Rear Shimmy Shake...

B_Regal Racing

0
Pro Racer
Apr 1, 2014
1,609
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...but I just do not understand why.

I've asked this before, but have to ask again with some better information and a few builds under my belt. I have a slight wiggle in the rear. Adding steer does not remove it. I can remove it by increasing the weight on the FDW, but this also slows the car down. In past races, I have seen cars shimmy like this; its not banging into the rail but just has a slight rear wiggle. Increasing the wheel gap on the rears exacerbates the wiggle, and I can almost eliminate it by decreasing the wheel gap. I was wondering if there is something I could do in my next build to eliminate this wiggle. Is it the drill, a slightly bent axle, weighting, wheels out of balance? It just seems to mean the less wiggle a car has (e.g. the more stable), the faster it would be.

Thoughts?

Car specs:
Body Thickness: 1/4"
Body Style: Ladder with Balsa ribs
Body Covering: Monokote
Fenders: Yes
Friction Plate: UHMW Polyethylene Film
Rear Axle: 3/4" from Rear
Rear Wheel Cant (drill): 3 degrees
Wheelbase: 4 3/8" (stock dist.)
Wheel Gap: a small as possible on rears
Wheels: DD4H 2g (Cheetah)
Axles: DD4H Dynasty Pro 92X
Left Rear: 2.430oz
Right Rear: 2.085oz
Right FDW: 0.485oz
Front Wheel Cant (drill): 4 degrees
Front Axle Bend: 1 degree (to adj steer)
Steer: 2 7/8" over 4'
 
BRR - when my SS car Happy was doin' the same thing the culprit was wide wheel bores in the rear wheels and the rear wheels were of slightly different diameter. Switching the rear wheels might help you too.
Maglev
 
Tighten the front end up.. Then change the rigjt front axle adding more bend.. This will be a good starting point. Shaking in the rear has nothing to do with wheel bored & wheels.. Another major factor is weight placement..Offset #, right front wheel # width of front of body and what side you narrowed on the front and if it was too much.. There is many variables.. I would start with my first two things and go from there. I personally can take the wiggles out of my car by staging it different. Also could be like of steer and cause it to fight to stay on the rail.. Hope I didn't lose you... I just gave A lot of info to AM..
 
~JBD RACING~ said:
Tighten the front end up.. Then change the rigjt front axle adding more bend.. This will be a good starting point. Shaking in the rear has nothing to do with wheel bored & wheels.. Another major factor is weight placement..Offset #, right front wheel # width of front of body and what side you narrowed on the front and if it was too much.. There is many variables.. I would start with my first two things and go from there. I personally can take the wiggles out of my car by staging it different. Also could be like of steer and cause it to fight to stay on the rail.. Hope I didn't lose you... I just gave A lot of info to AM..
I had to digest this and then disect it. The wiggle is in the rear, and it probably grazes the rail, but not "banging" into it. The front stays to the rail with no problems and between a 4 degree drilled cant and an extra 1 degree bend in the axle for steer, I have ~ 5 degrees cant in the front.

In most cars, I can usually remove this with the weight bias, which I usually do by moving 2 cubes around. In this case, that did not work. Maybe I needed more bias weight on this build?

One thing I did not understand was "tighten up the front end." For rear wheel gaps, I leave as little gap as possible (I have no way of measuring this), but for reference, it would be much smaller than the popular credit card sized gap. The FDW gap is ~ twice the size of the rear gaps. If this statement is in regard to the narrowing of the front end, I narrowed the front by a 1/16", and between the FDW cant and the narrowing, the FDW and the NDFW are 1 7/8" apart, just enough to clear the rail on both sides.

I can (probably will) change the sides in which the rear wheels are located if that would help. With this car, excessive rear wheel gap will make the wiggling worse, but I have cars where I can run a larger rear wheel gap (I don't, but could) with little to no problems. Just curious as to what caused this problem so I do not make it again in the future.
 
I'm a newbie to pro pinewood derby. Sorry. What do you guys use as a gap gauge between pine, and plastic? Credit Card too thick? Business Card? Thin sheet metal? Paper???
I have two new cars built,(4 more to build) and the faster one gets a shimmy towards the last few feet of the track, short/maybe the last two feet, but who wants to slam on the brakes at the finish line?
My test track is homemade/metal/32' long with a 4' drop to a 20' flat.
 
I use a business card it is about .010 thick.
smile
 
B_Regal---There are a lot of variables...but I have found the cause to the "wiggles" on my cars in the past is poorly drilled rear axles. I have learned my mistakes were not a perfect setup drilling rear axle holes. So after drilling rear holes, I use a trick I learned on another PWD site. I place a piece of clear shipping tape that covers bottom area of car at the nose. Insert a "test" set of axles& wheels to rear of car. Place some weight over rear axle and place car on tuning board. Let car roll slowly on board. I am mainly concerned the rear wheels stay at axle heads. Not too concerned if car doesn't roll straight, but straight is great! If a wheel walks inward on the axle, the slightest bit, I discard that block and start over.
With this process, I can usually correct any problems with adjusting steer. Just my 2 cents.
You may have already checked the rear axle holes for squareness.