20" wheels on mtb frame?

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20" wheels on mtb frame?

Postby Haste » Thu Sep 08, 2011 1:15 am

Seen alot do it...im guessing do to the epic torque? Does the front wheel need to be matched for better handling? How does a 20" wheel do offroad?
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Re: 20" wheels on mtb frame?

Postby sn0wchyld » Thu Sep 08, 2011 2:05 am

its going to do a lot worse over the bumps, because it'll roll into them rather than over 'em. and you only get epic torque from equilly epic low top speed, so its always a tradeoff. Smaller wheels also have a higher rolling resistance, so you'll have a reduced range (ignoring other factors like wind resistance at your top speed).

Im running a HT at 76V, 45A on a 26" wheel. The only time ive been unable to get up a hill is because I gave it too mutch and lifted the front wheel or the back wheel ended up loosing traction, yet I still have a top speed of around 50km/h. I dont think i really need any more low-down torque. unless you're looking to start from stall on a 20% grade without pedaling you shouldnt need to go that small.
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Re: 20" wheels on mtb frame?

Postby Miles » Thu Sep 08, 2011 4:31 am

sn0wchyld wrote:Smaller wheels also have a higher rolling resistance, so you'll have a reduced range (ignoring other factors like wind resistance
It's a pretty marginal factor, on road surfaces........... offroad is another matter.....

If you use a smaller wheel, you can use a smaller hub motor. :)

A 20" wheel on a frame designed for 26" wheels seems a step too far though....
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Re: 20" wheels on mtb frame?

Postby gwhy! » Thu Sep 08, 2011 5:10 am

:D This is my current ride ( for road work only ) I know its not a MTB but it is a frame for 26" wheels and believe it or not it is very nice to ride, only has a top speed of around 22mph but I dont have to peddle anywhere :wink:

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Re: 20" wheels on mtb frame?

Postby Miles » Thu Sep 08, 2011 5:19 am

It looks quite cute :mrgreen:

What's the bottom bracket height....?
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Re: 20" wheels on mtb frame?

Postby gwhy! » Thu Sep 08, 2011 5:29 am

Miles wrote:It looks quite cute :mrgreen:

What's the bottom bracket height....?


:D

Im not sure of the height, the crank arms are 165mm and there is about 50-70mm clearance peddle to ground. Its a trials frame so the BB is high to begin with.
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Re: 20" wheels on mtb frame?

Postby torker » Thu Sep 08, 2011 6:34 am

I have the same setup with a 9c 6x10=low speed winding. It does around 24 mph. at 60v . I plan on putting some bmx cranks on it soon. 155 mm. instead of 165. It rides pretty good with full. suss.
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Re: 20" wheels on mtb frame?

Postby dogman dan » Thu Sep 08, 2011 6:50 am

I think a normal mtb frame with 20" wheels would pedal strike a lot on a singletrack trail. But not so much if you never pedal of course. It would tend to drop deeper into holes too, but some trails don't have so many deep holes. Dirt roads should be no problem.

FS frames do have higher bottom bracket clearance in many cases, to make up for suspension sag. Dial it in hard, and you'll ride high for the most part.

I took a 24" frame, put a 24" back 5304 on it. Then put a suspension fork and 26" wheel on it. Because of the frame being intended for a non suspension fork and smaller wheel, it ended up reallly raising the front. It was an uncontrollable wheelie bike. Fun to ride, but steering was sketch with the front tire barely even touching the ground most of the time. You had to touch the front brake just before any kind of steering imput. I say match the wheel sizes, or at least don't make it worse adding 80 mm of forks.
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Re: 20" wheels on mtb frame?

