Hybrid Dirt / Mountain bike

[LegendLength]

Just wondering what people think about the future evolution of electric off-road bikes. I know there are currently some companies making this type of thing but I'm interested in what you think the ideal components would be.

For instance the power of current light hub motors is pretty small, say 3 kW / 4 hp. If you take a typical dirt bike like a YZ 125 cc it makes 25 kW / 36 hp at full throttle. I know a bike like that is only just enough power for a typical adult (70 kg / 150 lbs). Yet the YZ itself weighs 90 kg / 200 lbs so swapping the motor for electric might create a good weight saving. So would 20 kW sound like a close estimate for power needed?

A dirt bike will always need a transmission IMO because it needs to be able to scale fairly steep hills off-road (compared to paved roads). People who have tried dirt bike conversions using the existing transmission seem to say all 6 gears are not needed because of the flat torque curve. The estimate seems to be around 3 gears.

For battery capacity I think it doesn't really matter because even if you can only get a 5 minute run out of them it would still allow a bit of fun at places you normally couldn't ride a dirt bike. Would be interested to hear other people's opinions on this though.

The really difficult part IMO except for the transmission is getting the right components such as frame, wheels & suspension. There is a gap between mountain bike frames and dirt bike frames where it's hard to find any offerings. Trials bikes have lighter frames but they aren't the right shape for high speed. Wheels also suffer the same issue and again trials wheels are great for weight but they are made to take totally different loads (no side-loads for example).

I don't mean to ramble, I just want to see what people think about the future of this type of machine. If the transmission was solved I would build one straight away on a strong mountain bike frame but I wouldn't feel very safe. It seems like it will be a while before companies start to build hybrid frames of this type.

 

[amberwolf]

You might wanna talk to Mdd0127 about the prototype bike and designs he was working on.

 

[dogman dan]

I think eventually we will see something like a nuvinci rear hub for the shifting, but beefy for 10 hp or so. Then the chain will be larger to handle the load as well. Light rims and tires such as the trials bikes, but heavier than the beefiest DH stuff now avaliable. It's chain drive of course. Not knocking the greyborg motor, but personally if I had the budget for one, I'd still rather have the weight off the rear wheel for dirt use. Greyborg would be fine for mixed street and dirt roads, but not my choice for badass trails hard to ride a trials motorcycle on.

The idea is to have something with 5-10x the power of a street legal ebike, but keep it super light. Definitely can't have it weighing 150 pounds or you'd be better off with a regular dirt bike.

Most of you here at ES haven't ridden such a critter, but a really well put together dirt ride is quite a heavenly ride. Waaaaay under 100 pounds, preferably under 70 pounds, yet able to climb a 20% grade is pretty nice riding. My way is a compromise for sure, but I'd rather run a lighter hubmotor than have more top speed and raw horsepower. Long as I only pedal up the very very steepest sections of the trail I'm grinning. Those trails are very sketchy to ride faster than 20 mph, so a 25 mph top speed suits my terrain to a T.

Anyway, my point here is, I see no particular advantage to getting too much closer to the power of a 125 motorcycle unless you can do it while keeping it well under 100 pounds weight. It's the lightness compared to a motorcycle that rocks, not the power. BTW, I have owned a trials motorcycle, so I have experienced a pretty good gas ride. The 4 hp ebike that climbs like a goat rides even better in my book.

It's a lot like how I felt about ski's. The real fun began when I dumped heavy cumbersom DH gear, and started dancing through the trees on featherweight super agile telemark skis.

 

[amberwolf]

I think eventually we will see something like a nuvinci rear hub for the shifting, but beefy for 10 hp or so. Then the chain will be larger to handle the load as well. Light rims and tires such as the trials bikes, but heavier than the beefiest DH stuff now avaliable. It's chain drive of course. Not knocking the greyborg motor, but personally if I had the budget for one, I'd still rather have the weight off the rear wheel for dirt use.

