[Possum]

Bending a seat tube is a bit of a bitch.

First, you generally need to use a softer tube, like 4130, as the "better" tubing is a little more brittle and will generally kink/fail when you bend it.

Second, you generally have to create your own mandrel for bending a tube. You've got to decide on the radius you want, and the route a mandrel out of plywood or some other material with the correct tubing OD radiused into it. Fill your tube with sand, and the start bending.

You also (generally) have to use straight-gauge tubing, because you will kink your tubing at the butt interface on butted tubing (usually).

It's not generally something that a builder wants to invest the time in unless he/she plans to do it on a somewhat regular basis (like Curtis Inglis at Inglis/Retrotec).

I have 17.75" chainstays on my bike, with a 73mm shell. We still had to hand-bend the Zona stays a bit more than stock to get sufficient clearance for my 2.3 Resolutions. You may think that 17.75" is "long", but my bike manuals very well. There's more to a custom bike build than just the raw numbers on the drawing. Many other factors can add in to make this bike handle the NE singletrack.

With these wider tires and short stays, you also run the risk, as someone pointed out, of catching the cage with the side knobs of the tire.
^^looks like Ted and I were typing at the same time^^

 

[MMcG]

I have 17.75" chainstays on my bike, with a 73mm shell. We still had to hand-bend the Zona stays a bit more than stock to get sufficient clearance for my 2.3 Resolutions. You may think that 17.75" is "long", but my bike manuals very well. There's more to a custom bike build than just the raw numbers on the drawing. Many other factors can add in to make this bike handle the NE singletrack.

Your 17.75" chainstay frame - is it a Badger by any chance?

 

[Psychic_Pimp]

The main problem with the front derailer I've seen is tire contact with either the cable pinch bolt, and/or some other part of the cable leverage mechanism. Rotating the front derailer forward would free up a bit more space behind the seat tube.

If you look closely at the bike below you can see where the rear tire is overlapping the front derailer. I saw this bike at Interbike and the tire was firmly pressed into the front derailer. It may have been a preproduction sample, as the geoemtry on the Jamis website seems to long and steep(17.72", 73º) to have this much contact.

The other idea here was to not lock someone into an E-type derailer and help keep the seat tube out of the way if a slacker angle was used(which I'm all for).

If you're still referring here to the drawing you posted earlier, I do understand what you're trying to do and why. Here's the problem; the seat tube where a clamp-type derailleur attaches must be perpendicular to the bottom bracket or the cage won't match the chain rings' radius properly. Moving the bottom of the seat tube forward of the bb center to any appreciable degree would throw that off and necessitate an E-type der. It's a catch-22. Often the simple fixes just give you a different problem to deal with.

The frames I have seen with bent seat tubes still have a place to clamp on the derailleur that is directly in line with the bb. The Karate Monkey, Sinister Simon Bar, the new Specialized, and the fancy Scandal Mark posted all have this.

As an aside, it is possible that the problem you point out with the Jamis could be cured by simply using a low-clamp derailleur. My XXIX+G came with a low clamp and it was a pretty tight fit. I tried a high clamp like the one on that Jamis and it didn't help.

I have actually run into three different derailleur/tire interference problems on 29ers so far; back end of cage, cable pinch bolt, and the cable itself crossing too close to the tire.

 

[tozovr]

Bending a seat tube is a bit of a bitch.

First, you generally need to use a softer tube, like 4130, as the "better" tubing is a little more brittle and will generally kink/fail when you bend it.

Second, you generally have to create your own mandrel for bending a tube. You've got to decide on the radius you want, and the route a mandrel out of plywood or some other material with the correct tubing OD radiused into it. Fill your tube with sand, and the start bending.

You also (generally) have to use straight-gauge tubing, because you will kink your tubing at the butt interface on butted tubing (usually).

It's not generally something that a builder wants to invest the time in unless he/she plans to do it on a somewhat regular basis (like Curtis Inglis at Inglis/Retrotec).

I have 17.75" chainstays on my bike, with a 73mm shell. We still had to hand-bend the Zona stays a bit more than stock to get sufficient clearance for my 2.3 Resolutions. You may think that 17.75" is "long", but my bike manuals very well. There's more to a custom bike build than just the raw numbers on the drawing. Many other factors can add in to make this bike handle the NE singletrack.

With these wider tires and short stays, you also run the risk, as someone pointed out, of catching the cage with the side knobs of the tire.
^^looks like Ted and I were typing at the same time^^

FTW's process, as told by him...

