Who runs a 20" rear hub?

cajunjay · Mar 6, 2012

I want to run my 48v 500w rear hub motor in a bmx bike but not sure how to make it fit.

I havnt yet bought a bmx or a frame as yet until I clear up some of these issues. If you laced the wheel yourself or had somebody do it for you how did you get the right size spokes as they must be really short. Also was there any issues getting the motor to fit in the dropouts.

Also are there any other issues that I have overlooked?

Thanks

Ypedal · Mar 6, 2012

http://ypedal.com/eBMX/BMX.htm

BMX's are fun, be aware that " better " bmx's dont have 10mm slots for the axle.. so pay attention ( some are 12mm+ )

also , the rear frame is single speed 110mm, most rear hub motors are milled for 135mm with 7 speeds.

I used a front motor, and used washers to put it on the rear.. ditched the chain..

Alan B · Mar 6, 2012

Is there also a spoke count issue?

Timma2500 · Mar 6, 2012

Alan B said:
Is there also a spoke count issue?

If your hub is 36 spoke, it won't be an issue.

Chain Reaction Cycles list plenty of 36 spoke BMX rims:
http://www.chainreactioncycles.com/Models.aspx?ModelID=20495

Paul

cajunjay · Mar 6, 2012

Nice bike Ypedal.

Getting a 36h 20" bmx rim isnt a problem theres tons available.

My only issue is I dont want to ditch the chain as I need to be able to pedal aswell.

Ypedal · Mar 6, 2012

I have not tried this, but a front motor, with disk brake, ( would have to be Direct Drive ) flipped over and freewheel installed instead of a disk.. may work.

Brentis · Mar 6, 2012

I run a 20" wheel in a 26" bike.
That is something you may want to consider.
20" BMX' have a short wheelbase. Makes for a quick handling machine.
Putting power to it, only exasperates the situation.

Do you have lots of BMX experience?
If not I suggest you do a bit of research into bigger bikes(longer wheelbase).
Search 24" or 26" BMX. Also search dirt jump(er) bike.
You can still use a 20" wheel on these machines for the benefits of 20" wheel.
But do require some modification. (ground clearance/ pedal strike)
It will be a whole lot less modification than converting a front hub to rear.

What continent are you on?

Keep us posted.

Brentis · Mar 6, 2012

I just did a quick search & there are examples of 26" bmx bikes.
http://www.firemansbikes.com/texascruzer.htm
http://www.sycip.com/bikes_bmx.html

No affiliation, just a couple of US examples. :wink:
More out there.

Brentis · Mar 6, 2012

If you are handy, willing to be a guinea pig.
I am certain you could cut up a standard/bmx freewheel hub.

Then weld/have welded, just the flange with threads(axle removed) to the right hand side of a front hub motor cover.
Centered over the axle.
That way you can still have brakes.
Use a single speed freewheel & a lil' MacGuyver'in, should fit a 110mm dropout.

Hope that is clear

Dont forget what YPedal mentioned.
New/Premium BMX Bikes have 12mm axles. will not work with 10mm motor axle.

cajunjay · Mar 6, 2012

Thanks for the reponses

Ive found myself a trials bike frame with seat post that is for 24" wheels.

Im sure with some short cranks maybe 160mm this setup could work ok?

Head tube angle: 73 "

Head tube length 120MM

Frame length: 1020MM

The height of the central axis: 20MM

Fork length: 378MM

Fork width: 135MM

Color: black, white, green, orange

Weight: 1885g

Im in China btw

Brentis · Mar 6, 2012

That looks great. Rear disc brake to boot. :wink:

Battery on the top tube?
Are the dropouts 10mm?

The height of the central axis: 20MM

Not sure what central axis refers to.
How much $ ?

Brentis · Mar 6, 2012

One other consideration.

Head tube angle: 73 "

That is steep. It will be a quick/darty ride @ speed.
You see most folk' here on ES leaning towards a more relaxed steering geometry.
Like DH mtn. bikes

Just be prepared for what you build.
What top speed do you achieve with the 48v500w motor?

Mad · Mar 6, 2012

My 20" ebike.

neptronix · Mar 6, 2012

Golden motor magic pie in it's 20" cast aluminum wheel.

Many motors can come pre-spoked to 20" anyway..

zombiess · Mar 6, 2012

I'd suggest trying a kids full suspension steel frame Mountain bike and convert it to a BMX style. That's what I did. They are very addictive and always beg you to add more power. A high speed hub in a 20" wheel setup fed the proper amps is soooo much fun, but be careful of wheelies when changing geometry. I had one combination that was completely unridable at one point due to it looping me off the back several times by accident because I messed up the geometry so bad.

