Target: 35Mph, Trek 820 Frame, 36 OR 72V. What would YOU do?

[Pyrotrons]

Thanks to each of you for such a useful forum.

For my first-time build, I've got an older model Trek 820 that I'm converting. I need to cruise at 35Mph, but my battery options are limited to either 36V OR 72V. Already, I seem to have found myself between a rock and a hard place. Am I overthinking this?

36V:
25Mph seems to be a generally-cited max speed. Is procuring a low-turns-count motor my only option (Kv change) to get to my target speed of 35Mph? Currents will start getting high... 60A? 80A?. I assume that high-current controllers exist that are NOT rated for higher input voltages... because higher voltage MOSFET's would be a waste of power (higher ON resistance, Rds_on). I don't want to spend money on a custom-wound motor. Very little about this option seems practical.

72V:
The major issue here seems to be the 135mm drop-out width of my Trek 820. I'm unaware of any 72V motor with a drop-out other than about 150mm. Forcing the seat and chain stays out by 15mm doesn't seem too bad... I've seen it recommended for steel frames like this... what can of worms might I be opening?

Running 72V into a 48V hub motor?

I can drop a 48V hub motor right into my Trek. Provided that I can solve all thermal issues (by oil-filling, replacing with Teflon wiring and adding heatsink), where do I hit the point of diminishing returns when over-volting a 48V hub motor such as http://www.ebay.com/itm/48V-Electri...254627&hash=item339dcf9fe8:g:QG4AAOSwXetZUM1p

By "point of diminishing returns", I'm referring to saturation, eddy-current and hysteresis losses. Am I in uncharted territory with finding lamination thicknesses of various China motors? Overthinking. Help.

Regards,

- Pyrotrons

 

[Voltron]

Ive been running 72v ( 84v right off the charger) in a so called 48v motor for years with no problem.

 

[ScooterMan101]

To go that speed, or anywhere near that speed I would ... Only use a Full Suspension Bicycle !

 

[wesnewell]

I run my 48V 1000W motor on 88.8V/100.8V charged without a problem. The kit controller is probably limited to 63V max, so you'll need a new 72V 40A controller. Shouldn't be a problem unless you ride long distances at 35mph or try go up steep hills at max throttle. At 72V it will probably max out at ~40mph or a little more depending on weight. If you want to be absolutely safe I'd go for a mxus 3000 4T with 72V controller. It'll run 40mph all day long. The 48V 1000W mor will run continuous at 30 mph or a little more all day long.

 

[Pyrotrons]

Thanks to you three for your thoughts, I appreciate it. I'm hitting the GO button on a 48V hub motor as soon as I can print the drawings and put eyeballs and calipers on everything. For now I'm assuming that the motor will clear the stock derailleur (Shimano Tourney TY300) and cassette (Shimano TZ21, 14-28, 7 speed) but I've never done this before.

Of course I'll either be making or buying proper torque arms.

Planned Battery:
I'll use the LG cells from Ebay seller "alarmhookup", who advertises 36V/4.4AH (~4.1AH realistically) per pack (20 cells). We have one pack here in the shop and it tested pretty close. Final configuration will be five of these packs in parallel, wired in series with another bank of five in parallel, making for a realistic 72V @ 20AH. 1440WH/20lbs and 260USD shipped seems kinda heavy but also kinda great. It slightly bothers me that the gentleman (Richard Lloyd...much thanks to him...) that created a YouTube review video of this pack found that the BMS did *not* equalize an intentionally-skewed cell.

I'll lay out a PCB for ten XT60-Male connectors, so I can just plug the pre-terminated packs straight into the board. Maybe I'll put fuses on-PCB between the modules... that seems smart. And come to think of it, for Cycle Analyst, maybe I'll also put a 1mΩ shunt in there with a couple of screw terminals across it.

I'm fortunate to have access to a bandsaw and TIG welder, to make some 1/8" Aluminium sheet triangles for an enclosure.

