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Another frame design thread...

Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
Having tried just about every 26" street hardtail frame out there, I am looking at making a new type of bmxmtb. So here are the ideas I am having:

A) Higher Bottom Bracket Height- not only would this give you the ability to shorten the chainstay to 14.25 with ease, it would make the maneuverability of the bike much better. Ideal height would be around 350mm with a 465mm fork.

B) Super short seat tube- Less weight, way better clearance for tricks, and a tighter, stiffer frame. Ideal seat tube length: 230mm with a seatube angle of 73degrees so you could still slam the seat.

C) Spanish Bottom bracket to insure the shortest possible chainstays.

Here is a half-ass mock up on cad of this idea.


Discuss. Ridicule. Elaborate.
 
M

mattt

Monkey
Jul 21, 2009
126
0
texas
1) If you ever made a frame you'd know that 14.25 is waaaay not possible... for a 26. Even if you used a yoke (non tubes) at the BB you'd still only get to about 15.25... Using tubes without a yoke, you'll find that its very hard to squeeze a 2.3 tire that close and still have crank arm and sprocket clearance. And, referring to the post from yesterday, shortening the stays would actually allow you to lower the bottom bracket and get stability and maneuverability. Short and high would loop out very easy and is redundant.

2) Tighter stiffer... you'd have a stiffer frame if you used non "S" bent stays.

3) Its aesthetics, but I hate the look of a super short seat tube. The negative angle betw/ top tube and seatstays is probably physically weaker as well... I know its visually weaker.
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
mattt said:
1) If you ever made a frame you'd know that 14.25 is waaaay not possible... for a 26. Even if you used a yoke (non tubes) at the BB you'd still only get to about 15.25... Using tubes without a yoke, you'll find that its very hard to squeeze a 2.3 tire that close and still have crank arm and sprocket clearance. .
Click to expand...
My Eastern Betty Frame has 14.5" chainstays slammed, with room to spare, leading me to believe you could go shorter. No one really runs wide tires anymore for park or street, in fact, I run pretty thin tires. Intense MK2's 2.25s are very tiny.
 
Stoked

Stoked

Turbo Monkey
Nov 28, 2004
1,809
1
LI, NY
its cool you want to try something new. i'm not sure why a spanish bb would equal shorter chainstays? if you had a mid bb on the same geo you posted, the tubes would be shorter due to the increased shell size. mid bb may save some weight here.

but honestly choosing one bb over another because of weight is splitting cvnt hairs.

the tt/st junction looks like a p series bike. not visually the best.
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
Spanish BB is the narrowest BB, which means less space needed for the bottom bracket, bringing the rear axle as close as possible to it. The Seat stay wasn't really part of the whole idea, was really just seeing what it would look like. I didn't like the ultra low BB on the superco I rode.
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
mattt said:
the Eastern Black betty has "14.7" Slammed chainstay/ 15" to center"... Meaning 15" I dont know why theyd even mention the 14.7 part, its 15. And it has a yoke: http://www.pinkbike.com/photo/4052057/

A yoke is basically introducing stress risers and unnecessary/misplaced weight.
Click to expand...
That was the first gen Betty, they have since changed it drastically, new chainstay and new yoke, and a new headtube. However, my design will more than likely no make use of a yoke. But on the subject, yokes can be made ridiculously strong, if done correctly.
 
TheTruth

TheTruth

Turbo Monkey
Jun 15, 2009
3,893
1
I'm waving. Can you see me now?
mk1 betty's suck. They shall not be copied.


But I am saying that the frame you designed looks dangerously close to a superco. I have never dirt jumped with a super co, but from what demo9 tells me, he concluded that they are infact, dope.
 
M

mattt

Monkey
Jul 21, 2009
126
0
texas
The atomlab trailking is similar to what youve described... 14.4"L X13.3" height supposedly.

And I suppose if you flattened out the stays enough you could skirt the yoke-Tonic Howie... I dont know how theyve welded that close to the edge of a BB though without burning irreparable holes. Alot of manufacturing limitations come up when you go that short... raises cost... assuming you were thinking that far.
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
mattt said:
The atomlab trailking is similar to what youve described... 14.4"L X13.3" height supposedly.

And I suppose if you flattened out the stays enough you could skirt the yoke-Tonic Howie... I dont know how theyve welded that close to the edge of a BB though without burning irreparable holes. Alot of manufacturing limitations come up when you go that short... raises cost... assuming you were thinking that far.
Click to expand...
Trailkings are nice, but they aren't quite what I want. This wouldn't be a production thing, just one or two for me. Supercos are very nicely done, just way too low of a bottom bracket for my tastes.
 
TheTruth

TheTruth

Turbo Monkey
Jun 15, 2009
3,893
1
I'm waving. Can you see me now?
Now I will be serious...

May I ask what the TT length is? And what kind of head angle are you planning on having?

I thought the super co had a super far reachy feel to it. I would run this bike with flat bars. I like to be a bit closer to the front of the bike than leaned all the way back over the wheel. I like my short chain stays but a steeper seat tube angle. I do like the length of your ST though. I like the feeling of being high over the TT for maximum maneuverability.
 
