Brushless Kyosho S-RR

[Shark413]

I purchased a Kyosho ST-RR a few months back to replace my tired CEN TR Arena. I added an O.S. 28xz, JP-4 pipe, 300 oz servo, FASST, etc. The thing was so much better than my TR. Longer chassis, big bore shocks, chormoly drivetrain parts, excellent handling and powerful. It was the only nitro vechicle I actually drove, everything else was brushless (E-Revo, 1/8 buggy, Jammin CRT.5, Stadium truck). So after I split two fuel tanks and almost burned up my truggy, I thought about converting it to brushless and switching between nitro and brushless was throwing my driving off as well. The O.S. 28xz is an amazing engine so I had to have a good brushless motor I picked a Neu 1515 1y. I picked up a spare chassis on E-Bay and started the conversion. After turning my CEN brushless truggy chassis into swiss cheese I wanted to minimize the number of additional holes I had to put in the chassis. For all my previous conversions I used my own custom built motor mounts, this was mainly so I could set the motor back a little so I could use the mech brakes. For this conversion I decided to use the RC-Monster mount and a motor spacer. I only had to drill two additional holes in the chassis.

[Shark413]

Update:

As I was putting the conversion together I wanted to use the stock Kyosho chassis braces but they hit the can when using smaller pinions. So as I was removing the braces I noticed the motor flexed when I pressed on the side of the can. The motor mount was flexing a little and I figured this could not be good for the drivetrain. I decided to make a rear support bracket to support the motor and keep the flex down. The bracket had to provide complete support (side to side and up and down) and had to be movable so I could adjust gear mesh. I came up with a bracket that surrounds the entire motor can and also slides laterally for gear adjustments. Since I had to make a new servo mount I decided to lay the steering servo down and turned the braking servo 90 degrees for better gear access. I added a carbon fiber plate to the top of the servo mount for a place to put my rx. I then utilized the nitro motor mount holes to make a ESC mount.



Motor rear support bracket, also switched to King Headz chassis brace. Brace uses the radio box holes.


Servos and rx tray


ESC mount. Uses the nitro motor mount holes.

[Shark413]

I now have most of the truggy together, I added a carbon fiber battery tray, the front and rear suspension parts. Added a Futaba FASST rx, CCBEC, Center diff with a Losi 48T wide spur, Kyosho bonded brakes. Just waiting for a Mamba Monster Max to finish.


Almost finished truggy with a Neu Energy 4s lipo and Mamba Max installed ready for a test run.


Bottom of chassis, I only drilled to additional holes, added front/rear skids. Still waiting for the side guards to arrive.


Next to the nitro ST-RR.


The rear skid has a lip that protects the alloy suspension bracket.

Stock Kyosho ST-RR Kit
RC-Monster Kyosho Motor Mount
RC-Monster Hardened 14T pinion
Robinson Racing Losi 48T wide spur
King Headz chassis braces and rear suspension bracket
Futaba FASST rx
CCBEC
Mamba Max (coming soon Mamba Monster Max)
Neu Energy 4s lipo
Neu 1515 1y motor
Hitec 5955 servo
Custom made servo mounts and rx tray
Custom made ESC tray
Custom made skid plates
Custom made battery tray

[jnev]

Looks great!! Nice conversion! How are you liking the mechanical brakes compared to the motor brakes? Let us know how she runs.

[Shark413]

jnev, thanks. You and some of the other forum regulars really inspired me and gave me a lot of good ideas. That's what I like about this forum, a lot of great info sharing, positive feedback and friendly advice.

[DRIFT_BUGGY]

Looks good, carbon fibre touch looks great

[lutach]

I'll be looking at this thread often. I want to know how well the drive shaft and axles on the ST-RR handles brushless power.

[Patrick]

The 2 ST-RR's at my track haven't had problems yet on 4s trakpower packs. One has a 1515 2D (2050kv) and the other is lehner 1940 7t. They both have a fair bit of power for the track, but they haven't been doing on road speed runs with 6s or anything like that.

[Shark413]

It was really hot today in Socal, perfect weather to see how the E-STRR would hold up on the track. Without any adjustments to the suspension or brakes (from it's former Nitro settings) I ran one 4s lipo thru the truggy to get a feel for it and to see how hot things would get. Ambient air temp's high 90's. After 10 mins, ESC = 170 (a little hot), Motor = 132 (Great), Batt = 108 (not bad). Not bad considering how hot the ambient temps were. The brakes were way off, way too much front bias (almost flipped over everytime I braked). Suspension was a little too soft as well. I lucked out and a JBRL racer that I knew showed up and was impressed with what he saw and wanted to take a spin. After another 4s pack, he was stroked, he could not believe the power/torque a brushless motor could produce. He made some adjustments to the brakes and put in a few more laps. A pretty good first time out.

[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.