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williwoods

BMX frame geometry

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Just curious as I have been getting into vintage bmx and have not heard much discussion about frame geometry. I come from the MTB crowd and there is alot of attention paid to this including what guys started what geometry.

 

I noticed that it seems like in the late 70's things started to really go in another direction instead of the level toptubes and high bb's people like JMC and SE were setting the stage for what would become the new gen of geometry. I could be totally wrong but I would love to hear peoples opinions on geometry: what bikes geometry was revolutionary for the time which bikes geometry sucked etc.......... how has geometry changed over the years was it always for the best? What is considered an example of typical vintage geometry (actual measurements)

 

in the MTB world for example alot of people like the 90's style geometry. Low fork length (axle to crown length), shorter top tubes and longer stems.

 

anyway I guess I realized I dont know much about bmx geometry, please old school me!!!

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Guest BridgeCityBikes

Here is an extreme example to show the progression.

1978 to 2004

post-2503-1209497150_thumb.jpg

 

my own old bikes are 78 to 81. they get progressively more low slung feeling.

they start tall, short feeling in the front and chopperish, then get more low slung and stable.

 

the early 80s had it dialed in to perfection (for the track style of the times)

but the 1979 Sting had it all dialed in right off the bat. so I'd say Schwinn was at least 5 years ahead of the "masses" or maybe they were a leader in guiding the industry. (historical speculation on my part for the sake of the discussion)

Edited by BridgeCityBikes

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Here is an extreme example to show the progression.

1978 to 2004

pandaquadcompare_lg.jpg

 

my own old bikes are 78 to 81. they get progressively more low slung feeling.

they start tall, short feeling in the front and chopperish, then get more low slung and stable.

 

the early 80s had it dialed in to perfection (for the track style of the times)

but the 1979 Sting had it all dialed in right off the bat. so I'd say Schwinn was at least 5 years ahead of the "masses" or maybe they were a leader in guiding the industry. (historical speculation on my part for the sake of the discussion)

 

 

got any geometry data? Head angle, seat tube angle, TT length, wheelbase, Bottom bracket height.

 

Will

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Guest BridgeCityBikes

next week i'll dig up some bmxa scans where they show the outline of the test bike. that usually has all the specs. guess it'd be interesting to point out the major bikes of the day and their variations.

 

some might say a DY JMC had it dialed in, but i've never been on one to know.

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yeah just from looking at pics of built bikes here just by sight I would say that JMC was for sure ahead of the curve.

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Guest BridgeCityBikes

00031-1.jpg

00032.jpg

jmc6.jpg

BMXAJun80QuadSpecs.jpg

 

and here are Newschool measurements (taken from 2 common bikes at Dan's comp)

 

Standard 125R

chainstay 14.4

BB Height 11.5

headtube angle 74.5

seattube angle 71

 

Intense Sabot

chainstay 14.25 - 15.75

BB Height 11.75

Headtube angle 74

seattube angle 72

Edited by BridgeCityBikes

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This is actually a great topic and I was hoping it would blow up with various points of view.

 

Where is MotoBMX Matt? I seem to remember that he always had interesting things to say about geometry.

 

Seems like Rick T. would have plenty interesting to say as well, given they were doing a lot of experimenting in the earliest days of the sport.

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This is actually a great topic and I was hoping it would blow up with various points of view.

 

Where is MotoBMX Matt? I seem to remember that he always had interesting things to say about geometry.

 

Seems like Rick T. would have plenty interesting to say as well, given they were doing a lot of experimenting in the earliest days of the sport.

 

 

yeah surprised as well at the lack of response....however the items posted by Bridgecity are perfect thank you....keep em coming.

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Guest BridgeCityBikes

Rick T. has an interesting thread somewhere in history where he (or Reilley maybe) goes over the changes from Stingray geometry to the addition of bmx flavored mods.

 

didn't JU's dad do some clever mods, as well.

 

 

I have a Rampar R10 which all of us scroungers pretty much lived on. did anything you could ask but we were also child sized. Today when I ride the R10 the positioning of the cranks to the seat wears out my legs so i only putz around in it. usually barefoot, thanks to the rubber oldschool pedals.

 

The pics of Red Baron blasting jumps with the old geometry with his lanky frame is almost scary when you think of the balance issues at play.

 

 

Where's Spike, he still rolls early BMX geometry, let's have a critique, eh?

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Between my Champ and A&A they ride differently, the A&A having a slightly higher crank and less layback on the seat tube. The test for me is how they wheelie. I found a mid 90's Robinson on trash day and sorta fixed it up and I basically hated the way it rode, I ended up givin' it to Reilley. Looks wise, the 70's frames look way cooler, the new frames/bikes just look too small, but that is me. Hell what do I know, acording to the some guys on another site, I have no credibility and besides, after about 1979 I can't relate. I'd NEVER even consider getting anything new these days, I'll gladly stay in my mid-70's time warp.

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Guest BridgeCityBikes

the short top tubes brought your kneecaps in dangerously close to the gooseneck.

this was especially dangerous with the sharp cornered tufnecks.

 

with newschool geometry I don't think i've ever come close to jamming a kneecap.

which should count for a certain amount of credibility with the newer geometries.

 

Spike, when folks question your credibility please refer them to your seat clamp. How much more credibility should they need anyway?

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This Topic got me thinking and I broke out my tape measure.I have an 84 Torker 24'' and a 85 Redline 24" They both have a 21 inch toptube but there's 9 inches of space between the front of the bottom braket and the back of the front tire on the Torker and 10 inches of space on the Redline.The Torker in my opinion,handles WAY better and feels better,but I do tend to rub the fronts of my shoes on the front tire.The Torker also has about 1/2 inch more between the bottom braket and the center of the rear wheel.