Postby Tench » Thu Sep 08, 2011 7:25 am

The same has been going though my mind, i would trade some of my 40+mph top speed for some more torque but i think the cycle sourced 20" wheels are just too small. I have been looking at lacing some narrow 16 and 17" motor cycle rims on, these come somewhere in between the 20" wheels and 24's i use now which are wearing either 24x3 knobblies or 24x2.4 DMR moto RT's, there is a good selection of tyres available to suit the 17" rims not so much for the 16's though but the penelty will be added weight. I am going to go ahead and make another set of wheels up to see how it turns out, i have the rims, the tyres are on order and i have sourced the hubs.
Fitting a 16 on the rear and a 17 on the front will give me about a 15% drop in speed but an increase in torque, the difference in the rolling dia of front wheel is only about 8%, dropping the bike slightly more at the back also has the advantage of maintaining the trail measurement which would be reduced with smaller wheels. I do a lot of urban riding and going up steps is one of the reasons for wanting to change, that and being on the edge of the Peak district where it is all rocks, coming from 40 years of motorcycles (started very young!)with some off road racing of various disciplines i am finding it hard to put my faith in the push bike wheels pariculaly when charging up steps and want to be able to push harder and ride it like the enduro bikes i am used to. It seems daft to wait untill i have damaged my 24's before doing this.
My pedals have huge clearance so that is not an issue but i still might try some 155's or similar, other bonuses with these wheels would be the brakes work better because the smaller wheel is applying less leverage to them and the heavier rim/tyre will give more gyroscopic stability.
As far as i can see the only down sides are weight and speed of which i had more that i was using anyway.
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Re: 20" wheels on mtb frame?

Postby sn0wchyld » Thu Sep 08, 2011 7:34 am

Tench wrote:... I do a lot of urban riding and going up steps is one of the reasons for wanting to change, that and being on the edge of the Peak district where it is all rocks, coming from 40 years of motorcycles (started very young!)with some off road racing of various disciplines i am finding it hard to put my faith in the push bike wheels pariculaly when charging up steps and want to be able to push harder and ride it like the enduro bikes i am used to. It seems daft to wait untill i have damaged my 24's before doing this...



ah I understand now :P I was looking into mx rims initially but when I couldnt find any above 20inch or any under $300 I gave up (mind u i didnt look that hard).

I like the idea of fitting knobbies though and riding up stairs like it was a steep ramp, pluss once you have some thick rubber the diff. in final diameter wouldn't be as great as i initially thought. Still, Ive gone and bought myself some heavy duty DH rims so Ill have to wait and see how they hold up first.
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Re: 20" wheels on mtb frame?

Postby Tench » Thu Sep 08, 2011 8:35 am

You have to remember that bicycle wheels and motorbike wheels are measured differently, a motor cycle wheel is measured at the tyre bead surface and its diameter is the actual number of inches it is stated to be, a pedal bike rim is measured at the outside dia of its tyre. As an example a 26" bicycle rim measures 21 3/4" at the tyre bead surface, the 24" rims on my bike measure 19 3/4" and some 20" ones i have measure 15 3/4" so it would appear that you have to knock 4 1/4" off the size of a bicycle wheel when comparing it against the motor cycle wheels. so when i drop mine down from the 24's to a 16 rear and 17 front i will only be losing 3 3/4" on the rim dia at the rear and 2 3/4 at the front, some of this difference will be made up with the larger tyres so i shall end up about half way between 24's and 20,s.
The 16x1.4" rim with 36 holes which i have got to lace to the 5304 motor which has 36 holes was not the easiest to find, i think this size was used on the front wheel of the Honda XR75 so is a bit scarce here in the UK. Most of the more modern little crossers used 28 spoke wheels these days.
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Re: 20" wheels on mtb frame?

Postby MadRhino » Thu Sep 08, 2011 8:58 am

20 in bicycle wheels ride real bad in the mountain trails. It makes sense only if you are going to use a 16 in motorcycle tire, that is fitting OK on a wide BMX rim. Adding the extra weight of motorcycle rims and/or tires makes for the need of extra performance too, and few bicycle suspension will be tuneable with the rear wheel carrying the weight of a hub motor and motorcycle rim plus tire.

My idea of good handling in mountain trails can't accommodate that kind of weight, the hub motor is alot already, and a 26 in DH rim and tire is a must for my ride. I ride 66 pounds build on a Demo 8 now, and it is a major improvement on my 88 pounds A-Line. if anything to change, it will be for even lighter. Only a road E-bike can take advantage of motorcycle or smaller wheels, unless one rides easy trails and doesn't care much about handling.
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Re: 20" wheels on mtb frame?

