Dirt bike fork on an ebike?

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rg12
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Dirt bike fork on an ebike?

Post by rg12 » Dec 10, 2017 1:05 pm

Hey Guys,

I'm interested in building an ebike with an enduro frame (those fake bomber ones that everyone order from China).

The reason for a dirt bike fork is first of all for the fact the it enables me to have a motorcycle front brake without any adapters.
The wheels in the build are 18" x 2.15" dirt bike wheels anyway so instead of having an mtb hub I will just get a complete front wheel for dirt bike with a dirt bike hub that fits a dirt bike brake disc.
Also I noticed that new dirt bike forks are very inexpensive.
I also look for the front weight increase since once you go with an 80A+ for 20S you are going wheely all day, especially when going towards 100A or 24S (or both).

A few questions on my mind:
1. Will the fork be soft enough for a 45-50kg ride?
2. Will I need special adapters for mounting it on the frame?
3. Can I use an mtb handlebar with it or do I have to use a dirt bike handlebar? and if I have to use a dirk bike handlebar, will a rear regular mtb brake lever fit on it? and will mtb lock on grips fit on it?
4. Are there sizes to the forks? like for length and such? are there sizes for width which change the width of the hub or is it all one standard?
5. Is there an option to have a fork for double sided discs for a dirtbike? (yeah I know, overkill, but may look killer)
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Re: Dirt bike fork on an ebike?

Post by markz » Dec 10, 2017 1:24 pm

Try this search
https://endless-sphere.com/forums/searc ... cycle+fork

Comment - https://endless-sphere.com/forums/viewt ... k#p1304889

Also have you tried tradition regen braking on direct drive?

Apart from that, I remember one comment by someone here on E.S. that there are controllers that specifically have regen braking, it uses very little current to accomplish that.

I dont know how a motorcycle fork would fit on a bicycle, unless you weld on the matching head tube in front of the bicycles one. Maybe all thats needed is welding on a top cup and bottom cup, but maybe too the stem of the fork is too wide. So many variables. That one comment I linked too stated motorcycle brakes require the weight of the motorcycle and rider to properly heat up the braking pads so they work properly but I think you could use a 50cc-100cc or maybe even 250cc because of the added weight the motor and battery bring to the table.

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Re: Dirt bike fork on an ebike?

Post by rg12 » Dec 10, 2017 3:52 pm

Avid BB7? a 2 piston mechanical disc brake?
I got customers telling me they are frying pads and rotors a few times a week riding like crazy on roads with 2 piston basic hydraulic sets...

If you got wide tires which many now change to 18" motorcycle rims then you got alot of grip and with that grip your once strong brakes are no longer too strong and locking your wheel since you got tons of a grip.
My two piston hope tech pro's felt super strong on my 24" 2.5" mtb wheels, then changed to 19" motorcycle tires 2.75" which are 3.2" in the real world and the brakes feel so week since it's impossible to lock the front wheel.

I will take small motorcycle brakes for light dirt bikes to prevent the heating issues.

I have seen in google dirt bike forks on mtb's, so I know it is possible but I wander if the effort is not too big.
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Re: Dirt bike fork on an ebike?

Post by fechter » Dec 10, 2017 6:47 pm

Double crown forks made for bikes will fit the head tube. If you can find a set of triple clamps for a bike fork that has the same diameter as a MC fork, then you could just slide the fork tubes in and go.
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Re: Dirt bike fork on an ebike?

Post by litespeed » Dec 10, 2017 6:51 pm

I have Hayes Ace 4 piston on my bike (enduro frame, 20s15p, MaxE/QS205, 19, 2.75/17, 3.00 blah, blah blah) and I can lock up the front wheel no problem when ever I want. My bike is well over 70 mph and the good bike stuff works well. Maybe doing stoppies non stop might kill the front brake or warp the rotor but no problems for "normal" high speed trails and street runs. I'm 6'4" and 210 lbs....bike is in the 110 to 120 lbs range.

I know you can do the small forks from pit bikes and 80 size dirt bikes but until your over 20kw I think those might be overkill.....or at least my thoughts.

Good luck what ever you decide.

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Re: Dirt bike fork on an ebike?

Post by liveforphysics » Dec 10, 2017 7:59 pm

I ran a dirtbike front end (forks, wheel, brakes) and a superbike front end (forks wheel and huge dual disc brakes), and both felt great to me. If your riding style involves a lot of hopping you might not like it over bicycle parts, but they feel great and even a single dirtbike brake is better than any bicycle brake.
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Re: Dirt bike fork on an ebike?

Post by Chalo » Dec 10, 2017 8:01 pm

The crown race seat and upper steer tube won't be the correct diameters for a bicycle headset. You'll need to make adapter collars at least.

And you'll need to get used to a bike that's pig heavy in front and probably the wrong height and/or fork offset for optimum steering characteristics.