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bump...

 

just stumbled across this old thread, which I had been thinking about since the recent discussion of frame geometry. Thought you guys might like to have another look at all this as well.

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I Don't have the specs pulled yet but the D.Y. seems to have a lot in common with the Patterson Avalanche, longer front and shorter rear triangles IMHO. I think Pattersons were really ahead of their time in design, even though they look simple and clean there is some trick design in their racing frame sets. In example the rear triangle and how it connects to the BB shell and rake and pitch of the headtube.

 

I will update my post with some Patterson scans (gotta go Avalanche hunting).

 

O.C. Dave we were talking about this subject at Eldorado re: the 26" BMX frames and which forks work best member???

 

B-back L8R...

 

21

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The coolest frame geometry imho is the 82 Patterson Pro Long. Long, and low, with the seat tube sticking up quite high from the top tube. That frameset just looks fast standing still..

 

As for cruisers, the PR240 rates up there, but so far, the nicest, quickest feeling cruiser I have ever ridden is the JMC 24 (Clint, I know you read this... I *LOVE* that bike). The JMC just feels perfectly balanced.

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Yo Mike!

 

I've never ridden a JMC 24 in my life but I hope to experience what you've described. I had noticed that many 26" cruisers were designed to mimic the OM FLyer 2nd gen and now that I race on one I can testify that it is one of the best designed 26". Scott put some MO JO in those frames trust me! Just get it on the track and they can feel like a 20" snapping out of a berm. The landing gears offer very little rake and keep the handling very nimble. I pushed my OM so fast out of a turn I felt my rear 1.75 IRC tire feathering, but once I directed the front wheel the rear responded in symphony, this is a very high performance race tuned frame set. I Highly recommend you trying an OM flyer as well!

 

Thanks SCOT!!!

 

21

 

ps. Mike I really want to see what your 5" head tube Kuwi can do on the track, that has some agressive design to it, no?

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Yo Mike!

 

I've never ridden a JMC 24 in my life but I hope to experience what you've described. I had noticed that many 26" cruisers were designed to mimic the OM FLyer 2nd gen and now that I race on one I can testify that it is one of the best designed 26". Scott put some MO JO in those frames trust me! Just get it on the track and they can feel like a 20" snapping out of a berm. The landing gears offer very little rake and keep the handling very nimble. I pushed my OM so fast out of a turn I felt my rear 1.75 IRC tire feathering, but once I directed the front wheel the rear responded in symphony, this is a very high performance race tuned frame set. I Highly recommend you trying an OM flyer as well!

 

Thanks SCOT!!!

 

21

 

ps. Mike I really want to see what your 5" head tube Kuwi can do on the track, that has some agressive design to it, no?

 

Trust me, the JMC 24's hit the sweet spot. Interesting you note the OM Flyer. The first cruiser I ever rode, was a 1981 OM Flyer. It felt really good for a 26, and I still remember trying that out. I loved the feel of it, and my 1981 GT 26 feels very similar. The GT big wheeler has a very stable feel to it.

 

And I cant wait to try the Kuwahara 24. Jeff Haney told me it is the sweetest riding 24 he has ever ridden, and he has a few to choose from! Just his comments alone make me want to get out and try it. When it is together, I will do a ride report! :)

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SaWeeet!

 

The Race inc. RA-24 is no slouch either. The front head angle is steep and geometry seems similar to a Floval Flyer and you know SE don't play! The RA-24 is Uber light weight as well almost feels too light. I have been hesitant to take this one on the track for fear of da crack, snapple, pop! :lol:

 

21

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Interesting discussion. I have another thread related to top tube length for the big guys. Racing in the KOS has made a lot of us older, taller guys think about roominess on our rides. There's also always a thread that pops up every so often with someone stating "I'm too big to ride a 20." (I say you just haven't found the right 20" then...)

 

It's cool to see here how the rest of the geometry stacks up. Keep it goin'!

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Thread bump..ok ill start with what i think are the best early 80s geometry,maybe bias but i rode a lot of different bikes back then

20" i think the GT pro 81/83 had great geometry very stable at speed for 18.5 tt

24" the 83/85 GHP was a great handling bike,maybe not as fancy looking as others back then but built to win a race.

By 86/87 the frame that would change race frame geometry forever came out the 21" free agent limo..that had to be the 80s game changer.

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Rebel Racing stick tail's (with a 20" TT) ride amazing for a early 80's frame.  

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Rebel Racing stick tail's (with a 20" TT) ride amazing for a early 80's frame.

 

Ive ridden a 24" zeronine style backend Rebel,it was also a really nice ride that could be used on a modern track.The brakes are the main thing that let it down.

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Bitd we swapped bikes to ride with friends. BMX was just starting here in the UK in 79 so different bikes were turning up weekly. I had a Redline MXll and rode the hell out of it. Seems like they got the geometry spot on. Others were weird to ride also heavy to. Soon needed a layback post for the seat and eventually got a Proline. 19 1/2" top tube an 5" headtube was a perfect combination.

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I was lucky enough to speak personally with Chuck Smith, former partner of Skip Hess’. His exact quote; “Mongoose started as a combination of early Webco and Garry Littlejohn frame designs. “ Both companies seemed to have borrowed from motorcycle frame designs, much like Lynn Kastan with the first Tubular Offset Forks and the first Redline Squareback frames.

Edited by mongoosedrummer
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