 

[eMcK]

If you're still referring here to the drawing you posted earlier, I do understand what you're trying to do and why. Here's the problem; the seat tube where a clamp-type derailleur attaches must be perpendicular to the bottom bracket or the cage won't match the chain rings' radius properly. Moving the bottom of the seat tube forward of the bb center to any appreciable degree would throw that off and necessitate an E-type der. It's a catch-22. Often the simple fixes just give you a different problem to deal with.

The frames I have seen with bent seat tubes still have a place to clamp on the derailleur that is directly in line with the bb. The Karate Monkey, Sinister Simon Bar, the new Specialized, and the fancy Scandal Mark posted all have this.

As an aside, it is possible that the problem you point out with the Jamis could be cured by simply using a low-clamp derailleur. My XXIX+G came with a low clamp and it was a pretty tight fit. I tried a high clamp like the one on that Jamis and it didn't help.

I have actually run into three different derailleur/tire interference problems on 29ers so far; back end of cage, cable pinch bolt, and the cable itself crossing too close to the tire.

The drawling I posted was pretty sloppy and perhaps didn't get at what I wanted. The shorter tube would still be centered on the BB, mimicking a steep seat tube angle, while the actual seat tube would give a "vitual" angle of whatever was wanted by the builder or rider, effectively separating the seat tube angle from the front derailer mounting position. This can be accomplished with an E-type front derailer also, but I'm not a fan of the top-swing style derailer, mostly because they fill up with mud and freeze much more easily than the tradition swing type.



What I was after was a steep seat angle for derailer clearance and and a slacker one to position the rider a bit futher back, I was hypothesizing this was one possible answer that avoided bending tubes and an E-type derailer.

 

[Psychic_Pimp]

The drawling I posted was pretty sloppy and perhaps didn't get at what I wanted. The shorter tube would still be centered on the BB, mimicking a steep seat tube angle, while the actual seat tube would give a "vitual" angle of whatever was wanted by the builder or rider, effectively separating the seat tube angle from the front derailer mounting position. This can be accomplished with an E-type front derailer also, but I'm not a fan of the top-swing style derailer, mostly because they fill up with mud and freeze much more easily than the tradition swing type.



What I was after was a steep seat angle for derailer clearance and and a slacker one to position the rider a bit futher back, I was hypothesizing this was one possible answer that avoided bending tubes and an E-type derailer.

Ah, I think I understand for real now, I was not seeing that you wanted to mount the der. on the short tube. The length of the shorter tube would be rather longer than I had imagined, especially if you use a high clamp derailleur. Well, as an extra added bonus, the extra 9 or more inches of seat tube is sure to offend weight weenies.

 

[greenchris]

^^^^
Weight weenies don't ride steel bikes.

 

[Possum]

Your 17.75" chainstay frame - is it a Badger by any chance?

Yup. The baby blue one that you've seen on mtbr.

 

[MMcG]

Yup. The baby blue one that you've seen on mtbr.

Did you select the chainstay length or did Rob Pennell? BTW, that's a gorgeous bike.

 

[Possum]

The overall bike is pretty close to the 19" Dorothy, with a few tweaks that Rob and I discussed. The chainstay is the standard, and Rob and I discussed it a bit, and I decided to go with his recommendation.

The bike rides extremely well as a whole package. I have no problems with manuals or un-weighting the front end during seated climbing (I prefer to do as much climbing as possible seated if I have gears). There's no tipover fears, but I can get the front end as light as I want it with body position.

That's part of the reason that I keep driving questions about the chainstay length, b/c so many people keep talking about the need for short chainstays on a 29er. I've ridden short(ish) chainstays, and these are "longer" at 17.75, and the bike rides extremely well.

It simply made me re-think my whole "gotta have super short stays" thing.

Thanks for the compliments on the bike. I gotta get back out on it. I taco'd the front wheel in Arkansas and then started a bunch of work on my house. Gotta get that wheel built back up (Flows this time).

 

[MMcG]

Perhaps shorter stays are more effective for Single Speeds than geared bikes. Maybe the initial 17.5" that Ted mentioned would be fine. Perhaps there are other "tweaks" to the frame to make it easy(ier) to manual, wheelie, loft the front end. Perhaps a taller head tube might help with that?

I'm really digging this thread. Let's keep the thoughts and ideas and even the ramblings flowing folks.

 

[MMcG]

Just to update things a bit. I met Ted for coffee on Thanksgiving and he's got a lot of great ideas cooking for this frame and bikes in general. Super nice guy too!

 

[sportcult]

I'm also doing my research. I should be riding a bike with 16.75" stays by the end of the week. I'll report back what differences I feel compared to my current 17.9" setup...