I'm hoping everything else will fall into place (Cycle analyst wiring, twist-throttle, finding a charger, etc.)

 

[wturber]

I'm using the same batteries, just not putting them in series. I don't need/want to go that fast - other than downhill.

The ten packs I have had two different BMS connectors. One is the standard JST connector. What I've started doing is to install disconnects so that I can bypass the BMS and do a balance charge using the connector that ordinarily would connect to the BMS. The problem Is that I haven't been able to find a plug for the second, smaller kind of connector. My thought is to do a balance charge on the packs ever couple weeks or after a certain number of non-balance charges. That should keep a pack from getting too far out of balance and help identify any pack that might be going bad.

 

[Voltron]

The bulk charging with occasional balancing is a good plan... And don't be too thrown by the bms not balancing... Many of them have such low balance current it takes forever to fix a group that's way off. Often they're there more to cut off a low group during discharge than do heavy duty balancing.

 

[Raisedeyebrows]

To go that speed, or anywhere near that speed I would ... Only use a Full Suspension Bicycle !

Agree-personally I don't want to do over 30 mph on anything without FS. My Hardtail hubmotor bike will do 35 and at those speeds I need to stand up when I see any potholes, speed bumps, cracks in the road, driveway entrance curbs, basically anything bigger than a cigarette butt in the road. Failure to do so results in too much shock to the body and that's with decent suspension forks and a good steel frame.

Added:my FS bike is like riding on a pillow in comparison, bumps no problem and a lot safer feeling.

 

[Chalo]

Apart from suspension versus no suspension, the Trek 820 is a very cheap bike which is in no way competent to run routinely at 35mph.

 

[hppav]

any updates to this?

 

[Buk___]

the Trek 820 is a very cheap bike which is in no way competent to run routinely at 35mph.

If he had paid twice as much for it, would it be better suited?

 

[Chalo]

the Trek 820 is a very cheap bike which is in no way competent to run routinely at 35mph.

If he had paid twice as much for it, would it be better suited?

Probably yes, because it would be a model slightly less compromised by cost saving design decisions. It would still, by odds, be a general purpose recreational bike that aims to please as many people as possible at a limited price, and sell in volume. Not really the right tool for the job.

But if he had paid five or ten times as much for it, and if he'd chosen well from among his options, it would most likely be a more specialized machine with better specific abilities in regard to speed capabilities, braking, load carrying, and/or long term durability. You can pay the cost up front, or you can pay over and over again as you struggle to get things right. Or you can make your own, or you can give up. In the past, I've exhausted every one of these options. Now I suggest paying the realistic cost of what you're trying to do, or making your own (with all the background work and self-education that approach requires-- not always cheaper in the long run).

Most, y'know, simple folks exhibit better common sense about these things when you frame them in terms of cars. If you're getting a car for the purpose of going three times as fast as most people drive in their cars, at a much higher gross vehicle weight, do you choose it on the basis of overall lowest price? No, you do not. That would be stupid.

 

[Pyrotrons]

I appreciate the previous words of warning. Sometimes it's hard to gauge how far statements on the internet go, but you should know these things ring in my head more than anything else. It pushed me to procure what I think may be a slightly more capable frame... a Trek 4500. It's outfitted with hydraulic disc brakes that feel like Gold bullion and a front suspension that's so corroded that it might as well be welded. If I choose the 4500 the front fork gets replaced.

I was under the impression that the steel frame of my Trek 820 was something that was sought after for an ebike... perhaps that was specific to the strength and flexibility of its drop-out. Maybe the Trek 4500, though being substantially more expensive than the 820, is a worse idea due to its Aluminium frame. I do draw and machine and weld and understand the failure modes of these materials...I'm all ears for Trek 820 Vs. Trek 4500 comments.