Postby Miles » Thu Sep 08, 2011 9:47 am

A good reference for bicycle wheel sizes: http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3802
Nominal 20" usually refers to ISO 406
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Re: 20" wheels on mtb frame?

Postby Lemlux » Thu Sep 08, 2011 10:22 am

I've been considering running a 3.0" tire on a 20" velocity psycho rim which appears to have a 31mm wide bead mounting surface. This would appear to be equivalent to a 22" 2.0" bike wheel. The HT3525 hub simulates out to be much less overheating sensitive in hilly territory with a 150 kg fully loaded bike.

I'd planned on using 155 mm cranks with 2.35" x 26" or 700c wheels in front to raise the bottom bracket of a 2007 26" Trek Big Sur HT frame I bought this week, but Dogman's comments give me pause. I wonder if balancing a 72V 16Ah Headway pack on either side of a Surly front rack on a non-suspension Chromoly fork would keep the front end down. I also wonder if running 3.0" wide front tires would provide meaningfully more shock absorption than 2.35" tires. (The Surly Karate Monkey fork can handle up a 3.0" wide 622 rimmed tire and a 204 mm front disc.)

I ride 90+% on smooth road with most of the rest on hardpack with some loose sand on top.
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Re: 20" wheels on mtb frame?

Postby Ypedal » Thu Sep 08, 2011 11:10 am

Been there and done it, multiple times...

my chopper, Chaos, and RC bike all have rear 20 and front 24.

On paved roards ( 90% of my rides are on pave ) .. it's no problem at all, yes the pedals do come close but it's no problem on the Norco Chaos frame ( full suspension ) .. the RC bike with ridgid rear does hit easily but i dont pedal that bike anyways..

To make up for lost speed with smaller diameter rims, up the voltage to get it back !! :twisted: = bonus watts in the process !

I love having both feet flat on the ground when i come to a stop too..
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Re: 20" wheels on mtb frame?

Postby auraslip » Thu Sep 08, 2011 11:46 am

Been there and done it. Got the scars to prove it.

JK, Ypedal. But seriously, mixing wheel sizes up is a bad idea! I ran a 26" in the front and a 24" in the back. I ended up crashing while making a low speed turn. Wheel flop can be quite dangerous! One second your making a turn and the next the bike is falling over.

I've been thinking about this though: A bike with a higher front end would be much easier to make an effective fairing. Where a stanard MTB has you crouched over the handle bars would require a 2-3 foot tall fairing, a bike where the bars are at your chest would require only a very small fairing to cover the "pocket" of your body. I'd like to find a way to achieve this with out screwing with the handling of a bike. Because $ per $ and pound for pound, nothing can increase speed and efficiency like a fairing.

But I digress. You'll have no luck finding 20" or 24" wheels with disc hubs. Which means you have to lace them yourself. No biggie; it's actually kinda fun! However, it gets expensive buying spokes and hubs separately. Especially for odd sized spokes. I ended up scoring a free 24" rim and a cheapo disc hub from my local bike co-op, and then finding the correct sized spokes for only $20. However, if I was gonna do it right, I'd use a nice hub (sealed cartridges) and those are easily $50-$100.


I priced out building up 18" motorcycle rims with moped tires in this thread. After (custom) spokes, tubes, hubs, and the rest It'll end up near $250. But according to some calcs it'll give a overall diameter of 23.6" with a fat ass 2.75" tire.

20" is too small for comfort. 24" sounds perfect right?
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Re: 20" wheels on mtb frame?

Postby Ypedal » Thu Sep 08, 2011 11:48 am

Ya bunch of sissies !!.... :P

I can ride anything.. ( You should see my ex gf !! )
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Re: 20" wheels on mtb frame?

Postby auraslip » Thu Sep 08, 2011 2:20 pm

I can ride anything.. ( You should see my ex gf !! )


:lol:
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