LzREngineering

When my TC is going round the sweepers on track, I can often see the wheels shaking left to right quite violently, as if it's struggling to keep them pointing in the right direction.
I'm thinking this must be pretty bad for the car's stability - can this be caused by a weak servo? or is it more likely to be a suspension/wheel balance issue?

Btw. The servo I'm using is an old multiplex model, it was a good spec for its time (metal gear, digital etc) but have things moved on much?

Sean B.

iTrader: (13)

It’s most likely the drive shafts binding while at full steering lock. Do you have DCJ’s (ECS)?
You should be able to recreate it on a car stand on the bench. Give some throttle and turn the wheels and see how it acts.

LzREngineering

That may be the case, though I don't recall it being much trouble last time I had it on the bench. Although there's no lateral load on the tyres when the car's not on the track so that could be why.

I don't have double cardan driveshafts on the front BUT I also don't run a spool and my steering lock is not set to the extreme, so I'm hoping the driveshafts aren't a major cause of vibration. (The wheel movement looks too slow to be caused by the driveshaft but it's hard to tell without filming it on a high fps camera)

I am using the forward ackermann point on the steering knuckles, this would give less steering torque/leverage to the servo wouldn't it? I'm working on a solution so I can use the rear ackermann point and keep the same steering angles but increase the torque of the steering linkages. Just wondering if it's worth upgrading the servo at the same time?

nubs

iTrader: (2)

Doesn't matter if you have a spool. What matters is the angle of the turn. The "chatter"comes from the friction between the dog bone and the out drive of the gear diff. The sharper the turn, the more friction. Highly suggest getting double joints. It will improve control, speed up your cornering and lengthen the lifetime of the out drives.

tobyzhang

iTrader: (15)

I think it's a combination of mechanical flex, dynamics of the universal joints, and possibly the control algorithm of the servo you are using.

mechanical flex will just be the forces while the car is corning causing elastic behaviors in the suspension components.

if you're front drive shafts are not double cardan, it will cause acceleration and deceleration in the rotational velocity when the joint is rotated. This usually gives energy to the imbalance of the rotational components to cause various motions.

Third is the servo's own control algorithm. Some servo are more stable and reacts faster to envirnomental inputs and others are slower. Now a day servos are pushing hard on response speed and transitional speeds etc they often use anticipation algorithm and very high gains. This result is a servo that is very fast but will over shoot often. Hence you hear the chattering in servo when you apply force to it.

combine all of these in the system you get chattering mid corner like you observed. However if your car is still stable while the steering is doing all that motion, the car is still considered stable. You will lose ultimate grip or steering from that though so still not a situation you want.

glennhl

iTrader: (16)

I'm pretty sure the only way to fix it is to get a Rockwell Retro Encabulator.

https://m.youtube.com/watch?v=RXJKdh1KZ0w

gigaplex

iTrader: (2)

It's not caused by friction. A cardan joint doesn't rotate at a constant speed when bent, which causes oscillations. A diff instead of a spool can help dampen it a bit, but a double cardan joint is the proper solution. The dual joints cancel each other out with regards to the uneven rotational speed.

Skiddins

I would start by trying a newer servo and see if that solves it.

LzREngineering

Hoping to get hold of one soon! What's a good one to go for? Also gonna try a shorter servo horn to get some more torque.
Unfortunately double cardan joints aren't available on my car (x-ray t1fk05) but I don't think UJ vibration is the main cause in this case anyway due to the shallow steering angle (it doesn't chatter badly on hairpins, for example)

Skiddins

If you don't want to spend big money I'd go for a Savox

I've run standard UJ's in the front with a spool of far more recent cars like my old T4'17, and didn't notice anything strange going on with the wheels, hence my suggestion that I'd start with the servo.
Worst case, you now have a spare servo.

nubs

iTrader: (2)

Yes, it is caused by friction. CV joints travel at CONSTANT VELOCITY until the friction causes the joints to momentarily slow down, creating vibration and chatter. This friction takes place at extreme angles. And I misspoke earlier when I said the dog bone and the outdrive. I meant the axle and shaft. The exact same thing would happen to a DCJ if the angle were extreme enough.

tobyzhang

iTrader: (15)

Unfortunately the manufactures are lying when they say their universal joints are constant velocity. You can take a look at the wikipedia page: https://en.wikipedia.org/wiki/Constant-velocity_joint

DCJ is a true CV joint and will not create vibration the same way universal does when the front wheels are turned.

nubs

iTrader: (2)

Those bastards.

Then I apologize for my incorrect statement.

https://en.wikipedia.org/wiki/Universal_joint

snuvet75

iTrader: (16)

Just see if the wheels chatter on the bench when they are turned. when they do, check servo horns to see if it's moving. If not, servo is not the issue. If they don't chatter on the bench, it's tire issue. Either tire themselves, ackermann or something else around it. Process of elimination dude. Good servo always helps in many ways but unnecessary investment should also be avoided unless you have lots of expendable funds . In the same token, if you don't have a servo equal to or better than Savox, you're not doing it right. So you should buy one regardless of this issue.

LzREngineering

So, dont get a servo but do get a servo?
What sort of torque/angular speed specs are you guys running in terms of servo?