Safety is often situational; consider mine. The aforementioned Trek 820+48V hub+84V_charged build will be a commuter, and I have 2,400 miles on a previous (slower) ebike on the paths I'll be using it on. I know precisely where the potholes are. I also scuba dive, am a private pilot, have extensive experience with motorcycles, Corvette, trucks, trailers, boats, ice, mountains, gravel... you name it and I must say, 30Mph on the Trek 820 doesn't feel too unreasonable. I think 35 would be a bit sketch... 40Mph, too much for everything except a controlled top speed run. I wonder how many joules of energy an Armadillo contains at that speed?

 

[dogman dan]

The 820 is a good frame for a basic commuting bike. My idea of such a bike hits perhaps 25 mph, maybe 30 mph right out the gate, but 25 mph as the battery drains some.

Nuthin fancy, so nothing fancy needed, like disk brakes or high end suspension.

For 35 mph cruise, if you run 72v, you would want at least a better fork, but catch 22, a good new fork won't fit the frame, because the headset size is more common for cheap pogo stick forks.

The main thing you will get from the better frame, is a headset that fits a decent fork, and disc brake mounts. But,,, catch 22 again, really good forks now come in tapered headset.. So it might mean buying another damn bike, just to get the fork off it.

On the bright side, you can weld and modify the steel frame to your hearts content. You can weld on brake mounts, and even change the headset tube. Personally, if I see a good deal on a decent steel frame, I snap it up. I don't care if it has wheels or not, I want that good steel tube to weld on frankenbikes. I pay little to nothing for cheaper steel frames, like roadmaster or cheap mongoose.

For hauling ass, the first thing I'd do with that trek is lengthen it about 6-9 inches. Then it will track nice and wobble less at 35 mph. The longer bike will need shocks less, when hauling ass. You'll be amazed what 6" can do. Its possible to weld a bolt on attachment to lengthen any bike, including the alu one, the 6-9 inches.

 

[Chalo]

I was under the impression that the steel frame of my Trek 820 was something that was sought after for an ebike... perhaps that was specific to the strength and flexibility of its drop-out.

There's nothing wrong with the 820's frame. You could make it work fine if you so desired. But a bike is more than its frame. The parts that allow a Trek 820 to sell for $380 retail, brand new, are not designed or intended to run at traffic speeds routinely. That fork, particularly, is probably not helpful even at slow recreational pedal bike speeds, and could easily become a safety issue at 35mph.

 

[chuckufarley]

60v should get you to 35mph.

 

[Pyrotrons]

I want that good steel tube to weld on frankenbikes.

For hauling ass, the first thing I'd do with that trek is lengthen it about 6-9 inches.

I like how you think dogman dan 8)

This is all highly intriguing... especially welding on disc brake mounts. Maybe I'll do that, and use the nice hydraulic calipers and discs from the seized-fork Trek 4500 frame. Hmm. I guess the process would be:

• Mount Disc
• Assemble caliper assembly to the tabs
• Place calipers+tabs assembly onto disc...fiddle for best tab position against frame...
• Mark position, wire-brush the finish off to expose bare steel
• Tack Weld the tabs to the frame
• Remove the calipers so they don't melt
• Weld that bitch on

I can't get to the bike right now, but even so, I'll wonder if there will be something that I don't know, that I don't know. Dogman or anyone else... is there an alloy of filler rod that you'd recommend for TIG welding this? 2009 Trek 820. Definitely steel.

 

[JohnT818]

The 820 is a good frame for a basic commuting bike. My idea of such a bike hits perhaps 25 mph, maybe 30 mph right out the gate, but 25 mph as the battery drains some.

This! I converted a Trek 820 (2012 or 2013 maybe), and I’m pretty comfortable doing these speeds on my well maintained suburban streets. I top out at about 28 mph and haven’t spent enough time going down hills to know much about 35 mph, but I’d guess that doing it on a regular basis might be a bit risky.

I’m only 5’6”, so the short frame works great for me! I even opted for the XS version. I love the nimbleness of small frames, but I suppose they’re not ideal for high speed applications. I wonder if a longer fork would result in a slightly longer wheelbase and more rake for better stability without messing up the geometry too much.

