Mechanical Brake Big problem

[Shark413]

Ok I have several brushless conversions and most of large scale (1/8) vehicles use mechanical brakes. I like the feel and ability to adjust the bias, plus the fact it seems to take some load off the electrical system. For a few months everything was cool, the buggies/truggies ran great, faster than nitro, gobs of torque and sweet brake action. Then it happened, I was checking the motors (Neu 15xx series) and found a lot of play in my ST-RR truggy. I contacted Neu and they said send it in. Got an e-mail a few days later saying the front bearing was shot and needed replacement because of excessive force on the shaft and is not covered under warranty. Yikees!! So I paid $$ and got my motor back a few days later. I believe the reason there was "Excessive force" on the shaft was because of my mechanical brake setup. I was using the Kyosho semi-metallic pads and precision rotors, which in my opinion are the best brakes. Unfortunately they are also very fat, the pads are very thick, so the motor needs to be set well back to clear the pads/rotor. This means the pinion rides on the end of the shaft, and multiplies the force placed on the front bearing. My 1/8 buggy has the same setup and the bearing felt ok. But because of what happened to my truggy I removed the rear brakes from both and went to a hybrid rear motor brake and front mechanical brake, since most braking is performed by the front brakes I felt this would be ok. I set the motor brake (Mamba Max) to 75% and took my truggy out on the track, right away I could see that there was way too much brake and I could no longer control the truggy in the air, the brakes were like an on/off switch. The track has this triple jump leading into a sharp 90 degree left turn, normally you hit the last jump nose down and get on the brakes. But that day the last jump was destroyed by previous drivers lawn darting it and it was a crater. I figured I would try a few more laps and try and get used to the brakes before I lowered the setting. Big mistake, I came in a little hot and over cooked the last jump, I applied brake and they totally locked when I landed, which normally with mechanical brakes would not be a problem, but immediately the truggy was make bad sounds. I pulled in and found that I had lost 3 teeth off the pinion. I believe this was from having the motor brake completely lock the motor and then the hard landing with such a heavy vehicle just put to much pressure on the drive train so some had to give (pinion). I know this is a long story, but this forum has a lot of very knowledgeable people who have passed on some good info. I just wanted to pass on my experience for those that may be thinking of using mechanical brakes.

Broken pinion, spur gear and diffs were fine.






Motor spaced 12-15mm from diff mount to clear brakes. Put a lot of stress on the front bearing.

[What's_nitro?]

That looks like it took quite a lot of force to accomplish! I think you were speeding...

[Matthew_Armeni]

Sooo... the motor braking was what caused the teeth to shear off, not the mechanical braking, correct?

[Shark413]

Matthew_Armeni, I assume thats what caused the problem. I was not running mechanical brakes in the rear that day. Motor brake for the rear (motor/pinion to spur) and I was running the mechanical brakes in front. In this setup when the brakes are applied, even if there is an inbalance in stopping power between the front and rear the center diff will compensate and give a little, so the front should be isolated from the rear. So I believe the motor brake locked the rotor (or nearly locked it up) and when I came down hard after the jump the force transferred up the drive train to the locked motor and the pinion gave out. I ran mechanical brakes front and rear for several months and never had an issue with gears breaking like that. With a pure mechanical brake setup the motor freewheels (neutral) when the brakes are applied and the brake servo (3rd channel) applies braking force to the brake pads/rotors, so no stress on the motor/pinion/spur at all. I know most of you (conversions) are running motor brake only, what percentage (brake %) are you running? 50% is default and I was running 75% when the pinion gave out.

I have only used motor braking in my BL rigs and I have never had any damage like you have in the picture, including setups with more power than you are using, IMO the problem is more deep rooted

[MTBikerTim]

In all my brushless cars and my friends we all run about 25% and even that can be enough to throw the car into forward somersaults. 75% would be like hitting a brick wall. I don't run any 1/8th cars with electric brakes so it may be a bit different but I doubt it.

