Thoughts on building a 20" kids dirt jumper

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Thoughts on building a 20" kids dirt jumper
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flag Congested
Posted: Aug 6, 2013 at 21:48 Quote
My 8 year old son is really getting good at the local pump track. He's riding a 20" Specialized Hot Rock with gears and front suspension fork. Its been a great all around bike.

He's starting to really progress with his jumping.

I'd like to make a small dirt jump bike for him.

Maybe a small framed 20" BMX bike with dirt jump style handle bars, single speed and a front suspension fork.

Has anyone out there built or bought a bike like this?


Thanks
flag That1GuyYO
Posted: Aug 6, 2013 at 21:58 Quote
Why not just use a bmx?
flag dirty-old-man
Posted: Aug 7, 2013 at 5:53 Quote
+1 for the bmx

I'd go for 18 inch, a 20 inch will probably be too big for an 8 year old. I bought my 7 year old son a Haro 118 and it is even a little too large for him now, my guess is he can ride it till age 11/12.
flag Congested
Posted: Aug 7, 2013 at 10:22 Quote
Some of those 18" bikes look good.

I like to hot rod things.

I'm thinking a 20" bike that looked like the picture of Dirty-Old-Man's bike would be kinda cool.

I rode a BMX bike from the time is was too big for me until it was too small for me. Ha
flag PeixeCruiser
Posted: Aug 7, 2013 at 16:27 Quote
At 7 or 8, I would agree that you should probably be looking at BMX bikes. DJ's just simply aren't made for the little guys. I'd disagree about the 18" bike though.

There are perfectly good 20" frames that are built the perfect size for a little guy to learn on, although you see them a lot more in BMX racing than you do anywhere else. Try looking at frames they call "mini"

Here's an SE mini. http://www.sebikes.com/wordpressfiles/wp-content/uploads/2013/07/image43.jpg

Most decent manufacturers have a 20" mini.

A couple benefits of a mini over an 18"; You're already starting off with 20" wheels, so there's less time getting use to a new wheel size and more parts will transfer from the mini frame to a typical 20" frame as he grows and will end up riding a 20, so it will be cheaper in the long run to just replace the frame, then when he's too big for a wheels, replacing the wheels. Not to mention, these frames are built for that 7, 8, 9 year old kid who is just learning. Although I have seen a kid as old as 10 shred some jumps on one of these. (Not necessarily an SE, but a mini in general.)

The SE was merely the first mini I found on google. There may be better options in mini than this SE.

I wouldn't consider a DJ style bike until around 12, give or take depending on the size of the child. I don't know of any shock style forks that would drop low enough for a 20" frame. 24" is probably the minimum with the NS suburban, but that is still too big for an 8 yr old.
flag Congested
Posted: Aug 7, 2013 at 18:20 Quote
Thanks PeixeCruiser,

That SE Mini and the Red Line Mini's are what got me thinking of building a mini dirt jumper.

They look like the perfect size.

They obviously are for BMX racing and the rims/tires are too narrow for jumping.

I wonder what the widest tire size I could run on that narrow of a rim? I thought I read somewhere that the mini 20" rims are really an odd size O.D., they arent a standard 20" rim and they take a different size tire.

Do you know if I would be able to use a "standard" 20" bmx rim on that mini frame? If thats the case, then I'd be on the right track by replacing both rims and adding wider tires.

There are always some Mini's on Craiglist in my area. I just wasnt sure about the rim/tire situation.

I went to a local bike shop today to see if they had any 18" bikes. They had one left, a Specialized P series 18". It was very small. I think my son would outgrow that pretty quick.


Thanks!
flag PeixeCruiser
Posted: Aug 7, 2013 at 19:15 Quote
My take on it is that there is nothing an 8 year old is going to do in trails and dirt jumps that isn't done on a BMX track. They typically don't have the skill, nor the power to hit big and land hard, so it would be just fine as is. I take my own little 8 yr old nephew to the trails on his mini without an issue. My sister has since sold his mini and got him a full sized 20" that is a little big for him with the thought that in a year it will be just right for him and in 2-3 years he'll be ready for a new one.

That being said... I would think you could put a normal sized tire on the rims. They are suppose to just be 20x1.5 instead of 20x1.75. I don't see it being too much different than the 26x.1.75 vs 26x1.5 rims that are both used by hard hitting adults riding 2.something tires. Just the size is smaller so smaller area. That is something I'd have to actually try though and since my sister sold my nephew's mini I no longer have a bike to test that theory. I would try a 1.75 tire first and go from there. An 8 year old doesn't require a 2.+ size tire. 1.75 or 1.85 would be well big enough. You could also try 1.75 in the rear and larger in the front. Larger in the front gives you more stability where smaller in the rear is just to get you moving. I personally race with a larger tire in the front than in the rear.

