Midnight Pumpkin - Steering Mounts Breaking

[Jonrees 2]

Hi.  Newbie here probably doing something stupid but need a little advice.  I have 2 midnight pumpkins, the first I built last year (lockdown project) and can 100% stock with 5kg steering servo and standard servo saver as supplied in the kit.  This model has been great fun on rough ground, tarmac and even drifting on gravel.  the steering servo is great smooth, quiet with no electrical chatter at all.  No issues with this pumpkin apart from the body mounts.

I have just built one for a friend and this pumpkin came with a 15kg metal geared steering servo in the box.  After around 2 minutes of running on a smooth concrete surface I lost all steering.  When applying the brakes at speed these pumpkins always do a bit of a handbrake turn which will obviously put some strain on the servo but this has never been a problem on my original pumpkin.   I popped the body off and the steering mounts have both cracked leaving the steering servo flopping around.  This is the second time this has happened and I have replaced the chassis already when the previous one broken.

No idea what's gone wrong here.  Is the steering servo too powerful or has the servo saver failed?   Should I be using the Tamiya high torque 51000 servo saver or should I pick up a 5kg servo despite and remove the 15kg supplied with the kit?  I checked the chassis during assembly and it was solid with no cracks.   

Any help much appreciated.   I've loved my own pumpkin but feeling bad for a friend who's pumpkin is sat their looking sorry for itself.  Annoying thing about the chassis being wrecked is you have to virtually rebuild from scratch!

Any help much appreciated!

 

[topforcein 1764]

i would say the servo's are to powerful for a pumpkin/lunchbox as it is serious overkill just a standard cheapo one will do remember the front really has no weight over it to need such a servo plus these tend to spend most of there time on the back wheels anyway

just get your self a new chassis  and swop all over if you get a cheapo servo then the kit servo saver will be fine 

[Tamiyabigstuff 831]

^^^

What he said.

Basic servo is fine for this - probably having a servo too powerful is what broke it.

[MadInventor 3884]

The only other thing I can think of is that you've overtightened the screws mounting the servo mounts to the chassis and weakened the plastic round the screws. 

[Dakratfink 965]

56 minutes ago, Tamiyabigstuff said:

^^^

What he said.

Basic servo is fine for this - probably having a servo too powerful is what broke it.

On top of that, if your radio allows you to set end points try setting them so the servo is stopping before the mechanical limits of the steering itself 

[Bash 356]

I'd say that improperly set endpoints is the issue you are having. What transmitter are you using?

[Juggular 4961]

I wonder if you can cut up a 2mm aluminum plate and fix that... (I like ghetto fixes)

Servo saver should help a lot. I think of it as putting a rubber pad between servo and the chassis. Obviously, it's not that. It's a spring between the tie rods and the servo itself.  But the end result should be similar; the force from the ground is absorbed by the C-springs.

And I would definitely check what @Dakratfink said; see if the servo tries to go too far.  15kg is strong enough to break stuff. 

 

[silvertriple 3557]

16 minutes ago, Juggular said:

I wonder if you can cut up a 2mm aluminum plate and fix that... (I like ghetto fixes)

Servo saver should help a lot. I think of it as putting a rubber pad between servo and the chassis. Obviously, it's not that. It's a spring between the tie rods and the servo itself.  But the end result should be similar; the force from the ground is absorbed by the C-springs.

And I would definitely check what @Dakratfink said; see if the servo tries to go too far.  15kg is strong enough to break stuff. 

 

Yes, exactly what I have in mind when I read the initial post... And while at it, shocks brace as well to join the shock towers and reinforce the chassis.

I use a 3 kg plastic servo in my lunchbox, and it's actually largely enough for it with the standard servo saver, with probably less risk for the chassis. A metal plate to reinforce the chassis and servo attachement is a must with a high torque servo saver and a bigger torque servo...

[BattleTrak 58]

This stood out to me as a weak spot when I built my Lunchbox.  I cut and bent a piece  of 2mm aluminum to reinforce the area similar to what @Juggular described.

[alvinlwh 5891]

Excessive servo strength + incorrect end points and probably will be made worse if you used 51000. Remove one of them from the equation and you will probably be fine.

My Lunchbox using an extremely basic servo + basic no endpoints 27mhz TX from over a decade ago had been fine all along. When I swapped in a 51000, I also upgraded the radio gear to 2.4 with endpoints adjustments. 

[Jonrees 2]

Thanks all for the comments / feedback.  Apologies for the delay in responding.

I'm using an Absima CR3P on my original pumpkin and I've set the endpoints to the steering stops before it pushes against the chassis.  On the problem pumpkin I only have an Absima CR2 transmitter where you can't adjust the end points.  I set up the end points for the functioning pumpkin then marked out the turning circle on concrete and matched it with the second pumpkin where I can't limit the end points.  I reduced the servo throw to the turning circles were the same and neither pumpkin steering makes contact with the chassis.

The company I bought this second pumpkin from are still selling with 15kg servos as standard which from what you're all saving does seem crazy especially if you've had good service from a 3/4kg servo.  I love the metal plate upgrade, looks a great solution!  I have a new chassis on its way.

Any recommendations for a particular servo?  I have had a few steering servos in the past that have irritated me due to the electrical chatter and never sitting still.

   

[alvinlwh 5891]

1 hour ago, Jonrees said:

Any recommendations for a particular servo?  I have had a few steering servos in the past that have irritated me due to the electrical chatter and never sitting still.

   

Probably any 4/5kg servo will be fine. Mine is still on its original Acoms servo from 12 years ago, very noisy but it turns. Modern ones, even budget ones, are far quieter than it. 

[Jonrees 2]

cheers alvinlwh  

Just been looking through the 5kg servo provided with the first model and looks like a cheapo, it does't even have any branding on the case.

Nice to hear from Orkney.  I live in Essex now but lived most of my life in North of Scotland.  Still have a lot friends in Wick/Thurso and Kirkwall.

J

[BattleTrak 58]

UKMonsters have a really nice servo mount for this problem now.

https://www.ukmonsters.co.uk/lunchbox/index.html

[KEV THE REV 2307]

On 4/1/2022 at 10:47 AM, BattleTrak said:

UKMonsters have a really nice servo mount for this problem now.

https://www.ukmonsters.co.uk/lunchbox/index.html

Some good parts on that site