Bmx Geometry - In Trials Fashion

[zoster]

Hi!

can anyone tell me what a standard wheelbase for a street bmx is? (i find this top-tube measurements). also, what's the bb rise usually for a street bmx, and fork offset...

anyone...?

thanks

[JonMack]

Fork offset is usually about 45mm IIRC, bb height between 11.5 and 12", wheelbase isn't important because the most important measurement on the bike is reach, hence TT length.

[Trials Punk]

Go here for anything BMX BMX FORUM. Created by the two same great people who made this forum.

[zoster]

hey, thanks for the replys! why i am asking this is because i want to compare mod geometry with bmx geometry (to see if a hybrid between the two would be the stupidest idea in the world or might actually work, somewhat like a 24"er)

[Captain Scarlet]

bb height between 11.5 and 12"

wtf!?!

=D

What is all the business of the size your "rear triangle"?*

*Whilst I'm in the topic

[monkeyseemonkeydo]

wtf!?!

=D

What is all the business of the size your "rear triangle"?*

*Whilst I'm in the topic

BB height being the height of the BB above the ground...

Don't know what you mean by rear triangle size... main dimension there is the chainstay length, tending to be between 13.5" and 14.5". Seat tube length just dictates standover and 'look' of the frame.

[JonMack]

Pretty much.

So...

TT: 20-21.25" usually

CS: 13.5-14.5"

BB: 11.5-12.0"

Seat tube angle: usually 71º

Head tube angle: 74-75º

[J Trials 31]

hey, thanks for the replys! why i am asking this is because i want to compare mod geometry with bmx geometry (to see if a hybrid between the two would be the stupidest idea in the world or might actually work, somewhat like a 24"er)

Already done it. Looking at the bike right now. BMX aluminum frame with v-mounts, steel fork with v-mounts. Alex DX-32 style rims, v-brakes, homemade bashplate, WI freewheel. Pretty cool. Will get some pics when it's completely done.

[eskimo]

This will be interesting. Allot of problems i could see arising.

[zoster]

how's the top-tube measured?

from what to what?

and to J Trials 31 , C'MON , PUT A PIC NOW, I'M REALLY CURIOUS

Edited October 26, 2008 by zoster

[doopdoopbedoop]

how's the top-tube measured?

from what to what?

end to end.

[Pashley26]

how's the top-tube measured?

from what to what?

I believe it's from the center of the seat post tube to the center of the headtube ?

Edited October 26, 2008 by Pashley26

[Mark W]

Pashley26 is right - centre of tube to centre of tube. Chainstay length should be centre of BB to centre of axle, but people generally do centre of BB to inside edge of the dropouts.

The usual BMX fork offset is 35mm.

"Generic" street geometry is: 75deg head angle, 71deg seat angle, 13.75" back end (usually - most frames are significantly under that now), 11.5" BB at the very minimum - again, most are at 11.75". Seat-tube length is under 7.5" usually. Top-tube is anything from 19" to 21.25" on production frames. Weight is usually sub 5lbs.

[J Trials 31]

how's the top-tube measured?

from what to what?

and to J Trials 31 , C'MON , PUT A PIC NOW, I'M REALLY CURIOUS

Got a super old pic of it somewhere. I'll try to find it and put it up. It's not quite accurate though. A lot of stuff has been changed.

[eskimo]

Seat-tube length is under 7.5" usually.

Don't be so silly

[zoster]

Got a super old pic of it somewhere. I'll try to find it and put it up. It's not quite accurate though. A lot of stuff has been changed.

don't you have a camera handy?

[Steve-A]

Good luck if you can get it to work, I've been there and tried this.

An old fashioned mod frame, i.e. old monty, isn't far off a bmx for geometry. Its not perfect bmx geo but I was working on presumption that I'd be better of working from trials bike towards bmx, rather than from bmx towards trials, for ease of sorting brakes/bashguard etc. The problems I found were all to do with the bar and stem choice.

With a BMX bar and stem it pretty much rode like a bmx, not quite right but certainly not far off. With a mod bar and stem it rode just like a normal mod. Both bar and stem combos resulted in about the same sortta bar height, so its all down to the reach the stem gives really. The short bmx style stem was great to spin and bunnyhop to wheels, but it was horrible for trials, the bike was really difficult on the back wheel, nigh on impossible to gap any distance. And surprisingly I could never bunnyhop it that high to backwheel, wheels yes but not up to back. And obviously the mod reach stem rode just like a mod...

