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Old 02-17-2017, 08:31 AM   #1
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Default Suspension tuning: Stadium Truck rear end bouncing

Hey all,
I'm prepping my Stadium to truck for racing and I'm tuning the suspension. Unfortunately the rear end is bouncing and I'm not sure what's going on.

Setup:
ECX Circuit (I know, I know)
Schumacher minipin yellow compound truck tires (rear), red AKA foams, JConcepts T4 wheels.
Traxxas Big Bore shocks, stock Big Bore pistons, Losi Orange springs (2.9 rate).

When I drop the car from 8-12 inches, the front plants, but the rear end bounces (specifically, it will "hop" once). I started out with 40wt oil. Watching the suspension work, it seems like the tires are bouncing before the suspension compresses fully. I thought maybe the oil was too viscous (not responding quickly enough) so I switched to 32.5wt. This didn't eliminate the problem. With both weights of oil, dropping the car from a couple of feet (simulating over shooting a table top- hey, it happens) the car bottoms out and then bounces as before.

I haven't yet tried 30wt oil.

At the front, the truck has Schumacher stagger rib truck tires, blue compound with red AKA inserts. Traxxas Big Bores, stock pistons, Losi red springs (2.5 rate).

Race day is this Saturday so I only have tonight to make any significant tweaks. New tires/foams are out of the question before tomorrow so I'm going to have to make lemonade out of these lemons.

I appreciate any tips or hints.

Last edited by i_r_beej; 02-17-2017 at 02:35 PM.
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Old 02-17-2017, 09:37 AM   #2
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The only 2 oils I use are 30 and 27.5, 30 in the summer and 27.5 in the winter. The drop test isn't really a good test, because under race conditions the truck is moving and the forward momentum takes some of the impact. Also if you land on the throttle slightly it will bind up your drive line and that will also soak up your landing. For me it would be hard to tell what your suspension package is like without feeling it. I could bounce it and go yep to springy, or oil is too thick. Just remember the lighter the spring the lighter the oil, the stiffer the spring the thicker the oil. Most of the time if the car is bouncing all over the place its because the oil is way to thin and too springy or the oil is too thick and its bouncing up through the tires.
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Old 02-17-2017, 10:55 AM   #3
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Maybe your piston c clips popped out and your piston is just floating in the shock body? This is a common occurrence.

Take apart your shock shaft and make sure that your C clips are still holding the piston where it's supposed to be. Don't be fooled by checking it while they're on the car. Take the shafts out and grab the piston to see if it stays put.

If it's fine and truck still bounces, you might have too much weight in the rear End. The suspension shouldn't bounce much with 40wt oil
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Old 02-17-2017, 11:18 AM   #4
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Thanks @greener74. I'm planning on arriving early at the track tomorrow and running practice laps. I'll bring a couple of different weights and experiment.

@ZEe_NYC, the shocks are intact. I had them apart just last night and everything was in place. And you're right about the weight- this car is a rear-motor. I'm using a shorty and have pushed it as far forward as possible to try to offset the rear weight bias.
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Old 02-17-2017, 11:47 AM   #5
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I'm not really an ECX or Traxxas guy but, Most stadium trucks would run close to a 3.0 spring rate in the front with 2 hole 1.6 or 3 hole 1.4 pistons and 35 wt oil.
The rear is usually between a 2.3~2.5 spring rate with 2 hole 1.6 or 1.7 pistons and around 30 wt oil.
So did you mix up your springs or even swap front/rear shocks? The front shocks are usually 5mm shorter.
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Old 02-17-2017, 12:36 PM   #6
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You said the tires were bouncing... did you punch holes in the tires?
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Old 02-17-2017, 02:11 PM   #7
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Thanks @Eddie_E.
Part of the challenges in tuning the suspension on this car is that it has a pronounced rear weight bias so the front is VERY light. (I've added 30g under the bellcranks). So my efforts to get the front to sag may have sabotaged the rear suspension performance. Keeping your comments in mind, I'll revisit my setup tonight.

@Callaway, this is a 1/10 car and the wheels are vented. So I did not punch the tires, but that may be a last resort. The tires really only bounce once, like a hop.

Last edited by i_r_beej; 02-17-2017 at 02:37 PM.
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Old 02-17-2017, 03:55 PM   #8
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Quote:
Originally Posted by i_r_beej View Post
@Callaway, this is a 1/10 car and the wheels are vented. So I did not punch the tires, but that may be a last resort. The tires really only bounce once, like a hop.
I understand it's 1/10... it's a stadium truck. If the foams are closed cell, air won't flow through and vent out the inside of the wheel. So with closed cell foam we vent the tires. I've also heard of people punching holes in the foams and lining it up with the vent in the wheels if they don't want to vent the tire.

Also, if you're dropping it on your carpet floor, the padding under the carpet can make it bounce, but it won't bounce at the track. I race regularly at 2 carpet tracks... buggy, SCT, and truck.
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Old 02-17-2017, 09:42 PM   #9
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Sorry @Callaway- I wasn't trying to be snarky. I appreciate your suggestions!
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Old 02-18-2017, 06:59 PM   #10
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Quote:
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Sorry @Callaway- I wasn't trying to be snarky. I appreciate your suggestions!
How did the race go?
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Old 02-18-2017, 10:26 PM   #11
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Race went okay. I had a few issues with understeering and at one point the pinion fell off! (The Hobbywing 3650 motor has a shaft that's a bit short and the Robinson pinion was difficult to get fixed.) The truck handles very different from my 22 3.0! But I was able to get the handling dialed and it was pretty predictable after that.

Thanks everyone for the tips!
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