View Full Version : PN racinging gimbaled motor mount slop fix.

2011.10.15, 07:57 AM
Hi all.

Last week I traded a PN motor mount I wasn't using with the gimbaled motor mount. I did some reading before and saw that alot of people where complaining about the amount of slop in it. Being a mechanical designer myself I tought I take a look at it myself if I ever got my hands on one. Wich I did last week.

So I assembled it and I find the slop to be horrible. The amount of play on the rear axle was not to be ignored and I just could not see myself putting it on my car the way it was. To be correct, this was a second hand mount and I dont know how much time it spend on the track before. The guy I got it from simply said I dont use it because I can't get it work properly.

Anyway, I took a look at it and found that the slop is coming from the screw going through the mount and the T part. Both radial as axial slop.
I found in other threads here on the forum that some people tried to fill the holes with glue, others filed down the sides of the T to put a shim in between. But none of them ever stated that it worked good enough. So I'm thinking the best way to get rid of both slops at the same time: a bolt instead of the pin.

I got myself a set of M2 thread taps. Sadly enough the ones I got are not long enough to go through the T-piece in one time. So that obligated me to drill a hole in the middle and tap from both sides. Hoping that if I run a bolt trough it the threads would match and the bolt would go through.
Sadly and not surprisingly, it didnt.
T piece:

I know the hole looks horrible :o

Anyway my first plan was to use abolt through the T-piece and file of the thread at the end to fit into the motor mount. This wasnt an option any more. The next best thing I could think of was to cut of a piece of the original bolt and insert into the T-piece and have it torqued down by my bolt.
I had to file down the side of the T-piece a little bit to get the bolt to turn in without it binding.

So the pin covering one side, the bolt is covering another side but the problem now is that if the bolt is not torqued enough the slop remains. If it is torqued to much, the T-piece will bind. So I'm using the bolt (still need to make it the correct length) and a small contra nut. I torque the bolt just enough, and then fix it with the nut.

If you get the torque like it should be, ZERO slop. I mean absolutely nothing at all. The bolt is covering the axial slop, even if not torqued enough. And when it is torqued enough it is rigid enough to cover the radial slop to.

So for me it is good enough now to take it to the track.
What would improve it drasticly, the T-pice should be threaded in 1 time. The original bolt is good when the thread is lengthed to about 2-3 mm from the tip allowing it to sit in the mount at the end.
What would be even better, to have the holes in the mount threaded in 1 run and the bolt threaded entirely. Allowing the bolt to be torqued all the way into the mount eliminating all radial slop.

Now I know when the T-piece moves up and down it will shift because of the thread. But the pitch of the M2 thread is 0.4 mm. Considering the mount moves maybe max 10 this gives you 0.01 mm movement. Wich is a hell of alot less then the original slop.

My only point on concern at this moment is wear.
I'm wondering how long it would take for the thread to wear down and creating slop again, and how big the slop would be. Maybe this can be covered with using a bolt the is less hard then the alu mount. So if something starts to wear it will be the bolt and not the T piece and you can get a replacement for that.

I hope this will help out some people strugling with the gimbaled motor mount.
I also hope that maybe PN will take a look at this and if found good enough that they will provide a fix for all gimbaled owners having trouble with this slop. For existing mounts a threaded T-piece a little less wider and a bolt with longer threads on it should do the trick.

Thank you for reading this, any constructive critisisme or comments are very much welcome.


PS. Does anyone know how to scale down the pictures? They are kind of big :)

2011.10.30, 10:09 AM
Little update.

I found a M2 tap large enough to go almost trough the entire T-piece. So I shaved a little more from the sides and went trough it. Simply running a bolt trough it pressing against the pod was enough to take away all slop easily.
Sadly enough, when I put the mount to my chassis there was still slop in the axle coming from the chassis to the mount. I think i'm going to give on this gimbaled mount. By far the worsed PN product I have ever had.
I'll try the tri damper setup when a T-plate then.

2011.10.30, 05:40 PM
Interesting that nobody has replied to this yet. The way I see it, turning a screw hole into a moving part will drastically increase the amount of sliding friction you encounter in the gimbal joint, so you reduce play but increase stiction, and that's the reverse of what pan car development has been trending towards in the last few years (a little more play with links and balls, but greatly reduced stiction thanks to tube dampers). That friction on the soft-ish aluminum parts is going to make it wear fairly quickly, so you will have to replace T-blocks more often.

Although I'm not a huge fan of the gimbal setup (mainly due to parts count) I will admit that it has one advantage over any T-plate setup in straight-line acceleration, and that's one reason Grant worked on it for so long. The rear end squats predictably and plants down symmetrically when you yank the throttle unlike my T-plate car (also tri-shock equipped) that will head a tiny bit to the left or right if the rear tires spin. Perhaps that's the gimbal joint's internal stiction at work but it is a big redeeming quality that makes the car easier to drive, if not easier to work on. I definitely wouldn't dish on the gimbal system as a product, but as you've found, it takes some upkeep to stay running well.

Edit: Use the gallery here, or Imageshack, to resize your images without needing any software on your computer. ;)

2011.10.31, 02:24 PM
thank you for the reply.
About the firction, it isnt realy noticable. I also considered that fact when i started this, but I was happy to find that it is in fact very smooth with some realy thin oil.
The wear like i said in my first post I also tought about. But I will never find that out now :)
I also liked this setup. I love the engineering behind so I definetly wanted to give this a try. Shame I cant get it to work 100% like I wanted to.

Thx for the picture tip.