But, if you just have to make your bike more "moto" while also making it worse, go for it. Take care the fork you use doesn't allow the tire you put in it to contact the fork crown at maximum compression.
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Re: Dirt bike fork on an ebike?

Post by liveforphysics » Dec 10, 2017 8:58 pm

If you're used to the feel of the flex in bicycle suspension and wheels, and it's not breaking on you then you're good.

I broke even a fox36 fork, and melted down and wadded up the biggest downhill bike brake rotors before finishing races.

Most ebike applications have lower component stress levels where good bicycle parts may be fine, and feel so light and nimble. Where dirtbike front ends feel better is in hitting big drops and big step-ups, and doesn't feel like a wiggly noodle trying to keep your front wheel path tracking over big log piles.
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Re: Dirt bike fork on an ebike?

Post by rg12 » Dec 11, 2017 3:29 am

liveforphysics wrote:
Dec 10, 2017 8:58 pm
If you're used to the feel of the flex in bicycle suspension and wheels, and it's not breaking on you then you're good.

I broke even a fox36 fork, and melted down and wadded up the biggest downhill bike brake rotors before finishing races.

Most ebike applications have lower component stress levels where good bicycle parts may be fine, and feel so light and nimble. Where dirtbike front ends feel better is in hitting big drops and big step-ups, and doesn't feel like a wiggly noodle trying to keep your front wheel path tracking over big log piles.
Thanks for the reply, I remember your super moto front photo, that looked sick :)
Can you please reply to my compatibility questions from the main post?
You are the only guy here who actually done it...
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Re: Dirt bike fork on an ebike?

Post by liveforphysics » Dec 11, 2017 12:48 pm

I had to cut the head tube off the frame and lathe up a new one and weld it on each time I changed front ends.
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Re: Dirt bike fork on an ebike?

Post by Drunkskunk » Dec 11, 2017 1:04 pm

forks don't have a standard sized steer tube. you will have to adapt depending on what source you use. here's a list someone did of various axle and bearing sizes for Honda's http://www.hondatwins.net/forums/34-tip ... sizes.html

The handlebars on most dirt bikes is the same diameter as a BMX handlebar, but isn't comparable with MTB handlebars.
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Re: Dirt bike fork on an ebike?

Post by Chalo » Dec 11, 2017 5:07 pm

rg12 wrote:
Dec 11, 2017 3:29 am
Can you please reply to my compatibility questions from the main post?
You are the only guy here who actually done it...
I, however, have made triple clamps and fork crowns from scratch for both motorcycles and bicycles. Like I told you, the crown race seats and steer tube diameters won't match and you'll have to adapt them. Fortunately for you, motorcycle steer tubes are usually smaller than their bicycle equivalents, so you can simply make interference fit or split ring collars to reconcile the sizes. Or you can make a custom steer tube or set of triple clamps, according to the specifics of the fork and your ambition/ability to manufacture these things.
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Re: Dirt bike fork on an ebike?

Post by rg12 » Dec 12, 2017 8:17 am

Is the steer tube changeable?

Also, will the shock be soft enough for a 50kg ride?

EDIT: Is the diameter of a dirt bike handlebar where the grips and levers go the same as a mountain bike handlebar?
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Re: Dirt bike fork on an ebike?

Post by Chalo » Dec 12, 2017 2:38 pm

Most of the moto lower triple clamps I have worked with were press fitted with steer tubes. Those are difficult but not impossible to retrofit with replacement steer tubes. Some are clamped on, and those are easy. In either case, to change to a different diameter steerer, you either have to ream the triple clamps to a different size, or you have to machine a step in the steer tube where the triple clamp fits.

I have only seen motorcycle steer tubes that are smaller than commonly used 1-1/8" or tapered bicycle steer tubes. These can be sleeved over (at the crown race and separately at the headset/stem) rather than replaced, if they're long enough to fit the bicycle frame and stem.

In my casual observation, motocross forks tend to be initially softer, but with much longer and more progressive travel than bicycle suspension forks. So I think you wouldn't have a problem with them being too hard, but perhaps you wouldn't use as much of the suspension travel as normal. Keep in mind that moto guys usually want their forks to sag 25% from just bike and rider weight, which would promote terrible pedal-induced bouncing on a bicycle. On an e-bike, you'll be using more of that first 25% of travel for bumps instead of rebound.

There are lots more different springs available for motorcycle forks than for bicycle forks. If you don't like the spring rate you start with, you can change it.

P.S. - yes, the grip section diameter of MX bikes is the same 22.2mm as that of MTBs.
Last edited by Chalo on Dec 13, 2017 4:23 am, edited 1 time in total.
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Re: Dirt bike fork on an ebike?

Post by rg12 » Dec 13, 2017 4:00 am

Thanks alot man for all the info.
I plan on maybe doing the sleeve thing and designing a race for the steer tube bottom as the bottom crown is not flat and also drilling the top crown to the size of the bicycle steer tube to lock it with a regular bicycle screw that goes into the steer tube.
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