Pinewood Wobble Problem

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pketcham
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Pinewood Wobble Problem

Post by pketcham »

Need help with WOBBLE please!

We just finish out Pack race and the car had A LOT of wobble coming down. Seems to start coming out of the arch onto the straightway. I saw signs of this on the test track last night at the workshop we attend, but we pushed it this year and didn't have time to tweak. We were lucky and placed 5th, so were onto the District race. Still, if we can't get rid of the wobble, our times of 4.0309 won't hold up against season scout racers!

THE CAR FACTS
Wheels are moved out to maximum spacing front and back.

The CG/COM is pretty aggressive at around and .6" in front of the rear axle.

The car is set up to Rail Ride. The current alignment is agressive, allowing the car to turn in toward the rail 2" over 3'.

Rear axles have NO can't (nose of car is slightly narrower so I don't need to worry about rear wheels touching the center rail.

The dominant front axle has 1.5 cant and installed for [junk] camber.

My son built his car for the first time this year all by himself (with my supervision). Together, we've learned a lot and have done very, very well the past 2 years. Our track is a Best Track (49' horizontal). Last year my sons car took third in the Council race with a time of 3.856" Our design was almost identical to last years, but we wanted to try and squeeze a little more speed and it seemed like this was the place to do it. The COM for this car was at around .9"

Could my weight placement be causing this instability?

A couple other measurements for reference: the font of the car weighs 15 grams with wheels on. The cars overall weight is 143.00 grams or 5.05 oz. Wheels are lathe turned to center and make concentric. There are no flaws in the axels to contribute to this problem.

Any input from you all would be greatly appreciated. Especially focusing on the COM location as a possible culprit?

We've made it on to districts by the skin of our teeth with a time of 4.0309. We have a couple months to sort this out.

Thanks in advance to everyone kind enough to offer their insight!
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sporty
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Re: Pinewood Wobble Problem

Post by sporty »

I have really never gone less than 3/8ths of a inche forward of the rear axle whole/ slot.

And the drfit, well I usually measure at 4 feet not 3 feet.

15 grams on the front with wheels on. Thats right there on the edge of things.

You did reduce the wood on the RR side, 1/16th right ? I did not see that mentioned and I thought i better ask.

Can you pm me a pciture ? or two ?
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FatSebastian
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Re: Pinewood Wobble Problem

Post by FatSebastian »

:welcome:
pketcham wrote:Rear axles have NO can't (nose of car is slightly narrower so I don't need to worry about rear wheels touching the center rail.
Of course if the rear end is wobbling, you need to worry about rear wheels touching the center rail.

Rear axle camber not only helps with keeping the wheels away from the rail, it also helps with rear-end stability and minimizing rolling friction, that is, tread contact with track. I would suggest 2.5 degrees negative camber on the rear wheels if your rules allows. Also, did you evaluate your rear wheel alignment?

Agree with Sporty about the design being "on the edge of things." Advancing the CoM a bit and getting more weight on the front wheel will almost certainly help.
pketcham wrote:Wheels are moved out to maximum spacing front and back.
Not 100% sure what this means, but the maximum clearance between the wheel hub and the body should be about 0.035" (~1/32") - not excessive. This minimizes the opportunity for the wheels to slide around on the axles and promotes stability.
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whodathunkit
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Re: Pinewood Wobble Problem

Post by whodathunkit »

FatSebastian wrote::welcome:
pketcham wrote:Rear axles have NO can't (nose of car is slightly narrower so I don't need to worry about rear wheels touching the center rail.
Of course if the rear end is wobbling, you need to worry about rear wheels touching the center rail.

Rear axle camber not only helps with keeping the wheels away from the rail, it also helps with rear-end stability and minimizing rolling friction, that is, tread contact with track. I would suggest 2.5 degrees negative camber on the rear wheels if your rules allows. Also, did you evaluate your rear wheel alignment?

Agree with Sporty about the design being "on the edge of things." Advancing the CoM a bit and getting more weight on the front wheel will almost certainly help.
pketcham wrote:Wheels are moved out to maximum spacing front and back.
Not 100% sure what this means, but the maximum clearance between the wheel hub and the body should be about 0.035" (~1/32") - not excessive. This minimizes the opportunity for the wheels to slide around on the axles and promotes stability.
;)
What type of automobile can be spelled the same forwards & backwards?
pketcham
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Re: Pinewood Wobble Problem

Post by pketcham »

Thank you Shorty and FatSebastian for your comments.

