[Build Advice] Best setup for $1500 that goes ~50mph?

DukeofCrydee

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May 5, 2015
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Hi guys,

I'm looking for build advice. I have a store bought rear hub bike, 36V/10A and I'm looking to move onto bigger and better things.

I've got $1500 to burn and I'd like to build a bike that goes 50mph+/- with a geared brushless motor. I have a $500 mountain bike that I'm going to use for the frame. I need help picking out a good motor, battery, disc brakes, and controller, and would welcome any extra advice you'd feel like giving a newbie like me.
 
Going to a higher voltage will make these motors spin faster. The Kv of a motor is a rough approximation of how many motor-RPMs you get per volt that you apply. The geared hubs have an internal gear-set to spin the motor 5 times faster than the wheel. There's two problems I see with your stated goals if you use a geared hub:

1st issue: In a 26-inch wheel, 50-MPH is 647 wheel-RPMs, now times that by five, and the motor would have to spin 3,235 RPMs. Thats unloaded RPMs, so using wind-resistance at 50-MPH and a 250-lb rider + bike means you're likely to need 25% more unloaded RPMs to get that actual road-speed (or more), so...4,000-ish RPMs? The MAC and BMC have a high pole-count, so electrical switching frequency at 4,000-RPMs is above what they can manage...eddy currents?...inductance?...aliens?

2nd issue: The two biggest common geared hubs I know of are the BMC and MAC (very similar to each other). Both have poor heat-shedding at over 2,000W, and you will need a lot more than 2,000W to get 50-MPH (50V X 40A = 2,000W). so...the biggest geared hubs can't take the heat of a 50-MPH system.

I think 50-MPH will be unsafe on any bicycle and E-bike system that can get up to 50-MPH on $1500, but that's just me, however...if I was doing that, I'd research the MXUS 3000W at 72V using multi-star LiPo. At any speed above 30-MPH, a fairing will help...a lot.

http://endless-sphere.com/forums/viewtopic.php?f=6&t=66978
 
Best advice for going 50mph on a $500 bike is... don't.

Seriously, once you start upping the speeds you run into two main issues, both of which already touched on thanks to spinningmagnets. Requirements for both power and quality of components increase exponentially with speed. You really don't want to be going 50mph on a junk fork or a fixed tail bike. You also need high quality tires, wheels, brakes and a very stout frame.

You can almost certainly build something that will go 50mph for $1500. But you probably shouldn't. I have a bike that can hit 50mph, however it's using the best of the best bicycle components. My previous bike was along similar lines to what you're proposing (although it still cost me substantially more than $1500) and it was pretty sketchy above 35mph. Frame flex, oscillations, brake fade and suspension not being up to the big hits small potholes present at those velocities made it pretty unsafe - and that was based on a quality frame with decent components to begin with.
 
DukeofCrydee said:
I need help picking out a good motor, battery, disc brakes, and controller, and would welcome any extra advice you'd feel like giving a newbie like me.

First, read all of this, and the many other threads already asking your questions.
I'm a Noob and I Wanna Go 50MPH
Yes, it's a LOT of reading. Do that first and people will probably be able to help you better, cuz you'll need to know a lot of stuff to ask the real questions you need answers to.


Next, probably unless you only need to go down the block at that speed, you're gonna need a lot bigger budget. ;)`
 
Keep in mind that you need a top end of 60 or so to have a useful 50mph. As someone stated before forget the geared hubbies. Good brakes are extremely import with high speeds, and regen braking will greatly reduce the amount of brake maintenance required.
 
First off, I agree wholeheartedly with amberwolf ohbse and spinningmagnets. First I'll share a story from my experience, and then I'll give you my recommendations on making your dream possible (albeit with a couple major changes)

One time I was on a commercially produced gas moped, (~150lbs, 65cc), which had massive suspension etc, easily over 4 times as sturdily built as a bike. I was going down hill, and when I got past 50mph - it was insane. Insane. At 65mph the steering wobbled so much I was lucky to make it out alive. A regular pedal bike at 50mph is a good way to end up in the ER if you're lucky, or dead if you're not. Sure, someone, somewhere, might get irked, and post to chew me out about how great a 50mph ebike is, but come on. If you're running that kind of rig, you have a heavy duty frame (probably custom), with massive suspension, fatty tires, brakes etc. It's not really a bike at that point, it's a motorcycle, although maybe a lightweight one, and legally still an electric bike because you "can pedal it".

Either just realize you are talking about an electric motorcycle, which is totally fine and doable... (and ditch the idea of a geared motor - you obviously need to do a LOT more reading and research if you thought that was gonna work)...there are even electric hub motor motorcycle wheels commercially available...
...or downgrade your need for speed. An ebike is really best in the 17-30mph range, unless using beefier tires/components. Even with beefier stuff, 45 is really the upper limit IMO, and even that isn't entirely safe (it's only really safe for straight stretches with no risk of sudden cross traffic/stops).

