Modified steering brakes

[Peter Withers]

More news on UC no. 64, the ambulance. Having partially installed the motion kit I discovered that without a more sophisticated transmitter than my Spektrum DXe I was going to run into problems with the servos stalling as they tried to move the brake mechanisms past their mechanical limits. So a new plan was evolved.

I have added some extra linkage hardware to the pushrods which feature rubber buffers to give me some compliance. This means that the servos can travel to their limits without stalling, once the brake block is in hard contact with the disk the rubber compresses to take up the extra servo travel. I changed to compression springs to push the shoes back off and this revealed a flaw in the mechanism. The shoe was getting hung up on the edge of the button head screws and was not releasing until the motor was stopped. To fix this I put a light chamfer on the relevant faces of the brake shoes. after a bit of tuning of pivot locations, throws, angles, rubber hardness etc I came up with a solution that works well with the motor running on the bench. I need to put the tracks back on and try it in anger.

The photos show the new mechanism assembled to the axle. Since the photos the rubber buffer has been swapped from silicone tubing to hard rubber tubing and I moved the pivots (yet again) but the basic mechanism should be self-evident. The servo pulls the operating arm which pushes the shoe against the disc, the spring retracts things when you centre the steering. There is just enough room for the arm to clear the mechanism on opposite lock but I plan to make a modification to the servo connection to sort this out and make adjustment easier.

I'm suspicious that I'm now getting enough brake pressure to make the button cap screws in the discs unnecessary but I'm going to do some test driving first - one step at a time!

[John Clarke]

Like it. Looks good.
What are the chances of adding a transverse drive rod to control the other brake from the single servo?
I saw a chap on YouTube cut a model race car brake pad from a grinder slitting disk. I thought super idea to make a thin grippy pad to attach to the brake disk.. Though I've no idea how long the brake pad would last against it

[Dennis Jones]

When I raced 1/8th scale IC cars the brakes we used were a brake pad material with 2 steel plates either side of the disc operated by a cam and considering we were doing up to 70 mph I never had to replaced the disc or the pads. I would like to do something similar but there's not a lot of room. I wonder how much pressure would be required to push a steel shoe up against brake pad material on the disc which if it could be done would make a gradual braking system instead of a snatch.

Your setup looks good Peter I will follow with interest as I have parked my number 1 build on one side and started on the number 2 build.

Dennis.

[Peter Withers]

Like it. Looks good.
What are the chances of adding a transverse drive rod to control the other brake from the single servo?
I saw a chap on YouTube cut a model race car brake pad from a grinder slitting disk. I thought super idea to make a thin grippy pad to attach to the brake disk.. Though I've no idea how long the brake pad would last against it

A single servo with rod is a nice idea. It could live in the central engine housing. If the rod was allowed to slide through the servo connection until it came up against an adjustable stop you could precisely adjust the point at which either brake came on independently.

Thanks for the positive feedback, I feel encouraged...

[Peter Withers]

When I raced 1/8th scale IC cars the brakes we used were a brake pad material with 2 steel plates either side of the disc operated by a cam and considering we were doing up to 70 mph I never had to replaced the disc or the pads. I would like to do something similar but there's not a lot of room. I wonder how much pressure would be required to push a steel shoe up against brake pad material on the disc which if it could be done would make a gradual braking system instead of a snatch.

Your setup looks good Peter I will follow with interest as I have parked my number 1 build on one side and started on the number 2 build.

Dennis.

Thanks for the feedback, I'll keep you posted.

[Peter Withers]

I finally tested my brake setup over the last couple of days. The first test was a failure, the tracks were too loose. I took 2 links out either side (I'm now about 8 links less than the number specified in the build documentation!) and on the second try everything worked a treat. The main issue is that the differential is still very stiff and the poor old motor struggles at low speeds. The brake operation seems to be pretty smooth and I didn't see any instances of the brake not releasing in the admittedly short test drive. I'm really tempted to try the brakes with the screws removed from the discs (if I can bear the thought of taking the back axle off again, all those little screws...!).

Next step is to get the differential bedded in a bit better or go to plan B which is to swap in a geared diff from a 1/8th scale RC model which I think would suit the braked steering much better. From my RC experiences the ball bearing based diff is more of a limited slip thing so kind of works against differential braking.