The wheels were simply moved out to extend the wheelbase to the ends of the body.

The rear axels were drilled on a drill press at a near prefer 90 degrees square. They were not canted at all so there was nothing to do with alignment. I have never found the canting of rear wheels to help me much, but the stability is something to consider. At any rate, I checked the car on a test segment of track and the rear.

It almost seems as though the front is steering into the center-strip and then ricocheting off of it, which starts this pingpong back and forth. Is that even possible? I could be wrong here. It is hard to tell if it is the rear shifting or the front.

I'll post pictures later today to help illustrate.
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FatSebastian
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Re: Pinewood Wobble Problem

Post by FatSebastian »

pketcham wrote:The wheels were simply moved out to extend the wheelbase to the ends of the body.
Okay - thanks!
pketcham wrote:The rear axles were drilled on a drill press at a near prefer 90 degrees square. They were not canted at all so there was nothing to do with alignment.
"Near perfect" means "not perfect"! ;) Also, nail-type axles are not perfectly straight, you can usually rotate them and see some improvement, especially when running flat (uncambered) axles. I recommend that you go through the exercise of checking rear-wheel alignment anyway. It is presumed to be a common contributor of instability on aggressively weighted cars.
pketcham wrote:...ricocheting... Is that even possible?
Yes, if there is insufficient traction on the front wheel. We experienced this with an experimental car we built with ~1/2" CoM. It was slow; to control required extreme steer-in. Moving the CoM forward can correct this.
pketcham
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Re: Pinewood Wobble Problem

Post by pketcham »

Just to clarify, when you are referring to COM dimension, are you referencing this in a "wheels on" or "wheels off" state? I want to make sure I am referencing the correct information to you.
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FatSebastian
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Re: Pinewood Wobble Problem

Post by FatSebastian »

pketcham wrote:Just to clarify, when you are referring to COM dimension, are you referencing this in a "wheels on" or "wheels off" state?
Sorry. Wheels on (finished car), CoM distance relative to the rear axle line. You can convert between front wheel weight and CoM location using this equation.

Moving the DFW backward can also help.
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sporty
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Re: Pinewood Wobble Problem

Post by sporty »

Sounds like not enough drift or one ruff track.

But if yu have adjustable weight, you can move it about 1/2 inch forward from where it is now and should certainly reduce this or eliminate it also.
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psycaz
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Re: Pinewood Wobble Problem

Post by psycaz »

I tend think that Sporty may have nailed it I. The last post, not enough steer.

I would think somewhere between 2.5" and 3" would be it.

Do you have access to a test track? That would allow you to try to dial it in.

Maybe consider drilling second set of rear axle holes, making those with a 2.5 degree camber just in front of the existing ones.

That would let you try that as a fix as well.
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Stan Pope
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Re: Pinewood Wobble Problem

Post by Stan Pope »

Fellers who seem to know recommend minimum horizontal distance from rear axle to CM as 5/8" (0.625") on short tracks and 3/4" (0.750") on long tracks. From your description, I think you are running what they would call a "long track." Also you have changed from a 0.9" CM distance (above both recommendations) to a 0.6" CM distance (below both recommendations and into their "wobble zone".) Based on those recommendations, you "overshot" the CM move!

They don't say what the wheelbases are for those distances, and they might be entangled. You can compute the front wheel weight depending on whether the wheelbase is 4-3/8" (slots) or 5-3/4" (yours), and then reach your own conclusions considering the following paragraphs.

As I understand, the stability issue with CM location is the weight which it places on the front wheel(s). If the weight gets too low, it takes too much toe-in of the DFW to accomplish stability, and performance falls off or there is simply insufficient down force on the DFW to accomplish steering, and the car wiggles. But as the car is lengthened, the DFW downforce for a given CM distance decreases!

If you have time to experiment and you can extract a fraction of an ounce from the tail end of the car and move it forward, you can easily experiment with the effect of various CM locations before you commit to digging a hole in your finished car. Just tape it on top of the spot you would embed it! If you can't handle the math (try it first anyway), you can use my CM computation page which supports such computations for configuration changes.
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