If you use a NuVinci rear hub *in the wheel* it kinda negates taking the motor out, if keeping weight out of the suspension is your goal.

 

[dogman dan]

That's true. a beefier version of the nuvinci would be likely to weigh as much as a Mac does now. But surely it would be a lot lighter than a Greyborg or other version of a 5-10 hp hubmotor. The reason it would get done that way anyway would be a combination of simpifying a gear shift option on a chain drive, combined with the dilemma of where to put a gearbox when the pedals take the space where a motorcycle has it's gearbox.

The other way could be a few options, one could be a gearbox built right into the bottom bracket somehow. Another could be to just beef up normal chain and derailur stuff to take 10 hp.

I think to really hit the sweet spot of a high power dirt electric mtb, it's going to take at least a high low gear selection. My approach of a low speed hubmotor is fine for me. I like to ride slow. Others will demand a high speed mode for the flat roads and less twisty trails. But you'll still need that rock crawler mode for getting up the really steep grades. Right now, people just throw power at a steep hill, but lose a lot of the fine controll when riding lower speeds that I love about the slow winding.

Maybe the real solution is a 48v 96v switch on a slow motor. But you still get a honking heavy rear tire with a motor bigger than a 9c. I just fell in love with the lightness of an ebike. I don't see turning them into heavy motorcycles as progress.

 

[bzhwindtalker]

I am working on 4kw/sub 30kg bikes with gears, I am pretty happy with my current bike but there is room from improvement! Dirt/trails riding at 40mph on a lightweight ebike is a feeling you can't describe... you have to experience it.
I am using a DD hub motor as a mid drive trough the bike gears with a freewheeling crankset. I think 5 kw is the limit for standard bike transmission components so I'll stick with 3,5k peak for now, the gears makes the low end torque really nice while having a fast top speed.

 

[amberwolf]

The reason it would get done that way anyway would be a combination of simpifying a gear shift option on a chain drive, combined with the dilemma of where to put a gearbox when the pedals take the space where a motorcycle has it's gearbox.

That's why I want to put my NV as the pivot point for the swingarm. It takes it's weight out of the wheel (and allows for non-bicycle wheel options!), as well as reducing jackshaft requirements and losses.

 

[dogman dan]

Bueno idea. Sort of like the trike I had, that had a drum brake and hub, then another short chain to the drive axle. In your idea, the chain to the wheel would be long, and a short chain from swingarm to crank. Mabye even have the triple ring on the nuvinci instead of the pedals too?

Even at the lame 25 mph top speed of my dirt bike, it's just so much nicer than all that weight of a full blown motorcycle. I know they got lighter since the 80's, but still, no way do they come close to sub 70 pounds. Bumping HP up to 5-10 means mo battery and mo weight, but it's still just a fraction of a typical dirt bike.

For me, it really is heaven to go down twisty trails flicking the bike through tight corners with so little effort. In fact, I built 8 miles of singletrack deliberately too twisty to appeal to any motorcycle or quad. But it rides nice on the 70 pound ebike. You've seen the vids.

 

[lostrack]

In my experience, putting more than 5KW down using bike tyres is a challenge, they just spin and spin in the dirt. MX tyres are huge and have a larger surface area with knobblies.

 

[Drunkskunk]

I Like the Nuvinci, but I think the weight should be out of the hub, and There is something kind of primal about shifting through gears on a dirt bike that adds to the experiance. you could replicate the NuVinci's performance with a frame mounted CVT transmission, but I think Efficancy and Rider experiance would be enhanced with a good old fasioned clutch and gear type transmission.