Edit:

I also bodged a 100x35 stem onto some old mtb bars that had a 3" rise and ended up with a similar bar height and a stem length somewhere between bmx and mod. I found that i was too far over the front wheel for bmx, suffering the same flaws as trying to ride a normal mod. And it was still rubbish at trials.

Basically the conclusion I came to was that I'd just keep riding my mod in a streety manner. It was probably only 4 or 5 months later when I got my 24", because I was still looking to ride a bmx and trials combo really. I found it much easier to learn the bmx style moves on the 24 than on the mod. But interestingly enough I've recently got back into riding and gone back to mod, and I can do all the same moves on my XTP as on my 24".

I think once you know how to do the moves its not too hard to transfer them to a different bike. Obviously my XTP is a short one, so a bit better suited to spins etc than a long python or something, and I'm not running a super low stem and I've got a few more psi in my tyres than most mod riders.

A mod frame with sus forks, a modified mtb hub, gears and a seat, now that was a whole different matter

[Mark W]

Don't be so silly

Sorry, yeah, meant 8.5"...

[zoster]

So, steve-a, you would suggest a 24" for a trials/bmxey bike rather then anything else...?

[Steve-A]

Err, I think so.

I rode 24" for 4 years, and before that I rode 20' for 2 years. Over the years I have tried to ride a proper BMX, I never really felt at home. On my 20" I was managing to ride in a 'street' style way, but it wasn't always particularly pretty/smooth. The moves were all possible, and the mod was light and the small wheels meant it could spin well. But the long stem putting weight forward meant it was sometime hard to do things well. For example landing a fufanu to two wheels cleanly was hard, landing to back wheel was easy. It was all too easy to push down to land to two wheels and just smash into the floor all off balance. Once I bought the 24" I did find it easier to land tricks, the bigger wheels were more forgiving, and the shorter stem set-up of the larger wheeled bikes meant my weight could be in a more bmx style rearwards position. So landing tricks to wheels was much easier. That said physically spinning the bike was harder, it feels more clumsy and cumbersome.

I've now gone back to 20" and I can still do all the same stuff as on my 24" AND I'm landing smoother like I did on the 24".

I think what I've found, and this could well be just the way it works for me and not anyone else, is that once I've learnt the skills on one bike they are transferable. A mod IS hard to learn ride 'bmx' on but once you know how to do a proper bmx style 360 bunnyhop ( just to give an example) its not too hard to do one on a mod. Obviously this has limitation, a python is never going to be as good for street as a monty or short koxx frame, and like wise a short echo pure or equivalent is going to be much better than a Gu with its super high bb.

If you already ride bmx, then I'd say stick with a fairly short trials bike. Learn to transfer the skills, you'll know the feeling of how far you've gotta be leaning/weight shifting, it transfers quite quickly really, and you'll still have a true trials bike thats fun if you end up on a ride with some TGS boys. If this is going to be your only bike ,and your intending to learn to ride in a BMXy manner then definitely get a 24". When set-up well they can be almost as good as a true trials bike for trials and they ARE the easiest trials bike to ride in a bmxy manner.

Sorry lots of waffle there, but its not the sort of question with a direct answer. To throw a final spanner in the works.... You've gotta want to ride it and be comfy. I don't get on with stocks, I'm much more at home on 20". So even if stocks were the best option for street, I wouldn't buy one as I'd never be able to ride it as well as the technically less suited 20".

[zoster]

Err, I think so.

I rode 24" for 4 years, and before that I rode 20' for 2 years. Over the years I have tried to ride a proper BMX, I never really felt at home. On my 20" I was managing to ride in a 'street' style way, but it wasn't always particularly pretty/smooth. The moves were all possible, and the mod was light and the small wheels meant it could spin well. But the long stem putting weight forward meant it was sometime hard to do things well. For example landing a fufanu to two wheels cleanly was hard, landing to back wheel was easy. It was all too easy to push down to land to two wheels and just smash into the floor all off balance. Once I bought the 24" I did find it easier to land tricks, the bigger wheels were more forgiving, and the shorter stem set-up of the larger wheeled bikes meant my weight could be in a more bmx style rearwards position. So landing tricks to wheels was much easier. That said physically spinning the bike was harder, it feels more clumsy and cumbersome.