[Armortek]

Peter - instead of changing to ball diffs (which will probably not be able to handle the torque - or if they can, will require frequent maintenance) either run the diff on a power tool to loosen it up or change the viscosity of the grease in the diff.

[Peter Withers]

Peter - instead of changing to ball diffs (which will probably not be able to handle the torque - or if they can, will require frequent maintenance) either run the diff on a power tool to loosen it up or change the viscosity of the grease in the diff.

OK, thank you for the advice, particularly on the grease. I'll try running my diff in and re-test.

[Dennis Jones]

Peter,

With regard to the screws holding the back axle in, I reversed mine so the nuts were on the inside and I drilled a small hole in the drive sprocket level with the cap heads so I could use and allen key from the outside. The cap heads can't be seen anyway as they are behind the drive sprocket. Whist testing I have only put 3 in.

When you installed the disc did you pack it out as I found it was floating loose and also had the nuts come undone when I went in to reverse and put brakes on as then the disc just undoes the nut. I locktited both nuts in the end.

Not sure about removing screws on disc as it would the be aluminium shoe to steel disc, I think some form of brake material fixed on to the disc might be the answer unless Armortek already tried this (Kian ?)

Dennis.

[Peter Withers]

Peter,

With regard to the screws holding the back axle in, I reversed mine so the nuts were on the inside and I drilled a small hole in the drive sprocket level with the cap heads so I could use and allen key from the outside. The cap heads can't be seen anyway as they are behind the drive sprocket. Whist testing I have only put 3 in.

When you installed the disc did you pack it out as I found it was floating loose and also had the nuts come undone when I went in to reverse and put brakes on as then the disc just undoes the nut. I locktited both nuts in the end.

Not sure about removing screws on disc as it would the be aluminium shoe to steel disc, I think some form of brake material fixed on to the disc might be the answer unless Armortek already tried this (Kian ?)

Dennis.

Thanks Dennis, I already did the hole in drive sprocket mod (from your previous post?) and yes it helps considerably, I just struggle to handle M2.5 fasteners these days...

My discs are a very tight push fit on to the threaded part. I found that they were lightly rubbing on the housing behind so I shimmed them to get a bit of clearance. And yes I found they unscrewed so they and the nuts are now loctited.

Regarding the disc and shoe material I am wondering about a different shoe material, maybe something like a bicycle brake block of the right kind of hardness cut down to suit? I am concerned that the soft aluminium shoes will rapidly wear as they are forced onto the screw heads as the brake is applied. That is my main reason for wanting to eliminate the screws.

Pete W

[Peter Withers]

More progress on UC 64 (the Ambulance) and the steering brakes. I now have removed the screws from the discs, added some friction material to the brake shoes, bedded in the diff (as recommended by Armortek) and it all works! I have reliable steering. The motor is still struggling slightly but some of that is down to the fact that the moving track friction goes up as it slews across the running surface. It is OK on concrete and smooth surfaces but didn't cope with grass (although a mower might help there).

The friction material is some form of rubber-like material that experimentation showed to be best in the current setup. It looks to be something like Viton or similar and I'd guess it is around 75-80 shore hardness. The fact that the brakes spend very little time rubbing before they lock suggests that wear should not be too rapid. It was interesting to note the wear on the aluminium brake shoes when I removed them even after a very short test drive. I secured the material to the shoe with superglue, it bonded incredibly well, almost as well as skin

Next stage is to tidy up the installation and then time to look at the rear sprockets which don't mesh well with the tracks when the tracks are under tension.

[Dennis Jones]

Hi Peter,

Just seen Viton on Ebay, what thickness did you use ? Would it be advisable to remake the shoes in steel ?

Dennis.

[Peter Withers]

Hi Peter,

Just seen Viton on Ebay, what thickness did you use ? Would it be advisable to remake the shoes in steel ?

Dennis.

The piece I had was around 2mm thick, it was probably an offcut from work (I design equipment for the oil industry, viton is a favourite for O rings etc). I personally was happy to stay with the aluminium shoes, the glue bond with the viton was superb. While this makes the shoes thicker there is more clearance because the screws from the discs have gone.

Pete W

[Mike Keenan]

Found some clutch liner from my RC Heli days about 1mm thick and almost indestructible perfect for brake pads on UC it can be bought from most Model Shops and takes very well to metal with two pack resin glue

[Dennis Jones]

Have you tried it yet Mike ?

Dennis.