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Anti-Outsourcer
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Discussion Starter · #1 ·
As some (most?) of you know I hijacked another thread a couple weeks back with a few questions I had. While some people were a bit aggressive... :rolleyes: you were all quite helpful. I browsed through some of your blogs that go over your techniques//designs/daily happenings and I must say, very nice :thumbsup:

So, running with the idea of your chronicles, I'm starting a thread to follow my adventure. The thread title should explain pretty well what my goal is in doing so... Hee-hee :D

To start things off, this is what I have drawn up for my prototype dirt jumping frame. I will be fillet brazing it from a combination of butted and straight gauge 4130 tubing.
It is drawn in 1/4" = 1" scale, and these are the geo's I've come up with.
41.5" wheel base
HT angle ~69deg
ST angle ~66deg
CS length ~16" (from cent BB to Axle)
Center of BB is 1/2" below axle line

I haven't drawn in the rear triangle as I still have some figuring to do on what kind of stays I will be using/making.

Thoughts/Comments/Suggestions?

 

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Just use bikeCAD

I like drawing stuff on paper too, but it's not really that useful in the end. Just go use the free version of bikeCAD instead.

I'm not the world's best dirtjumper, but I do have a DJ bike and at least occasionally ride it as intended. So these are just ideas, not criticisms. What you have looks fine to me.

-24" wheels kick the crap out of 26" ones for adolescent stupidity type riding. 20" supposedly are even better, but every time I ride a BMX I fear for my life. If you're *just* going to DJ, I'd do 24, personally. I think my DJ bike has a <40" wheelbase, and it's great for the pumptrack and tight double rhythms.

-A steeper STA makes it easier to shorten up the rear end. Not sure if you want the 66 deg for a specific reason, but I just put mine at a standard 73 and put a skinny saddle on it to keep it out of the way.

Honestly, design for a DJ bike is pretty simple. Start practicing your joinery!

-Walt
 

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Cheese, steepen the ST angle. It will make fitting the seatstays in a LOT easier. I think mine was 72 or 73 and it feels fine. Remember, you're never really sitting on the seat except when standing around BSing.
I wouldn't bother with the DT gusset, but I would also move the tube down a bit closer to the edge of the HT.
I would also lower the BB but that is a personal geo preference. 1/2" drop = 13.5 high (correct me if my math sucks) that is way too high. Think sub 11.
 

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Anti-Outsourcer
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Discussion Starter · #4 ·
Walt:
I'm going with a 26" because I already have some parts laying around that need homes, most notably my Marz DJ1 that I'm deeply in love with <3 Also because our urban/dj scene around here is really layed back, so I'll be doing a lot of cruising and shooting the breeze. I roll out my 20" whenever my MTB is layed up, and while the lil guy is uber-snappy and a blast in general, I'm definitely more at home on the 26er.

The CS length is more or less on par with a Specialized P2cromo (~15.4") so I think that should be adequate? I'm still yet to work out a yoke design. I'm leaning towards having some plate milled up for me, but there is still some research to be done on that. So pending that, the extra room might be nice to have.

Joinery practice should begin any day. I have a buddy with 2-3 OA setups (I couldn't tell you why) and he'll be loaning me one. Only challenge left will be finding tanks, but we have a welding/gas/blacksmith shop in town, so I'll hit them up in the near future.

BYO:
I think you added a half instead of getting rid of it... so 12.5" BB height at center... yes? (or am I wrong now?)

The gusset/DT issue was in an effort to eliminate ANY possible fork/DT problems... Hopefully this will be the rig to learn barspins on...

And going back to the first part of this post, I need the seat to be at least usable and allow for seated riding from time to time...

Aaaand I've decided that since 3-4 of these will probably be made, I'm going to fab up some kind of a jig/fixture/thingamabob to make them all end up roughly the same shape. I'll post about that as soon as i have enough of an idea on it to even formulate questions haha.
 

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I'll chime in here as I think I'm one of the only builders on this forum that builds mainly "DJ" as these guys call them , we prefer to call them freestyle MTB's round our shop same difference .

A few suggestions : First off Do not lower your DT closer to the end of your HT unless you plan on making this a dedicated rigid fork frame . You need a minimum of 15mm-20mm from the bottom of your headset cup or bottom of internal headtube for fork crown clearance if you want to be able to do bar-spins on this frame .

As others have said steepen the seat angle , I would not go as steep as a 73' that will move the majority of your body mass into the front triangle area and thus make the bike harder to bunnyhop and to rear wheel tricks like rear wheel fufanu stalls .
70-71.5 seems to be the range on most decent bike right now .

CS: you can bring this in a lil bit if you steepen your SA just a lil.
Depending on chainstay and yoak design you should be bale to get the CS into the 15.50 range with 26" wheels

BB to ground : keep it at 12.50 dont go any higher than possibly 12.75/13.00" with big tires . Any taller than this and your bike will start to ride funny it will raise your balance point and make tricks much harder .

BB drop : try not to go much over +19mm BB drop or it will make the bike impossible to 360 .

Wheels size : 24's are fun for the skatepark and on street but for bigger DJ's the 26" wheel larger dia has proven itself to be the mainstay now with most hardcore 20" DJ kids moving up to 26"s

Gusset : research gusset design and which one os going to work best for your brazing method , also mating a angled gusset to a straight gauge tube can effect frame strength the gusset will act like a can opener on your DT if you run if straight back from the HT ( ala 90's hoffman bikes style ) I suggest a simple plate gusset Bontrager style and please for the love of frame building LEAVE the ENDS OPEN !!!
I have seen people say they have added Bontrager style gusset's to their frame but have welded them al the way around , this is not how KB did his gusset's and it's a flat out insult to call a fully welded gusset Bontrager style ( sorry for the rant )

Good luck !
 