I've now gone back to 20" and I can still do all the same stuff as on my 24" AND I'm landing smoother like I did on the 24".

I think what I've found, and this could well be just the way it works for me and not anyone else, is that once I've learnt the skills on one bike they are transferable. A mod IS hard to learn ride 'bmx' on but once you know how to do a proper bmx style 360 bunnyhop ( just to give an example) its not too hard to do one on a mod. Obviously this has limitation, a python is never going to be as good for street as a monty or short koxx frame, and like wise a short echo pure or equivalent is going to be much better than a Gu with its super high bb.

If you already ride bmx, then I'd say stick with a fairly short trials bike. Learn to transfer the skills, you'll know the feeling of how far you've gotta be leaning/weight shifting, it transfers quite quickly really, and you'll still have a true trials bike thats fun if you end up on a ride with some TGS boys. If this is going to be your only bike ,and your intending to learn to ride in a BMXy manner then definitely get a 24". When set-up well they can be almost as good as a true trials bike for trials and they ARE the easiest trials bike to ride in a bmxy manner.

Sorry lots of waffle there, but its not the sort of question with a direct answer. To throw a final spanner in the works.... You've gotta want to ride it and be comfy. I don't get on with stocks, I'm much more at home on 20". So even if stocks were the best option for street, I wouldn't buy one as I'd never be able to ride it as well as the technically less suited 20".

cool! thanks a lot for the post (and it's good that you gave a long answer).

i already ride 24", i'm no expert in trials nor bmx, but i saw some live bmx action recently and it seems having 20" gives you a lot of maneuverability, and also you can bail easier, so i thought a bmx-trials hybrid might be a good idea. i'm not dropping it right yet, i'm super curious what J Trials 31 will say about his new bike.

any videos you might give a link to that have streety mod riding?

[Steve-A]

Youtube Video ->

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[J Trials 31]

Alright, finally took a pic. Been doing some riding on it for a few days, as it's the only bike up and running. It's not completely done. Need a new headset as that's bodged. Need some trials vee pads front and rear. Been trying to do some Akrigg style riding on it. Floater 180's on to logs, manuals up to rear and such. Rides pretty cool. It's a little heavy and the parts spec wise aren't all that good at all. Also, added a homemade bash. With some steel and a bolt through the frame. So far it's not cracked. Had a screw through the bottom part, but it always came out so I just bolted it. If it's held up for as many years as I've beat the crap out of it in BMX, I think it will hold up some more. Running some Shimano v's. Soon to be with some Moe pads or Plazmatics. Regular bar and stem setup. Rear wheeling is a challenge, but I believe I'll get it down with some new pads. Haven't tried gapping it at all. Too scary to do that with really no brakes. But I have rear wheel almost all the way around the yard. Manuals and bunnyhops like a beast of course. And spinny stuff is fun. I'll shut up now and let you look at the pic. Tell me what you think.

[omgnoseat]

I remember a video with some guy on a echo mod riding at skate parks. He was doing some very interesting moves. His riding was still more trials orientated, but it was cool anyway. Anyone have any idea what this video was? Can't seem to find it on youtube.

[Tom_]

Alright, finally took a pic. Been doing some riding on it for a few days, as it's the only bike up and running. It's not completely done. Need a new headset as that's bodged. Need some trials vee pads front and rear. Been trying to do some Akrigg style riding on it. Floater 180's on to logs, manuals up to rear and such. Rides pretty cool. It's a little heavy and the parts spec wise aren't all that good at all. Also, added a homemade bash. With some steel and a bolt through the frame. So far it's not cracked. Had a screw through the bottom part, but it always came out so I just bolted it. If it's held up for as many years as I've beat the crap out of it in BMX, I think it will hold up some more. Running some Shimano v's. Soon to be with some Moe pads or Plazmatics. Regular bar and stem setup. Rear wheeling is a challenge, but I believe I'll get it down with some new pads. Haven't tried gapping it at all. Too scary to do that with really no brakes. But I have rear wheel almost all the way around the yard. Manuals and bunnyhops like a beast of course. And spinny stuff is fun. I'll shut up now and let you look at the pic. Tell me what you think.

Isnt that just one of those ultra cheap BMXs with the bars tilted forwards?