BUT if I was going to attempt this (50mph+ ebike) - here are my tips:
1. If you are set on a 2 wheel upright bike, go with 2wd (hub motor in front as well as another motor powering rear wheel) Why? Not just traction, although that is very good for safety at high speed as well (notice race cars have wide tires), but also a major benefit is the spinning weight in the front wheel will stabilize your steering at high speed. You do not want wobbles after you sink $1500 into your beast machine and realize it's an overpowered moped which you can only ride safely up to 40mph. Avoid the pain of overbuilding, only to realize it was wasted. This may also making steering more difficult, because there is some resistance, but at 50mph, you need all the stabilizing you can get up there. Non spinning weight would stabilize without causing the steering difficulty, so maybe load the batteries on the front fork somehow? (I've never seen it done, but it seems like a decent idea to me, as long as it's not in the way of steering)

2. Personally I would want a low-profile recumbent with full fairing/shell. Human powered vehicle land speed records are always set on shelled recumbents, for a reason.
Aerodynamics. Much easier to reach high speeds. The current record: 83.33MPH Yes you read that right. MILES per hour, not kilometers. Just pedalling. Can you freaking imagine? At that point, they are basically lying down flat.
At 25mph wind resistance becomes significant. At 50mph you are spending more energy pushing air out of the way than you are hauling weight across the ground.
This is a really helpful page about wind resistance speed increases: http://phors.locost7.info/phors06.htm
The table at the bottom shows that at 30mph you are using 1.16 horsepower. At 55, you are using 7.14 horsepower. Think about that. From 30 to 55 isn't even doubling speed, but the jump in horsepower, from 1.16 to 7.14 means it's using more than 6 times as much energy. It's really ludicrous. Hence getting a more aerodynamic setup. That's how folks can do 83mph on human power (less than 1hp, more like a bit over 1/2 a horsepower). Upright bikes are air hogs.
Another thing that's great about low -to-the-ground recumbents, is they FEEL fast. It's like a go-kart. 30mph feels as crazy as you want to go - everything is wizzing past and you lose all desire to go faster. Also the lower center of gravity helps you feel secure, so if you did push the speed, you don't feel as exposed as on an upright.

3. There are way better motors out there, if you are thinking of a geared motor, you need to read a lot more. DD for speed all the way. I'd suggest crystalyte, followed by 9c. Do your research. Also, mid-drive could work, but less simple to install, and usually chain noise is audible.

For ebikes, I recommend sane speeds around town - up to 25, with capability of 30-35mph bursts as needed. If you want more than that, you might consider just getting a regular motorcycle. If you hate the exhaust, convert it to alcohol.

Also, one last thing - I make sure you run your build by folks on the forum before you run out and get parts. I don't want to be responsible for encouraging you to do something crazy. Read up a lot before you build something that pushes a device designed to go 17-30mph (bicycle) to 50mph. They aren't designed to be powered, let alone that much power. Imagine the front fork ripping into shreds from metal fatigue as you hit a dip on a downhill at 60+mph. It can happen. Hairline fractures can be invisible on the surface, and you could be dead in seconds.
 
I'd say you'd be interested in my favorite ongoing project. The guy just runs around having fun with it without having spent so much as $1,500, so I would think, yet I think he must be able to crack 50mph. Note it became more and more motorcycle like.

http://endless-sphere.com/forums/viewtopic.php?f=6&t=63428

But this guy has cracked 50mph with the $300 Yescom kit and a replacement controller. And yes, he's probably asking for trouble doing it.

http://endless-sphere.com/forums/viewtopic.php?f=6&t=59005
 
Wow, I was not expecting that, thanks Dauntless! Sure am glad I replied to the topic just so I could get the email alert and read your post. That thing looks like it would work really well, and my initial reaction was wanting to get one. But the more I thought about it, the less safe I think it is to use it at 50mph. Looks really cool though, minimal, has a great seat.

Personally I wouldn't trust my life to a vehicle with an original MSRP of $450 going 50mph. Sure, it's built for "motocross" but at much lower speeds than 50mph.
Something can look very strong and sturdy, but after a lot of road vibration at 50mph, weld joints are vulnerable to snapping if they aren't done properly with good metal. Something you don't want happening at speed. There's a lot more momentum at 50mph than 18mph. Still, motocross is pretty harsh on a bike, so who knows.
It could be fine, but if it were me, 50mph would mean using a vehicle/frame/parts designed to travel that fast, or custom built by someone who builds them.

Still, reading reviews etc should give some idea of whether they are prone to cracking, and how cheaply it's put together. It could be a very cool ride, if it worked out.
Hopefully something like that will be available affordably with beefier motor, LiFePo4 and decent components. $1500 would be on par with 50cc mopeds.


[editted because I was bit too harsh on the bike/idea]
 
Hey folks, I have built many motorcycles, from superbikes to little trail bikes. my all time favorite, was a 100cc 4speed, yamaha street bike. It went 72mph, and weighed about the same as my ebike now, 100lbs. even on that tiny motorcycle I used larger hubs, brakes and forks. Consistent speeds of 40 to 50, hammer the bejesus out of bearings, and everything else. if you use bicycle tires, you will crash. I used some cheap, $30, cheng shin? Tires that allowed me to drag my elbows, and brake very heavily. I would hate to hit the pothole I hit today at 50mph, no bicycle whell would have survived, let alone forks, bearings etc. Those cute little head bearings, wheel bearings, will be be dangerous. Bicycle brakes, even hydraulic discs, will be very dangerous. I am not a safety nazi in any way, but busted forks at 50, will kill ya. on top of the safety issue, you, ll save money by building heavier. Bicycle parts will require constant replacement/maintenance. I, ve destroyed alot of bicycle stuff, over the years, just on pedal power. it is necessary to carry extra parts when touring on a bicycle, I have had to replace many bearings on the side of the road. Flats, bent rims, collapsed rims, etc etc. That is touring at 10-20 mph. Don, t do it.
 
40 mph on that budget, 35 mph usable, is not hard on that budget. 50 mph will take mo money.

Not that I advise it, no cop that sees you whizz by at 40 mph on a bicycle, :shock: is going to think, " No need to stop that guy and talk to him."

Cheap kit hub motor, replacement but still cheap ebay 72v 40 amps controller, 72v 10 ah of batteries from Hobby King.

Armor up, those bike tires are pretty sketchy at 40 mph.
 
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