50mph Diamondback RC Build

MrBoots

100 W
Joined
Dec 22, 2009
Messages
109
Hey there. I know some of you have been following my on-going RC build and fortunately, I was able to take it out for a spin today right after work. I'm basically running an HXT outrunner mounted on the rear rack, feeding it 30v, 16ah of LiPo juice. Thanks for Burtie, Jeremy Harris and all the other folks who pioneered mounting hall sensors onto outrunners.

I took the bike out to a long, broad stretch of road along a river. Speed limit here is a posted 35mph but some cars routinely zip by close to 50mph so I was being extra careful today. To break in the new system, I pedaled out up to about 12-14mph before gently pressing the throttle. Power was immediate and within a few seconds of 1/2 throttle I was already reaching close to 30mph. I don't have a GPS to confirm any of these speeds, but just going by my butt-meter lol. As I was going riding, a car passed me up in the left lane. Just to gauge how fast the bike could go relative to the car, I went all the way on the throttle.

Power delivery felt pretty frantic and within a few short seconds I matched the car's speed and then was even reeling it in. By this time, the speed was scary fast and I'd assumed somewhere around 42-43mph. I def feel like I can wring out about another 10mph out of it but not sure I'd wanna test that theoretical limit. I really only built this bike for commuting but I severely miscalculated my gears on top of having far less rolling resistance than I had planned.

Anyway, here are some older photos of the build while it was still-in-progress - I'll have some new ones posted whenever I get a new camera since my current one is trashed. Again, thanks to the ES community for the wealth of information available here!

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img_7828.jpg
 
MrBoots said:
I severely miscalculated my gears on top of having far less rolling resistance than I had planned.

Well done on the build mate always good to see another rc motor build, looking at
the gearing though it does seem a tad 'tall'

KiM
 
50mph on a commuter hardtail with V brakes? :shock:
 
Yeah I know, I should've drawn-up the set-up a bit better. I originally planned to run only 6S on my disc-equipped, front-suspension MTB with a 130Kv outrunner. Instead, I ended up picking up a 200Kv outrunner, running it at 8S on 700c tires.

Right now, I guess I have a few choices: get a bigger sprocket OR reduce the batter pack to 6S OR just restrict throttle to 1/2. I'd rather not order another sprocket because I ordered this one from King's Sprockets - which did a fabulous job, by the way - and would have to shell out money for another one. The issue with the second one is that 6S is extremely close to the LVC of the controller and modifying it looks really tricky. I already replaced the FET's on this one and performed the shunt mod which were both a royal PITA for me.

This bike will probably only do around 30mph at half-throttle so that's quite manageable. I guess in that respect, it's very much like modern cars: most of us drive cars that are capable of going 130+ MPH, but how many of us even use 50% of our cars' performance capability? Even with the tall 10:1 single reduction gear ratio, it feels like it has plenty of torque. Yes I realize a big disc brake upfront would be ideal but staying under 30mph, the V-brakes should be adequate. I rode with the same set-up (Avid SD7's with Koolstop thinline pads) on a FS Slalom MTB and it worked beautifully under severe riding conditions: fast downhill descents, wet and mud.

Anyway, I plan on putting up a video soon. A friend and I are planning to do a car vs. ebike commuter race across town (total of 8 miles) so that'll be a good opportunity for me to shoot one.
 
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