Brushless Kyosho S-RR

[RC-Monster Mike]

Cool conversion - I have a comment on your rear motor support, though. I think it is a bad idea. Any flex in our motor mount will be pretty minimal, and the entire center diff assembly will "flex together" as a single unit and maintain its integrity without the rear support. It will flex far less than the chassis itself in actual use for sure.

In use, the chassis will flex at the point between the front of the rear chassis brace and the back of the center diff(where there is no bracing) - this causes the rear of the chassis to flex independently of the motor/diff assembly all day long - by supporting the rear of the motor in this area you are introducing excess force on the rear of the motor, and therefore the motor mount, motor mount screws, etc. (when you land a jump, for instance, the rear of the chassis will flex upward and push the rear of the motor upward - twisting the motor and motor mount far more than it will flex on its own).
This forces the motor, motor mount and motor screws to absorb the load of the entire truck as it lands and the chassis flexes, instead of only the weight of the motor(the motor and motor mount are now a chassis brace). Part of the idea with our motor mount strategy is to keep the motor and center diff together as a single unit - independent of chassis flex.
I think supporting the rear of the motor like this will create a problem in the end - perhaps the motor mount screws will be the breaking point, but the motor mount and motor itself will likely take the most punishment (the aluminum motor can will take a lot of force at the mount screw location).
Anyways - just food for thought. I just figured I would take the opportunity to explain the thought behind my motor mount design strategy, and why I think this particular rear support implementation it is a bad idea. :)

[Shark413]

Mike, I completely agree with everything you said regarding the chassis flex. It is part of the design of most 1/8 scale buggies/truggies and improves handling. My setup is a little different, using the thick (1/8 inch) carbon fiber battery tray stiffens the center section a little more than the nitro version. And the King Headz braces are stiffer than the stock Kyosho braces. Also I am using a rather thick spacer between the motor and mount to allow clearence for the thick Kyosho bonded brake pads/rotor. This adds to the leverage the motor has on the mount. I noticed a lot of lateral (side to side) flex in the motor mount between the motor and diff, which caused gear mesh changes. This concerned me more than up and down flex, which could have been fixed with a simple bumper under the motor. Your logic regarding flex is correct since nitro engines are mounted to the chassis separate from the center diff and chassis flex would cause movement between the engine and the center diff. I may try something else, my center diff is sitting on a 3mm spacer (for clearence for the 49T spur) and I may try adding a 3mm plate instead with the center diff/motor mount attached to it as well as a rear motor brace. This would tie all those parts together and still allow for some chassis flex. Thanks for the feedback, that's why this forum is the best.

[Shark413]

Mike, I was thinking about what you said and may be a "U" shaped rear motor brace would work. The sides of the "U" would prevent side to side flex but the open design would allow up and down flex (when the chassis flexes in the middle).


The "U" would provide side-to-side support but allow the motor to flex up and down with the chassis.

[Shark413]

Ok, more thinking. The "U" brace may work but it is a lot more work. I think what I will do is anchor the outside portion of the motor mount to the chassis. Currently the motor mount is only held to the chassis by the two inboard screws. If I anchor the outside part of the mount that will eliminate all side-to-side flex, and still allow some up and down chassis flex. I also have a small press and a micrometer, I will put some pressure on the center of the chassis and see where/what flexes and by how much. Crude, but I guess it may provide some idea on the flex points and how flex I need to deal with. A small hard rubber dot/bumper under the motor close to the mount will provide additional motor support if needed. Please forgive the crudeness of the drawing.


Front view of the motor mount. Normally mounts to chassis with two inboard screws. Want to add one to the outside to reduce flex.

[Shark413]

Well as soon as I got home from work I took the truggy and removed the rear motor brace. I placed blocks under the front and rear ends and applied a few pounds of pressure to the center diff. WOW, there was about 1.5mm of up and down deflection at the motor (the end of the motor can moved 1.5mm from level when the chassis flexed). This was just from about 5 lbs of pressure. So with the rear brace installed this deflection would have been transfered to the motor/shaft/bearings/gears. So I think I will just add another attachment point to the end of the motor mount to eliminate the side-to-side flex. Thanks Mike for taking the time and letting me know about this issue.

[Shark413]

Ok, after taking everything apart I was going to make a new brace but after checking the flex I notice the center diff supports were a little loose. I am using a 49T spur so I had to raise it to clear the chassis. Kyosho supplies some plastic spacers for this purpose, but they were not designed to support the diff and a brushless motor. The plastic is too soft, and gives, which adds to the flex. Mike's mount is a great piece of engineering and many have used it without issue. So I decided to stick with Mike's mount as is, I will machine some new diff spacers out of 7075 aluminum which should help with the center diff flex. Most of the force from driving a off-road truck is up-and-down, not side-to-side, so I think the mount will be fiine once I make new spacers.

[Shark413]

Well no more machining for a while, my bad machining skills blew the gear box in my mini mill. But I was able to finish the spacers the old fashion way, saw, dremel and files.


Broken gear - Mini Mill


Carbon Fiber spacer for front diff support


Aluminum spacer for diff/motor mount.

[Shark413]

Ok, got the mud guards and installed them.

[Shark413]

Ok, I finally was able to get (bribe) my nephews to come out to the track and shoot some video of my brushless ST-RR in action. Still need to fine tune the shocks and brakes.

http://www.youtube.com/watch?v=GC9rYX8hLPk
Updated with a better quality video