As far as using the 1.75 rim, I would think that would depend on the frame and you'd never know till you tried. The only issue would be the rear and there being enough clearance between the rim and the chainstay. I had a bike once that I could put 1.75 in the rear, but could not put a tire larger than 1.75 or the tire hit the chain stay. Mini's are made too different for me to tell. If I'm at a race sometime with someone I know personally who has a mini I might be able to test that theory, but I only race on the weekends and this weekend is a National that I'm not going to, so it'll be a couple weeks at least before I can test that for you.

If it were me, I'd try using the wheels are they are originally purchased and seeing how large of a tire would actually fit. I always thought they were 20" just like any other 20", so it would just be the clearance. I wouldn't worry about the rim size unless he actually bent one. I believe they're rated at about 90lbs and most 8-10 year olds don't come close to hitting the weight limit.

If you can get a cheap one off craigslist, then that would probably be a good starting point. If not, I can give you the number to a vendor I know personally. I'm not sure what he has right now in the used stuff, but my nephew's mini was bought off him for $160 complete, so wasn't a bad price. He might also be able to answer other technical questions that I wouldn't have the answer to without trying and testing and seeing. If you'd like the number I'll shoot you a PM.
flag Congested
Posted: Aug 7, 2013 at 19:58 Quote
I like the Mini idea a lot.

There are a couple on CL right now.

We're running a 20x 2.125 on the Hot Rock. I picked up a donor bike as part of a trade and its got a 20x1.75 on it.

Maybe I'll take that wheel off and use it as a gauge when I find a used bike.

Thanks again for the info!!
flag erikthefatty
Posted: Aug 7, 2013 at 21:55 Quote
Scott makes some nice kids bikes.

Link to whole product list.

Sounds like you want to keep things smaller than 24", but this one looks solid.

Unsecure image, only https images allowed: http://dfp2hfrf3mn0u.cloudfront.net/227/227867_31990_tif_raw_2.jpg
Link to bike./
flag dirty-old-man
Posted: Aug 8, 2013 at 1:45 Quote
My son as both an 18" freestyle BMX and a 20" racing BMX so I can easily compare the two. The racing BMX has a smaller frame, is faster, lighter but a lot less sturdy, but as PeixeCruiser says it will be sturdy enough for the average 8 year old jumping (they do it on the race track as well of course), but when he hits a curb a few times, the rim will get damaged without a doubt, so if you go that route expect to replace a rim a couple of times a year. The geometry of the racing BMX is purely built for speed, the freestyle geometry will allow for better handling.

I don't think starting of on a 20 inch with a smaller frame will be an advantage in the sense that he will already be used to 20 inch wheels if he upgrades to a larger frame. The geo on a 18 inch BMX is often very comparable to a 20 inch BMX, they are smaller bikes for smaller riders but share the same feel. Eventually upgrading every part when he grows won't be cheaper as buying two completes, high quality after market parts are expensive.

Also the thinner tires roll very well on nicely groomed and often partially paved BMX racing tracks, but the average dirt spot is usually more dusty, more muddy, more sandy depending on the time of year and the thin tires will not be ideal on a less then ideal track.

You cannot put a substantially bigger tire on a racing BMX, the frame has no room for that option.

Those Scotts will do fine for commuting, but are way too big for an 8 year old trying to work it on the trails. It is not only in the weight, if it is oversized it will feel a lot heavier than a smaller bike with the same weight.

For safety a smaller bike is easier to bail on, and take my word for it, save some money for proper protection and a especially a full face helmet, we have learned about that the hard way (don't ask, believe ;-))
flag dirty-old-man
Posted: Aug 8, 2013 at 1:47 Quote
Cult CC18 is a great bike btw, really light
flag erikthefatty
Posted: Aug 8, 2013 at 2:07 Quote
That makes sense. I really have no idea or reference for the size of an average kid at any age. My girlfriend will make fun of me for it on occasion.
I think you probably have your options pretty well dialed in, but I figured I'd mention that Kona has a couple kids bikes with 24" and 20" too. Here's their Shred 20". I'd imagine it'd be easier to outgrow these than some of the other options discussed.

Unsecure image, only https images allowed: http://www.konaworld.com/images/bikes/med/shred_20.jpg
flag cmc4130
Posted: Aug 8, 2013 at 9:27 Quote
There's good stuff in this thread:

http://forums.mtbr.com/families-riding-kids/mountain-bike-vs-bmx-6y-o-489339.html

Including this "mini DJ"
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flag Congested
Posted: Aug 9, 2013 at 17:46 Quote
Thanks for all the advice.

How do I post a picture on here?
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