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Old 08-12-2014, 03:32 PM   #1
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Default A Question of Balance

Still trying to learn how to tame this beast. I feel like I'm half way there but don't know where to go next.

After a few test days I've finally gotten a car that steers well at speed. I'm very happy with my front end performance but I'm still fighting a wagging tail at power on. I'm familiar with the Learn Setup and RC Crew Chief apps and have some idea of particular changes and their effects.

The question I have is where to spend my next few test sessions. I'm very reluctant to start to free up the front in an attempt to fix the rear. Even if I'm going to some non-standard rear end set ups, I'm inclined to spend time getting the rear hooked up rather than work to get both ends of the car to equally well, or poorly

So am I on a fools errand working only on one end of the set up or do I double down on my current strengths? I do realize that in the end I will have to work on over all balance, but with one end working well and the other out to lunch I'm not sure where it is best to spend my time.

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Old 08-13-2014, 09:11 AM   #2
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Its very difficult if not impossible to offer any constructive advice without knowing where you are starting from setup wise. If you are using RC Crew Chief then I always start with a neutral setup and then work from there. The best way to get there is to use the Mid Corner Graphic on the Handling page and adjust to get the push-loose bar graph neutral. Then you should have a good place to start from.

If you want use the "Car Manager" to export the Car File and send it to me. [email protected]
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Old 08-13-2014, 01:33 PM   #3
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Bob: Thanks for the reply. As far as set up, I could post one but it would not be particularly drivable. As of yet, the new car is not acting as theory might predict.

I started with the basic asphalt set up from the XRay site. The car would push going in and through the middle then in out on power. I changed the springs, roll bar, shock position, roll center, arm sweep, and camber settings until I ended up with a front end that sticks like glue, ( RC Crew theory says I should have an entry push ).

I've centered the CG about 49/51 as best as I could tell, worked with various chassis flex, suspension settings, springs, droop, ride height, track width, check tweak and aero but have yet to keep the tail under it for power on out of slow speed corners. The SW and other tuning sources are telling me to loosen up the front end to balance the car, ( Captain Obvious ). All I get with that approach thus far is a car that pushes before it spins out.

So, back to the basic question. Do I have to break the front end and back up to a car that simply drifts nicely and rework from there or am I better off continuing to work to get the rear under it before I start compromising to get the best average performance?

I'm getting back into this after a few decades of time off for bad behavior. When we first started working with 4wd, we had one way front bearings and ran a touch of over drive so kick out wasn't a problem. What we were taught was you never broke one end of the car to get the other end working. With the new car designs and equal ratio drives, my old ways may have been overcome by events. Thus it is an honest question on the approach to use, not on the particular conditions and set up.
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Old 08-13-2014, 03:56 PM   #4
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I have never seen a prediction in RC3 that shows a corner entry push for a TC. That tells me there is something wrong with the model you are using. It's very easy to export the car file and send it to me. Takes less than a minute. If you don't want to do that then send me a setup sheet.

I don't want to offer any advice unless I know where you are at.
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Old 08-14-2014, 09:38 AM   #5
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If I bought a car sight unseen and it had the behaviour you describe, I would check these things first, in this order:

How tight is the rear diff?

What is the ride height front and rear?

What is the rear toe in?

You didn't tell us what class you're racing either, so we're really in the dark here.
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Old 08-14-2014, 07:00 PM   #6
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Quote:
Originally Posted by niznai View Post
If I bought a car sight unseen and it had the behaviour you describe, I would check these things first, in this order:

How tight is the rear diff?

What is the ride height front and rear?

What is the rear toe in?

You didn't tell us what class you're racing either, so we're really in the dark here.
Thanks for the ideas. I have in fact checked the diff, ( 1K gets me a diff snap and 3K seems to overpower the track ), the ride height & droop, ( did find I was dragging the tail on power which caused part of the problem ) and having been running between 2 1/2 to 3 degrees of rear toe.

Your reply did give me the impression that I should look towards the rear for a problem which does answer my question. After thinking about your suggestions and reviewing my set up tests, I'm leaning towards a problem with belt tension or something making the car behave more like an overpowered 2wd than pulling with 4.
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Old 08-15-2014, 09:31 AM   #7
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What do you mean by diff snap?

The rear end stepping out means your rear wheels can't follow the front for whatever reason. One of the reasons is a slow to react diff, but yours seems okay (if it is built correctly, which I assume it is).