 

[mdd0127]

If you put the nuvinci low in the frame, in a position where it spins much faster than it would if installed in the rear wheel, it will see less torque to output the same power. Then, you gear up the pedal drive to match the motors speed range at the nuvinci input, then bring the overall gearing back down with the final reduction. This way, both the motor and the pedals can take advantage of the gearing options the nuvinci provides independently or assisting each other and the nuvinci will survive at 12kw because it's never seeing too much torque. :wink:

 

[amberwolf]

Bueno idea. Sort of like the trike I had, that had a drum brake and hub, then another short chain to the drive axle. In your idea, the chain to the wheel would be long, and a short chain from swingarm to crank.

On mine, it's actually a normal-length or even shorter chain on the swingarm. Probably same length as a typical single-speed bike would have.

The longest chain is from cranks to NV. On mine it is actually planned to be a hella long chain, because it will go around the whole frame space, so that it doesn't pass thru the spaceframe that the motors and batteries go in, unlike on CrazyBIke2 where the wider spaceframe in it can't all be used, due to the chain passing thru there. Can't stick a big battery box in, simply because the chain cuts into it and can't be guided laterally around it. But if it's moved *above* the area, it will clear it all.

Mabye even have the triple ring on the nuvinci instead of the pedals too?

Possible, I suppose--hadn't thought of that. Have to ponder....but I actually don't plan on a triple at all--just a single so I don't need a big tensioner / slack-taker-upper for any of hte chains.

 

[dogman dan]

True enough about the tires, my 1500-3000w dirt ride chews up the tires pretty fast, but not too fast yet with 25 mph top speed. 40 mph in the dirt would be even worse.

So for sure, going to 10 hp means some more weight, better rims and tires.

 

[mdd0127]

True enough about the tires, my 1500-3000w dirt ride chews up the tires pretty fast, but not too fast yet with 25 mph top speed. 40 mph in the dirt would be even worse.

So for sure, going to 10 hp means some more weight, better rims and tires.

I wore out my Maxxis downhill tires in 5-600 miles and never really felt very safe on them. I got some Bontrager FR3's to replace them and have been really impressed. I haven't been riding too much lately because my neck is all screwed up but I've put at least 200 miles on these tires and there is virtually no visible wear. They also do pretty well on the road because of the center ridge. I wish they made them wider and thicker but with the same tread and compound. For a 26" bicycle tire though, I don't think these can be beat.

http://bontrager.com/model/07939

For rims, I'm using Sun Rhyno lites and have beat them pretty hard with no issues. After about 1400ish miles of mixed useage, my rear wheel needs some tuning but I built and laced it myself and never really spent much time truing it in the beginning. I ride pretty hard and my bike is a 75 lb hard tail with a 200 lb rider so I'm pretty impressed with the rims too. Since I built the bike though, I know it's limits and try to pick lines that won't tear stuff up too bad. I'm sure that a rider with no concern for, or knowledge about the capabilities of the components would tear things up pretty quickly. http://www.mtbr.com/cat/tires-and-wheels/rim/sun/rhyno-lite/prd_352684_139crx.aspx

Another cool side effect of having a lot of fun while trying not to beat the bike to death is that I don't do any damage to the trails. The typical MX mentality that is still hanging around is really screwing a lot of stuff up for everyone, not just in the marketing/development of lighter bikes, but also the public's perception of what bikes like this would be used for.

As far as the overall concept and topic of the thread goes, I really think that the secret to the ultimate two wheeled fun machine is keeping the bike under 75 lbs and pedalable with 10-15kw on tap that runs through at least a three speed trans or nuvinci. The bike I have now is 75 lbs, software limited to 8kw, accelerates fast enough to scare 250lb, KTM 950 adventure riders and as long as you keep your speed above 10mph, it will climb pretty much any hill. Usually, momentum is needed to get up a hill like that anyway unless you're in Moab, climbing slick rock and have insane balance skills so it's pretty much a dream off road. If it ran the motor through the nuvinci, had the nuvinci mounted in the frame, and had rear suspension, it would be PERFECT.

We'll definitely see some bikes that are between the regular dirtbike and mountain bike sizes soon. It only makes sense. Check out some FX85 videos for some good examples of what something like this can do.