WestCoastHucker

WestCoastHucker

RETSUBDUHT
Oct 23, 2003
3,560
710
daydreaming about a new rhythm line.....
from what i've been able to tell from most short chainstay bikes, the bottom bracket size is irrelevant(ie. spanish vs. mid vs. euro). the issue is with the bottom bracket width because the tire typically becomes pinched between the stays before it bottoms out on the bb shell...
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
TheTruth said:
Now I will be serious...

May I ask what the TT length is? And what kind of head angle are you planning on having?

I thought the super co had a super far reachy feel to it. I would run this bike with flat bars. I like to be a bit closer to the front of the bike than leaned all the way back over the wheel. I like my short chain stays but a steeper seat tube angle. I do like the length of your ST though. I like the feeling of being high over the TT for maximum maneuverability.
Click to expand...
That was also a matter of concern but I thought it more of a preference thing. If the bike was going to be strictly dirt, I'd give it a 69 degree HA with a 555mm toptube. But seeing as how it will be for everything, I'd even it out with a 70 degree headangle and a 540mm toptube. I also realized that I should be more specific when I am referring to BB location. The idea is to raise it above the axle line, which isn't dependent on the wheel or tire size.

My gripe with higher seattubes is that I have lots of trouble doing tricks over the toptube on my 26, but they are effortless with a 20. I don't see why a dj/park bike needs all of that seattube.
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
WestCoastHucker said:
from what i've been able to tell from most short chainstay bikes, the bottom bracket size is irrelevant(ie. spanish vs. mid vs. euro). the issue is with the bottom bracket width because the tire typically becomes pinched between the stays before it bottoms out on the bb shell...
Click to expand...
This idea will allow for wiggle room when the tire is slammed, but will limit the size of the sprocket to 27t. Which is fine, considering I typically run 27/12 and would love for that to become 25/9.
 
TheTruth

TheTruth

Turbo Monkey
Jun 15, 2009
3,893
1
I'm waving. Can you see me now?
Dirtjumper999 said:
That was also a matter of concern but I thought it more of a preference thing. If the bike was going to be strictly dirt, I'd give it a 69 degree HA with a 555mm toptube. But seeing as how it will be for everything, I'd even it out with a 70 degree headangle and a 540mm toptube. I also realized that I should be more specific when I am referring to BB location. The idea is to raise it above the axle line, which isn't dependent on the wheel or tire size.

My gripe with higher seattubes is that I have lots of trouble doing tricks over the toptube on my 26, but they are effortless with a 20. I don't see why a dj/park bike needs all of that seattube.
Click to expand...
Ahh, see, i am a man of pedal separation anxiety
 
demo 9

demo 9

Turbo Monkey
Jan 31, 2007
5,910
46
north jersey
Ill be brief on this 1, i had a custom bike(i didnt make it myself, but i had it made to custom geo)
On the general topic, ill mention a few things, while it was awesome, keep in mind-you cant change it, i went with a design very similar to Cru's, but with a few tweaks to make mine smaller, it was great, i liked it. A problem i had was selling it, as cool as it sounds, "custom" sucks, i got 100 bucks for mine, and that was an absolute steal for me, im surprised i was able to get that much.

In regards to BB heights, i know you had said that you want a higher BB-i do NOT recommend it for a few reasons, See the higher it is, sure you can get more leverage and better hops, but for anything with speed and pumping, you can throw your stability away, i am not saying to go super low, but i think the current standard of MTB has the nail on the head.

I have a supeco, i LOVE it. In regards to that and chainstays, the chainstays are 15" as was my simple and that was as short as the builder could make it, and he was even surprised he could get them that short. We all know its a trade off, i really think you reconsider a high end stock hardtail frame before you go for it yourself. going with very "extreme" measurements gives you good ______ but kills your ______. Putting bigger tires on your truck will make it have MORE ground clearence, but LESS handling, something to consider on bikes, is designing your bike for a 360 hop worth possibly loosing your dirt jump tailwhips?
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
True that, but the only real extreme adjustment I am making would be the seattube. The BB I am only raising about 10mm above what my current set up is. I am not worried about selling it. The BB rise would still be very low in comparison to the BB rise on a BMX bike, So effectively your location in relation to the bike would be the same as on a 20.
 
Tetreault

Tetreault

Monkey
Nov 23, 2005
877
0
SoMeWhErE NoWhErE
looks good and YES you want a positive bb (above the axle line) its similar to what demo said but from my experience the opposite, the gains from a higher bb on a dj and strett bike FAR out weigh the negetives. also how would a higher bb affect tailwhips in the air? thats like saying it will be harder to do no footers with a higher bb?
 
P

poekie

Chimp
Mar 21, 2009
59
0
demo 9 said:
Ill be brief on this 1, i had a custom bike(i didnt make it myself, but i had it made to custom geo)
On the general topic, ill mention a few things, while it was awesome, keep in mind-you cant change it, i went with a design very similar to Cru's, but with a few tweaks to make mine smaller, it was great, i liked it. A problem i had was selling it, as cool as it sounds, "custom" sucks, i got 100 bucks for mine, and that was an absolute steal for me, im surprised i was able to get that much.