 

[wturber]

One of the things that convinced me to finally settle on a basic hard-tail DD hub as my commuter was this video below of a guy in Portland who has put over 10,000 miles on his e-bike. He's not a speed demon. Though he seems to routinely take his bike over 30mph. His typical speed when conditions permit seems to be between 22-28mph) This is his car replacement, general purpose, transportation bike. His use seems to be pretty much (maybe a tad faster than) what dogman dan was describing for what the 820 frame is suitable.

[youtube]2_zVFn6x3jg[/youtube]

So an 820 appears to be fine for a building a commuter bike that will exceed 30mph. But perhaps not if you are going to sustain speeds of 35mph for long periods of time - something that seems like a questionable activity (road use) on an e-bike to me regardless of frame.

This is a video of a 26 mile trip that he rode. You can see his speeds on the display overlay in the lower right of the video.
[youtube]fsmwqGoiS4Q[/youtube]

 

[Voltron]

Just for reference, here's what 7 minutes of jamming around town looks like at 25 to 35 mph. The extra 10 mph really really makes a difference in the bumps and how fast things are coming at you. If you plan on doing it on a sustained basis, your brakes and suspension take a beating, and your battery mounting gets really important, esp if carrying a big enough battery for decent range.

[youtube]gux0etlFN18[/youtube]

Its less rough than it looks as the rider, soaking up the bumps with your legs while you stay in a tight aero tuck...( thats my style anyway...lol)... but with the camera mounted to the bike, it really shows what the bike is going thru.

Then theres this.. which again shows how important brakes are at high speed near cliff edges... in the final minute the road switches to old rutted blacktop, and is another good look at how bumpy 35 is on anything other than perfect pavement, and how on it with your focus and reflexes you have to be.
[youtube]XShgunSoW5o[/youtube]

 

[Voltron]

And re. the 820, when it was the early day of people throwing kits on crappy Huffys and such, the 820 was a sturdy workhorse compared to them. But as things have moved along, there's more and better choices out there for cheap.

 

[wturber]

And re. the 820, when it was the early day of people throwing kits on crappy Huffys and such, the 820 was a sturdy workhorse compared to them. But as things have moved along, there's more and better choices out there for cheap.

For instance ... ?

Right now, I'd build a commuter on a Trek 820 frame in a heartbeat. But if there's something better/cheaper, I'm all ears.

I ask because I recently built a basic commuter and ended up with what appears to be a fairly sturdy aluminum mtb frame. But, frankly, I was just looking for something from a "real" bike maker so I'd be assured of having a decent frame. I figured that was a better compromise than buying a cheap new bike. But if there is another alternative, that would be good to know for myself down the road and for others.

 

[Voltron]

No, I'm def thinking used also. But there are so many reasonably priced lightly used ones on Craigslist, in my area anyway which does have an abnormally large bikes section, that will have disc brake mounts and a better fork, and the newer larger seatposts and handlebars that all help stiffness and weight.

I just did less than 5 min of looking just now and found this GT for $75, and the K2 29er for $200




Totally different groove than a commuter, but I nabbed this one for a future conversion for $350 a couple of weeks ago!



And there's nothing inherently wrong with the 820 frame structurally. Its just getting harder and harder to fit modern parts onto as the standards change.

 

[Raisedeyebrows]

To go that speed, or anywhere near that speed I would ... Only use a Full Suspension Bicycle !

Agree, otherwise you'd be beating yourself to death and a substantial pothole could be very dangerous.

 

[Voltron]

Finding a setup that actually likes it at those speeds regarding the wheel balance and whatnot is important too. It gets annoying and dangerous if the bike really makes you pay attention all the time to keep it going the right direction...

[youtube]sb2bFfwDAIU[/youtube]

A little lower speed, but a lot bumpier trail with frost heaves and ravines. This sort of thing is prob the one of the most important characteristics I look for in a build, and its sometimes a hard thing to tell by just looking at a bike.

[youtube]nD3s0gEmiLs[/youtube]