prefer using front low rear high, and if the turn in is insufficient I compensate with the spring positions front and rear.

(Disclaimer: I'm a newbie which is not yet a year old!)
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Old 04-26-2002, 04:37 AM   #6
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dw - my arms are converging inboard.

My refernce to positive camber was that I'm running the tyres quite upright to get even wear, and as the chassis itself actually rolls, the tyre is acquiring positive camber in relation to the surface, although not to the chassis itself.

The reason I started fiddling with the rear roll center in particular was because the cars tail had a tendency to twitch a lot on power and mid corner when the grip was low, regardless of the shocks settings. Moving to a more angled top link, i.e. a higher roll centre, but also giving more negative camber as the suspension was compressed, improving the mid-corner wheel angle, seemed to make the car a lot more driveable under power, and also increased rear end grip, giving me a bit of understeer.

I then tried angling the front upper link more, and that seemed to have a less drastic effect - If anything, it made the steering feel more inconsistent and less direct.

I then stiifened the rear springs a bit, to try and counter the understeer, and ended up with a car which seemd to have a nice balance, but was pretty un-smooth, and slow as a result.

Does this sound about right?
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Old 04-26-2002, 06:26 AM   #7
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an you confirm if you are running on a wooden floor or not?
It would be helpful to know how little overall grip there is.
Also what tyres and inserts.
Generally if grip is really low tyre wear is not an issue so camber angle is only relevant from the point of view of handling.
If you are getting even wear on the tyres by running them upright then the wheel can't be gaining a lot of positive camber in the corners. Otherwise the outside edge would be wearing.
Try drawing a line across the face of the tyre. This will allow you to see where the tyre is wearing more easily.

One critical thing I forgot to mention is suspension droop.
At the point when (if) the inside wheel reaches full droop the roll centre moves to the centre point of the inside wheels contact patch. Roll centre is then unaffected by the suspension linkage.

Rear roll centre changes will always have a bigger effect than front ones on our cars for the simple reason that there is more weight on the back axle.

Another thing which I don't know if you can do on a pro3, but if you can move the cells forward. This will make the car less responsive and stop the rear stepping out on power. At the same time you will find that under power you will have more steering.
Under braking steering is reduced.
You will actually have less forward grip coming out of the corners. Many people don't notice this though because of the increased rear sidebite.

It is very difficult to get a car to be smooth when grip is very low.
The only answer is careful throttle control. Adding exponential to your throttle on the transmitter can help. Don't run any drag brake. It may even be worth knocking your trim forward so you get a tiny bit of throttle in neutral.
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Old 04-27-2002, 04:12 AM   #8
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Yeah it is slippy floor, running yellow schumacher minipins with schumacher blue foam inserts.

Grip is very inconsistent - the evening will usually start quite slippy, and by the end of the night the grip will have come up so everyone's a couple of seconds a lap quicker - normally about a 15sec lap.

Although there's not much grip, i still rip through a set of tyres in about 12 runs.

That's an interesting point about the roll centre switching to the wheel's contact patch when it reaches full droop - perhaps if I increased the droop - currently about 2mm beyond ride height - I would get a smoother reacting car.

I can't switch the battery position.

I'll try experimenting with the throttle exp on the tranny - I already have the speedo's torque control on minimum (20A), but that's hardly had any effect on the car (which is among the quickest on power at the club)
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Old 04-27-2002, 06:50 AM   #9
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sosidge: by running a higher roll centre, you can afford to run less camber (like 0 or like -0.5 degrees). This means in a straight line you will have more contact patch on the ground.

While in the corners, the extra camber gain by a higher roll centre should be able to compensate for running very little camber.

The downpoint here could be an understeering car out of the apex. This may not be bad as it will allow you to lay down the power.

My 2 cents.

~Alvin
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Old 04-27-2002, 01:54 PM   #10
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Tyres sound right.
I've not run scale saloon much on wooden floors but what I found with my buggy was that any tyre compound other than Schumacher yellow was useless although blue would do at a push. Minipins gave most grip at the back. Minispikes were better on the front although didn't work on the rear as well fro some reason.
Have you or anybody else locally tried the schumacher Rally tyres?
They may give comparable grip but flat blocks shouldn't wear quite as badly.

I think you are overly concerned with roll centres. I found my b3 to be relatively insensitive to roll centres in those conditions.

What helped more with my B3 was tightening the diff. This allowed me to accelerate harder and gave more steering although you had to drive the car quite point and squirt and slide it about. Trying to be smooth didn't work.
1/8 scale circuit cars have similar problems because they have more power than traction. That is why they use solid rear axles and one way "diffs". This would also suggest that tighter diffs could be the way to go. I would think a one way diff would be too extreme though.

BTW when I say add exponential to your transmitter I should have said which direction to go in. You want to set it so the initial throttle takes longer to kick in. About 10-20%. You probably realised that though.

The changing grip levels may be down to rubber being deposited. There is another possibility as well. Some of the cleaning agents used on wooden floors aren't ideal for traction. They leave a thin film which is removed as you race. What my local club used to do was mop the floor down with just water before racing. They'd swear that this improved the traction when it dried. I don't know if it really made a difference but we never had much difference in lap times through the day. (Except for the first round when the track was often damp still.)

Anyway good luck getting your car sorted.
Wish I could race and wasn't stuck at uni preparing for finals.
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