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Evil is really the guy to listen to here, but some things sound odd with what Cheez says.

I'm a trail rider and part time downhiller but I grew up skateboarding in real skateparks and still show up to session from time to time. I've known lots of park riders (bike) and jumpers and can manage (on bike) to do 50/50's on minis and febles on benches. Pretty basic stuff. But I can ride carve and pump anything around.

If you can't do barspins now, then you are a novice rider at best. It makes little sence to build a 26" dirtjump bike if you don't have a 20". I tried to ride a 24" for years and gave up in favor of a 20" for park and street. It's a HUGE difference in performance and is very easy on the pocket book. 26" jump bikes are all the rage right now, and that is fine, but most of this is just cashing in on kiddies that don't know better. Take a look at who kills it at the park and they are on 20s. Learn on a 20" then take it to a 26" and you will be much farther ahead of the game.

Yes, 26" dirtjump bikes are more fun to cruise around on, but the 20" really is where the game is and will be again when this fad is over.
 

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Anti-Outsourcer
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Discussion Starter · #7 ·
"If you can't do barspins now, then you are a novice rider at best."
Oh yeah? well yo mama's a novice rider at best, foo!!!
just kidding.
But no, I'm well versed in crazy hardtail shenanigans. 30' gaps, wallrides, fufanus, pumptrackage for days, etc :thumbsup: However two brake lines, front and rear mech cables and no detangler of any sort make barspins a little tricky to do lol. I'm actually quite confident that I can do them, assuming the bike can. I'm thinking I'll build this as a 1x9 with a hollow bolt headset and tape the lines going to the rear.

Predicting your next move, I know that a single speed would be better about the lines, and a dedicated SS wheel and blah blah but it really just isn't a good solution for my situation and what I plan to use it for.

I'm building a 26" dirt jumping do-it-all frame and that's final :p

So aside from trying to talk me out of building the bike I've set out to build (lolz) good advice, and notes taken.

PS: Evil, you're with Nemesis right? Are you guys out of Sac, CA?
 

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CheeseSoda said:
Evil, you're with Nemesis right? Are you guys out of Sac, CA?
YES my shop is just outside SAC in Folsom .

Oh i'm not trying to tlak you out of it , sounds like a decent build jsut adding some advise so your first projest comes out a lil better , always here to help :thumbsup:
 

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I'm not sure if this is a dedicated DJ bike, or an all around hardtail that you want to use for a little jumping but will still be able to handle gated racing and some pump track action. If it's the later, the I stand by my original geo recommendations.
The dim from the DT to the bottom of the HT on my bikes is 11mm and it will clear a Pike 454 with the really big floodgate knob. Probably the worst clearance fork on the market.
The BB really needs to be low for the bike to corner well. I stand by 11"
16.8 is DH bike long. Sub 15" should be your goal. 14.75 is achievable with a yoke but is twitchy at speed if this will be used for 4x.
I largely based my geo off the Cannondale Chase. It has an 11" BB and Aaron has no problem throwing 360s.
Again, it's all about intended use. Horses for courses.
 

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Anti-Outsourcer
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Discussion Starter · #12 ·
I tried that BikeCAD GUI and it wouldn't render any of my dimensions, I assume because they're "out of the ordinary" for a mountain bike... I should have expected the Man to try and keep me down.
That failing, it looks like I'm going to draw up v1.3 by hand :thumbsup:
Shall we make a list? :D
I rode a friend's brand spanking new Black Market Riot today (21.5 TT) and reeeaaallllyyy liked the feel of it, excepting the fact that it was uncomfortably short in length when standing. So, I averaged out the riot, the Chz v1.2, and your suggestions, and I've come up with:
70deg HT
68deg ST
41" WB
15.5" CS
12" BB Height

How would we feel about lengthening the headtube by about .5" on top, dropping the DT junction down .5" so the DT only has one mitre instead of two, and then add a plate gusset on the sides (and maybe underside)? Sort of like this:
 

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Most Delicious
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I designed a DJ bike with a 66 degree seat angle and it looked good on paper (err, CAD) for all the same reasons, but when it was done it felt pretty funky.

How about just making the head and seat tube angle parallel? That might simplify things a touch for your mitres and for building your jig (since you want to build a couple).

I think good advice for beginners is to start with a design you like, and don't change anything by more than 1 degree or 1cm. Just massage the design a bit in one direction and see how it ends up comparing.
 

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BikeCAD should be able to handle any of those parameters (it can render some wildly unusable dimensions). Sometimes you need to hit enter/return in one of the text entry fields for it to process your input. It might help to get the latest Java download from Sun, if you're having problems with it.
 

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Bike Dork
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BikeCAD Design

I'm one of those guys that has actually bucked up for the pro version, so I whipped you up a quick frame. I can only post a JPEG, so if you want the actual BikeCAD file drop me a PM with a good email address and I'll send it on. You can open my file in the free BIkeCAD online program. This is really based on the limited info you gave us. More details and you get more accuracy.

Bicycle tire Bicycle frame Bicycle wheel Bicycle wheel rim Mode of transport
 

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"collaborative doom"

not a bad name for the first run of frames! ;)

Wouldn't work as well for the company name... then again the first surlys were called "1x1 rat rides" :D
 

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Anti-Outsourcer
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Discussion Starter · #19 ·
Whoever it was that mentioned hitting Enter in BikeCAD, that definitely was the problem.
I've been futzing around with it for a few days, and drumroll please...





The fork is approximated but close, and everything else should be spot on. Does she look like a winner? I think my wheelbase went out the window... I don't have the slightest idea how to convert front center + CS to get WB... Well, I kind of do... But I just finished my 5th semester of trig, and I'm not doing anymore. The wheelbase can be whatever it damn wants to if it means no trig.
 
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