[Mike_D]

In every brushless vehicle i have ever had i always use motor brakes and have never had an issue like that either. I usually set my MM brakes at about 50% - 60% and that seems more than enough stopping power.

[RC-Monster Mike]

Originally Posted by azjc

I have only used motor braking in my BL rigs and I have never had any damage like you have in the picture, including setups with more power than you are using, IMO the problem is more deep rooted

Agreed. I doubt the mechanical front brake had anything to do with the failure. By supporting the rear of the motor on the chassis, the chassis flex will affect gear mesh(chassis flexes between the rear brace and the center diff - the front and rear of the motor is supported on opposite sides of this flex and assumes the load in this configuration). Not saying this is the root cause, either, but it is likely a factor. An aluminum front diff bulk will strengthen the mount the way it is(front plastic allows some flex, but at least everything attached to the diff flexes together without the rear brace). The way those teeth broke(at the very bottom of the teeth), my guess is that the gear mesh got jammed tight from flex/mesh change and was a factor in the failure. That pinion is likely a year old as well, so fatigue likely played a role in the failure as well (our current pinions use better steel than that version as well).

I use 50% brake strength as start point, and then dial in(out) the rest on my radio. I also use a brake curve. I don't think mechanical front brake will actually do anything for you - a hybrid setup with mechanical rear would allow you to effectively tune bias, but a front mechanical brake only won't allow you to get more rear brake or a rear bias. Motor braking by itself will allow the diff to bias the braking power, but the diff is usually unloaded somewhat to the front when accelerating, so when you hit the brakes, the front "feels it first" anyways and a mechanical front brake alone is counter-productive IMO.

[Shark413]

Mike, that was an old picture of the motor/mount. I just wanted to show the spacing needed to clear the Kyosho brakes. In another post you mentioned that bracing the motor at the rear would cause problems when the chassis flexes. I agreed with you and removed the rear brace before the truggy was ever run. So the stripped pinion must be due to some other problem. I find it hard to believe 75% motor brake could shear off metal teeth like that. I only have this kind of problem with my truggies (Kyosho ST-RR and CEN TR), I have stripped a few plastic spurs but only this one pinion. My buggy has not suffered from this type of problem. I appreciate all the feedback.

[Shark413]

Still experimenting with the brakes, I will turn the motor brake way down, disconnect the front mechanical (remove the servo 3rd channel wire from the rx) and run motor brake only, to see how it feels. I guess I will need to wait for the Ofna Hyper 9 before I can go with full mechanical brakes again. The Hyper 9 has the rear brake mounted at the rear diff so motor clearence won't be an issue. I have also been playing around with making a rear mechanical brake mount that works similar to the Hyper 9 that I could try out on my Kyosho's. Just not enough hours in the day.

[lincpimp]

I have a feeling that Mike's slipperential will help with pinion/spur issues, and drivetrain shock on landing. On the mech brakes issue, motor brakes do work reasonably well, but hard braking does cause the weight to shift over the front wheels, thus unloading the rears, and thus the diff, limiting braking. The hybrid setup with rear brakes, and motor braking sounds the best, but I would prefer to use the slipperential center diff before realy racing hard...

[MetalMan]

Originally Posted by Shark413

Still experimenting with the brakes, I will turn the motor brake way down, disconnect the front mechanical (remove the servo 3rd channel wire from the rx) and run motor brake only, to see how it feels. I guess I will need to wait for the Ofna Hyper 9 before I can go with full mechanical brakes again. The Hyper 9 has the rear brake mounted at the rear diff so motor clearence won't be an issue. I have also been playing around with making a rear mechanical brake mount that works similar to the Hyper 9 that I could try out on my Kyosho's. Just not enough hours in the day.

Why wait for the Hyper 9?

http://www.rc-monster.com/forum/showthread.php?t=12295

[Shark413]

Metalman, thanks for the info. I knew there were other buggies out there that used a rear diff brake setup. I am just trying to get one to work on my Kyosho truggy.