In regards to BB heights, i know you had said that you want a higher BB-i do NOT recommend it for a few reasons, See the higher it is, sure you can get more leverage and better hops, but for anything with speed and pumping, you can throw your stability away, i am not saying to go super low, but i think the current standard of MTB has the nail on the head.

I have a supeco, i LOVE it. In regards to that and chainstays, the chainstays are 15" as was my simple and that was as short as the builder could make it, and he was even surprised he could get them that short. We all know its a trade off, i really think you reconsider a high end stock hardtail frame before you go for it yourself. going with very "extreme" measurements gives you good ______ but kills your ______. Putting bigger tires on your truck will make it have MORE ground clearence, but LESS handling, something to consider on bikes, is designing your bike for a 360 hop worth possibly loosing your dirt jump tailwhips?
Click to expand...
I get what you're saying but compared to a bmx, even the most 'extreme' geometry'd mtb still feels sluggish and those guys are still going way bigger on everything, just check the size of the average trails, racetrack or street huck those guys are doing..says enough to me about how much stability you need.
 
Tetreault

Tetreault

Monkey
Nov 23, 2005
877
0
SoMeWhErE NoWhErE
looks good. maybe steepen the seat angle in order to achieve a little more tire clearance to the seat tube? just a thought.

also are you going to building the frame yourself? if not you are you sourcing to build it for you?
 
IH8Rice

IH8Rice

I'm Mr. Negative! I Fail!
Aug 2, 2008
24,524
494
Im over here now
jonKranked said:
Jeff's bikes are always odd geometry compared to the stock stp. He comes from a very strong trials background and his frames reflect that. I got to ride his custom trials STP once. Was an awesome bike.
Click to expand...
that STP is a production frame:
The chainstays are adjustable so you can make the length whatever you want from just over 15 to 16. My trials bike is actually longer because the tire is taller. That is a production frame.
Click to expand...
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
Tetreault said:
looks good. maybe steepen the seat angle in order to achieve a little more tire clearance to the seat tube? just a thought.

also are you going to building the frame yourself? if not you are you sourcing to build it for you?
Click to expand...
I'll be making it myself more than likely. I was thinking the same about the seat tube, its at 73 degrees right now.
 
Tetreault

Tetreault

Monkey
Nov 23, 2005
877
0
SoMeWhErE NoWhErE
Dirtjumper999 said:
I'll be making it myself more than likely. I was thinking the same about the seat tube, its at 73 degrees right now.
Click to expand...
it seems from your posts in the thread me and you have similar outlooks as to what we want in a frame. since your tt is so low and you dont plan on sitting anyways i see no problem making the seat angle more drastic then usual 74 or even 75. it will also give you more clearance to get the seat a way from the tire. the way it is now it looks like you will need quite alot of post sticking out to even clear the tire. if you go with a steeper seat angle and find it just feels off for pinching you could always go to a laid back post later.
 
Dirtjumper999

Dirtjumper999

Turbo Monkey
Feb 13, 2005
1,556
0
Charlotte, NC
Yea, the seatpost may go quite a bit more forward actually, since toptupe length won't be a big issue at that point. It's not like the seat would get in the way of barspins being that low.
 
slimshady

slimshady

¡Mira, una ardilla!
Dec 20, 2007
5,421
3,276
A long time ago, in a galaxy far, far away...
mattt said:
1) If you ever made a frame you'd know that 14.25 is waaaay not possible... for a 26. Even if you used a yoke (non tubes) at the BB you'd still only get to about 15.25... Using tubes without a yoke, you'll find that its very hard to squeeze a 2.3 tire that close and still have crank arm and sprocket clearance. And, referring to the post from yesterday, shortening the stays would actually allow you to lower the bottom bracket and get stability and maneuverability. Short and high would loop out very easy and is redundant.
Click to expand...
Hey Matt. Me and my mate we just built a 26er (still at the proto stages) and gave it a CS lenght of 15" using straight tubes, no yokes/plates/whatever. I'm using Intense MK2 2.25" tires. And I still have plenty of room back there. I mean, I would not hit a muddy trail with the thing, but I sure can ride street, park and dirt jumps with ease.
 
M

mattt

Monkey
Jul 21, 2009
126
0
texas
yeah, I guess if you were running tires that small itd be a little easier. I was looking at it from a production standpoint. Most people, myself included, would want a larger dirt capable tire on a hardtail... Also, I particularly hate intense tires. I am of the opinion that the 26" wheelsize is the most hindering factor for making a purely "street" MTB. I personally would spec a 24" with a 14mm rear and vbrakes.

I dont think the benefits of shorter stays is worth sacrificing proper engineering and tire clearance/versatility
 
M

mattt

Monkey
Jul 21, 2009
126
0
texas
thats what superco have done:

http://www.ridesuperco.com/2010/photos/charger5.htm

But the thickness of the downtube that the seat tube is connected to is probably 1 mm, as that is the thickness that supertherm comes in. I would personally use thicker tubing, or a gusset of some kind at this weld. This design would also make it a tad more difficult to build... As a normal frame build would start at the BB/seattube weld...
 
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