Dynahead servo chassis flex

[Golden]

I’ve now built my Dynahead - and what a lovely process.

However - I’ve noticed that there is significant chassis flex from the servos (I’ve set up four wheel steer - the front and rear servos twist the body slightly…..) - and in particular, on fill lock, the rear left wheel seems to over pivot and then touch the left centre wheel. 
 

This seems to be caused by chassis flex - are there any chassis braces or other hop ups which might help? I’ve used the ds 3225 25kg servos - so might have over done it??

[OldSchoolRC1]

I used the same servos, they work great! I had to adjust the endpoints quite a bit, they were moving way to much initially and flexing the mounts as you describe.  I made a small extension for the rear bellcrank using some scrap aluminum (I don't have a pic unfortunately, I'm sure I could get one if it is helpful.) It was over pivoting and locking up the servo - It's so much smaller than the front bellcrank the the geometry is wierd. It seems to work much better now. 

 

[Golden]

Many thanks for this. I think the bell crank is shorter at the rear to limit the throw slightly, so as to change the geometry as you say? Be great if you wouldn’t mind sending a pic of what you did though if you don’t mind? 
 

I have dialled in the end points - but think I might knock-up a simple plate from some aluminium to see if that helps with rigidity on the rear servo. I am wondering whether the servos are simply too powerful though, despite restricting end points?

I have temporarily set up the four wheel steer with a Y connector for the servos - but am going to hack my flysky to see if I can set up four wheel steering using separate channels for each servo, as I wanted to play around with the crab feature - so might be able to further limit the rear steer (without ruining the front steering) I did give it a go with just the front servo - but the turning circle was horrendous!

[Busdriver]

7 hours ago, Golden said:

Many thanks for this. I think the bell crank is shorter at the rear to limit the throw slightly, so as to change the geometry as you say? Be great if you wouldn’t mind sending a pic of what you did though if you don’t mind? 
 

I have dialled in the end points - but think I might knock-up a simple plate from some aluminium to see if that helps with rigidity on the rear servo. I am wondering whether the servos are simply too powerful though, despite restricting end points?

I have temporarily set up the four wheel steer with a Y connector for the servos - but am going to hack my flysky to see if I can set up four wheel steering using separate channels for each servo, as I wanted to play around with the crab feature - so might be able to further limit the rear steer (without ruining the front steering) I did give it a go with just the front servo - but the turning circle was horrendous!

I have a 4 wheel steer module on my Dynamog. It allows 2wheel, 4 wheel, rear wheel and crab steer. The first two are as you would expect. Rear wheel is practically useless as you need a football pitch to turn in and crab steer makes the middle wheels drag. I think crab steer only really works on 4 wheeled vehicles unless of course all six wheels steered!!

[Golden]

1 hour ago, Busdriver said:

I have a 4 wheel steer module on my Dynamog. It allows 2wheel, 4 wheel, rear wheel and crab steer. The first two are as you would expect. Rear wheel is practically useless as you need a football pitch to turn in and crab steer makes the middle wheels drag. I think crab steer only really works on 4 wheeled vehicles unless of course all six wheels steered!!

Ah that’s annoying re crabbing. Really want to give that a go. I wonder if it’d run ok (on flat grass) without the middle wheels…. (Although kind of defeats the purpose of the six wheeler!). 

 

[alvinlwh]

.

[Golden]

Thanks. I’ll adjust a bit more.

[Busdriver]

6 hours ago, Golden said:

Ah that’s annoying re crabbing. Really want to give that a go. I wonder if it’d run ok (on flat grass) without the middle wheels…. (Although kind of defeats the purpose of the six wheeler!). 

 

It will crab but it kind of drags the middle wheels. If they freewheeled it might work better 

[Golden]

I think I’ve sorted the problem. Have adjusted end points - but the wheels would still push past the end points - so I added a screw to the servo linkage and it acts as a stopper against the plastic moulding in the chassis (which houses the retention screw for the linkage pin…). It does feel like I’ve lost a bit of steering angle - bit I think some of that was gained from chassis flex!

[Golden]

[Golden]

[Willy iine]

Endpoint adjustment should electronically prevent your servo from moving past that setting without any mechanical stops.. unless you are changing the sub-trim after you set your end points as they tend to parallel the amount of sub-trim applied (this is different from steering trim after you set sub-trim).

25kg is a powerful servo, I use it too on various cars.  Definitely set the end points correctly.  

Also, people may tell you otherwise, but I do not worry about having complete symmetry between left and right steering end points.  Some cars just do not line up perfectly and the servo is not smart enough to know this.  It will just follow your center and end point settings within the total servo travel range as if it was centered.  GL!

[OldSchoolRC1]

On 1/19/2022 at 6:16 PM, Golden said:

Many thanks for this. I think the bell crank is shorter at the rear to limit the throw slightly, so as to change the geometry as you say? Be great if you wouldn’t mind sending a pic of what you did though if you don’t mind? 
 

I made the crank much longer, it was moving too far and locking up....even if I turned down the endpoints to 30% or so. It's much better this way, I get nearly full throw now with the servo and the crank moves perfectly.  Hope this pic works, I took it from the opposite side of the rear servo so it was clearer. My camera does not macro well. 

[Golden]

Ah that’s a great idea. Wouldn’t have thought of that. Might give it a go - as steering is slightly limited - probably got another 10mm of turn before the wheels would be in danger of touching with the way I’ve limited the movement. Thanks for posting!