There shouldn't be that much difference between 3k and 1k diff (both pretty low), should be okay unless you're on a really low grip track.

Toe should be okay, if need be, go to the higher value, but not really a biggie.

How much roll do you have? Is it enough to make use of some camber? If so, add say about 1.5-2 deg negative (we're talking rear end here) and take it from there. If not, maybe move the shocks in one hole (at the top) and add the camber.

When you're happy with ride height add some antisquat if car still drags its tail on power.

I don't believe the belt tension would have any effect, unless it was too tight or too slack. That checked, I don't think it can bias drive.

These are the kind of things I would be looking at.

As for software, I hate the stuff. If I liked it, I wouldn't get my hands dirty screwing away with real (toy) cars. Leave software for kids who want to play video games.
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Last edited by niznai; 08-15-2014 at 10:03 AM.
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Old 08-15-2014, 10:31 AM   #8
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i'd say if the difference between the two ends are that big, you might want to just try and fix one end instead of bringing down your front grip.

anyway, these are the first the things i'll check:
- tires
- ride height
- chassis (check the rear sides (L&R), the area near the motor where it begins to taper towards the suspension blocks)
- front droop
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Old 08-15-2014, 11:22 AM   #9
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Originally Posted by niznai View Post
What do you mean by diff snap?.
Diff Snap is a corner reaction to having too much differ action. The powered tire drives the car to the point where it looses traction.. The car turns great in the corner up to a point then the rear end jumps out with the loss of traction. For a better and more extended definition I would refer you it Carroll Smith's excellent Engineering to Win series which also covers the cures.

Having mentioned belts got me thinking. Back in the day, we learned to run the belts as loose as possible to reducing friction. I did a quick check and even with my rear belt set as loose as possible, the front belt still has significantly more slack on part due to the longer length and part to my setting it up for smoothness. I know the car has much less grunt than the field, so I may be getting front belt slip and not know how to detect it. It would certainly explain a lot of ghosts if it were true. I just tightened it up and will try it at tuning day tomorrow. Thanks for the prompt. Next round is on me.
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Old 08-15-2014, 11:35 AM   #10
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Quote:
Originally Posted by Minami View Post
i'd say if the difference between the two ends are that big, you might want to just try and fix one end instead of bringing down your front grip.

anyway, these are the first the things i'll check:
- tires
- ride height
- chassis (check the rear sides (L&R), the area near the motor where it begins to taper towards the suspension blocks)
- front droop
Thanks for the advice, I'll take it. I'm pretty sure i did have some height issues at one point but hopefully those have now been addressed, ( at least my tape tell tales say I'm no longer dragging ). Certainly worth rechecking.

The only reason I discount tires is the car is reacting roughly the same on different tracks using different tires. It's something I'm bringing with me before the tires are installed. Interestingly enough, the tires do wear evenly across the face, side to side and front to rear.
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Old 08-15-2014, 01:11 PM   #11
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Quote:
Originally Posted by PACaSteel View Post
Diff Snap is a corner reaction to having too much differ action. The powered tire drives the car to the point where it looses traction.. The car turns great in the corner up to a point then the rear end jumps out with the loss of traction. For a better and more extended definition I would refer you it Carroll Smith's excellent Engineering to Win series which also covers the cures.

Having mentioned belts got me thinking. Back in the day, we learned to run the belts as loose as possible to reducing friction. I did a quick check and even with my rear belt set as loose as possible, the front belt still has significantly more slack on part due to the longer length and part to my setting it up for smoothness. I know the car has much less grunt than the field, so I may be getting front belt slip and not know how to detect it. It would certainly explain a lot of ghosts if it were true. I just tightened it up and will try it at tuning day tomorrow. Thanks for the prompt. Next round is on me.

I don't understand why is the front belt tension dependent on the rear belt tension. You should be able to adjust independently unless you're running a single belt car (which I suspect you don't).

If the belt skips, you'll hear a clicking noise. Drive past the rostrum at full chat and slam the brakes. If there's belt slip you'll see it this way immediately.

As for the diff problem, I think I understand your problem. Seems like your rear end doesn't load enough under acceleration. Another suggestion would be to move some of the spacers on the rear hingepins behind the arms (effectively moving the arms forward a bit) to have more weight on the rear. This is perhaps one of the first things to try.
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