# Dirt Jump bike ABC



## Verttii (Aug 20, 2009)

I've ridden AM/DH style rides for couple of years now, and due to recent "jump sessions" on our local hill, DJ/Street bike seems to be my next bike to get. My use would mostly be something like 50% DJ, 30% street, 20% trail/general riding on various environment...


So selecting a proper bike is the next step, and with that I have some questions to ask...

Yes, I have read the sticky threads and used search, but many questions are still open. Also most of the threads here seem to be oriented more of bike/model suggestions, where I'm at the moment more interested about the "parameters" of a DJ/Street bike and how those affect the ride, jumping, handling, etc...

So, here we go:

1. Wheel size, 24" or 26", what kind of differences there are between the two sizes if we think about dirt jumping? 24" wheels I guess would be a bit lighter and stronger, where 26" rolls better on uneven terrain? Is there noticeable difference when rotating 360° or tailwhips in example? Smoother landings with 26"?

1.1 How strong the wheels needs to be in real life? I'm not trying to fit any crossmax SLR:s to a dirt bike, but does the wheels need to be really strong made? If all goes well, the landing is usually smooth and I have no intention of riding in "rough" terrain, so "perpendicular stress" should be relatively low, however, I don't have any idea how big "lateral stress" there can be ie. if landing too sideways..?

2. Frames, I guess that the geometry is more or less always the same and there are no major differences, chainstay as short as possible, head angle 70...71°, so are there any bigger differences between the frames than the color? What about the material, most of the frames seem to be steel(cromoly), why not aluminium? 

3. Brakes. 6" disc as back brake, but should I have front brake too? Are there some disadvantages having brake in front wheel too..?

4. Cockpit, how wide handlebars are usually used? Advantages/disadvantages of having narrow or wider bars? Flat or riser? Stem quite short, something 50mm-ish? How does the handlebar height affect jumping(comparing flat vs. riser bar), if any..?

5. Cranks, on AM bike I use 170mm, maybe shorter for DJing? Do I get any benefit of shorter cranks?

6. Weight of DJ bikes seem to be relatively heavy, usually something like 12...13kg on complete bikes, that is about the same as my full suspension AM bike weighs... What makes these so heavy? I'd suppose bike this small and simple should easily go below 10kg...


Huh, many questions there.. 
Yes, I know that getting a DJ bike isn't that difficult, buy one and then change components to what is your personal preference, if needed. But the thing is that I would like to know how these variables affect the ride and especially jumping and landing the jumps... 


So if you have answers, opinions, ideas regarding some of the questions, free feel to share!


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## ZenkiS14 (Aug 25, 2008)

Verttii said:


> 1. Wheel size, 24" or 26". I've ridden both, and I really just prefer the 26" its smoother, and more stable in the air by a long shot. I have a 20", 24", and 26" actually, all built for dirt jumping, and I ride the 26" ALL DAY, and rarely will bring out the others to ride myself.
> 
> 1.1 How strong the wheels needs to be in real life? *They need to be strong, but not like a DH wheelset or anything. Cause really, if you are doing it ride, it should be a very smooth transition from take off to landing and be fairly easy on it. That being said, I ride a set of 30mm wide Spank Rims with a solid rear axle (non-QR) and a 20mm front hub (DH style), both with DT double-butted spokes. I have never broken a rim beyond repair, just minor truing every few rides or so. (im a perfectionist)*
> 
> ...


I answered your questions in *BOLD* above. I've been riding DJ's for about 4 years now, pretty heavily. LOVE IT. also love building bikes!!

Here is mine for reference:


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## twistedshifter (Jun 10, 2010)

Thin


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