Started with this, very similar to a cheap Kent steel frame bike I picked up to make another one. Brand new entire bike is $150, got both of mine used for $20 and $40 (second one is being modded now).

Changed a lot of stuff including the rear shock to get the ground clearance I wanted, powder coating, BMX handlebars, better linear pull rim brakes (good upto about 40 mph max, you really need good big disc brakes after that if you want some resemblance of safety and the ability to panic stop quickly and controllably).

Now it's 8" extended swing arm, 26" front fork, 203mm hydrualic disc brakes and way fast. Rode it to work today and find it more comfortable than my full size mountain bike. I think my full size just needs higher handle bars to solve the problem of comfort.

Brentis · Mar 6, 2012

Didn't think of whipping it out. :wink:

Ah, a kool breeze.
:mrgreen:

cajunjay · Mar 6, 2012

Thanks again for all the responses Ive got alot to think about before I buy a frame/bike to run the 20" wheel so gonna give it some thought.

Another question I have which seems pretty difficult is that I can buy a 20" rim with 36h but how the hell do I find the right size spokes. I know there are methods for calculating this but even when I have the actual figure needed the spokes are gonna be really short.

Where did you guys buy the from? If you hadnt bought your motor already laced that is :mrgreen:

d8veh · Mar 7, 2012

An idea that might work, but there's a few ifs.
You might be ale to use a front motor flipped over with an adaptor for the free-wheel if your frame is symmetrical, if the axle is long enough and if you don't need a disk brake. Then switch your chain-wheel to the right side. You'd need a left-hand free-wheel, which I believe are available and the threaded flange off a flip-flop hub or left-hand drive hub.
Here's one I did. I made the adaptor by cutting a pressed steel hub in half and knocking out the middle bit, which leaves the spoke flange and thread. Then drill the flange to screw it to the 6 disk holes.

John Bozi · May 15, 2014

you guys that have put 20" on a 26" bike please give me some workarounds for the pedal strike. I can't find anything in cranks that come remotely being able to answer 5 centimetres I have chop off.

dogman dan · May 15, 2014

Grin cycles would be a good place to get your spokes. Or a 20" motor. Lots of the 20" hubs are radial laced. But I see a cross pattern on the pics above. Easier to do with a smaller diameter motor like a gearmotor. I would guess the rim got some drilling at an angle.

Pedal strike, don't pedal in the corners. Shortest cranks you can find.

cwah · May 15, 2014

Mad said:
My 20" ebike.

what front and rear suspension did you use?

amberwolf · May 15, 2014

John Bozi said:
you guys that have put 20" on a 26" bike please give me some workarounds for the pedal strike. I can't find anything in cranks that come remotely being able to answer 5 centimetres I have chop off.

The only solutions I can give you depends on how your bike is made.

If it is a suspension bike, you could add an extender to the shock or shock mount so that the dropouts are forced downward by the amount of difference in size (3", probably) between your new wheel and hte original, to regain the frame height (and to fix your steering angle back to what it was meant to be).

If it is a hardtail, you could add plates to your dropouts (bolt or weld on) that give you new dropouts that distance below your originals, to do the same thing.

On my CrazyBike2, doing what you did doesn't cause me any problems becuase of the length of the bike, htough it does make my cargo pods close enough to the ground in turns that the turn radius is much greater (closer to a car than a bike!) now.

John Bozi · May 17, 2014

amberwolf said:
If it is a suspension bike, you could add an extender to the shock or shock mount so that the dropouts are forced downward by the amount of difference in size (3", probably) between your new wheel and the original..

This is what looked like a while ago. I am on 18s2p on different controller too.

I looked on google for shock extenders, I only seem to be getting mopeds or car pictures. I don't quite understand where they go what they even look like.
I get images like this:

or like this

Thanks amberwoolf I hope this is a good option. Does the extender mean I lose travel?

dawrench · May 18, 2014

Standard bicycle cranks are 170mm long. Look for SINZ cranks in lengths of 120-135mm. Other manufacturers as well. Sometimes marked as BMX cranks.

Get ones to match your BB - square taper or ISIS.

Find them on ebay or the Used Parts Page (that's where I found mine).

John Bozi · May 19, 2014

dawrench said:
Standard bicycle cranks are 170mm long. Look for SINZ cranks in lengths of 120-135mm. Other manufacturers as well. Sometimes marked as BMX cranks.

Get ones to match your BB - square taper or ISIS.

Find them on ebay or the Used Parts Page (that's where I found mine).

had a good look - sinz in smaller sizes are all 5 bolt hole...

there 4 hole ones are very expensive.

I am currently looking at http://endless-sphere.com/forums/viewtopic.php?f=6&t=49136&start=700#p896042

Who runs a 20" rear hub?

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