 

[MMcG]

I'm also doing my research. I should be riding a bike with 16.75" stays by the end of the week. I'll report back what differences I feel compared to my current 17.9" setup...

Going for one of the Origin-8 Scouts are ya?

 

[sportcult]

Yeah, I wanted to get tozovr's Simon Bar frame, but that didn't work out, so I opted to go ahead and try the Scout. It's cheap and if I'm going to try short stays, might as well have the option of going as short as possible.

 

[MMcG]

Yeah, I wanted to get tozovr's Simon Bar frame, but that didn't work out, so I opted to go ahead and try the Scout. It's cheap and if I'm going to try short stays, might as well have the option of going as short as possible.

Cool - send me a PM as to where you are getting it from.

Best,

Mark

 

[ÆX]

Welcome to the Monkey!

Your comment on the Fox fork makes me wonder if somewhere down the line we'll see a Fox 29er Talas model on the market.

soon, hahaha

 

[ÆX]

what about 2x5. putting the granny in the middle position.
i am all about it. 94mm bcd is back with fsa 4 bolt!
44x29 front with 12-16-21-26-32 dishless rear wheel. plenty of
range. then w/o a granny so far into the center your f der should
clear any fat tire w/17 cs no bent seat tube.

 

[MMcG]

what about 2x5. putting the granny in the middle position.
i am all about it. 94mm bcd is back with fsa 4 bolt!
44x29 front with 12-16-21-26-32 dishless rear wheel. plenty of
range. then w/o a granny so far into the center your f der should
clear any fat tire w/17 cs no bent seat tube.

This is interesting stuff. ARe these new cranks for 2008?

 

[ÆX]

This is interesting stuff. ARe these new cranks for 2008?

the carbon ones are yes. 2x9 is not new really,
but i think it would work for 29er app real well.

4x94 bcd is new.

used to be 5x94.

 

[MMcG]

I thought the 4x94 bcd was new.

Not sure about the five speed rear cassette though? Why not 2x8 or 2x9? Won't work as well?

 

[ÆX]

I thought the 4x94 bcd was new.

Not sure about the five speed rear cassette though? Why not 2x8 or 2x9? Won't work as well?

it would work fine. i am just so sold on dishless rear and
minimizing the amount of parts you need to ride a bicycle.

i like ss but for all around geared bike i have found i don't
need 9 speed cass.

 

[MMcG]

it would work fine. i am just so sold on dishless rear and
minimizing the amount of parts you need to ride a bicycle.

i like ss but for all around geared bike i have found i don't
need 9 speed cass.

I hear ya. I think the goal for this "project" however is to design a frame that anyone could put components on and ride it.

With that said - your 2x5 drivetrain idea sounds pretty damned cool.

 

[ÆX]

I hear ya. I think the goal for this "project" however is to design a frame that anyone could put components on and ride it.

With that said - your 2x5 drivetrain idea sounds pretty damned cool.

well a 2x9 could bolt on too. the main concern an any short cs 29 geared bike is f der to fat rear tire. there are only two way of going about it.

rotate the seat tube around the bb center.
move the f der out from the tire by
A - widening the bb width.
B - only run 2 rings in the front.

no miracle cure out there that i see.
pretty cut and dry.
alex

 

[indieboy]

running two chainrings on mtns is the way to be son

 

[Ted Wojcik]

I measured a WTB Exiwolf 2.3 and got a rolling diameter of 29.375 with about 45 lbs of air. Here is the beginning of our drawing showing a chanistay length of about 17.25 without any magic. Remember the first of Fat City bikes had chainstays of 17.25" and started the tradition of New England Single Track. As we go on we will try to have a weight distribution close to a 26" wheeled bike using 16.75" chainstays. With the use of Paragon sliders, we can set the distance from the tire to the seat tube to a minimum distance.
Stay tuned.

 

[sportcult]

What's the range on the paragons? It would be cool if the sliders were placed such that the chainstay length was 17.25 with them all the way back for those running gears, while single speeders could run them shorter.
What went into your bottom bracket drop number of 2.25?

 

[MMcG]

I like something along the lines of sportcult's idea. Paragons at 17.25 for gears, but maybe with the ability to slide them in even more for single speeders or 1x8 or 1x9 or 1xwhatever riders.

 

[Ted Wojcik]

What will happen is the sliders will alloy a few different tire sizes without rubbing on the seat tube. I think we will end up using an e-type front dérailleur to keep clearance. As we go on I'll talk a bit about B.B. drop and it's effect on stability. I think 2.25" drop with give about a 12.3 B.B. height with tire squish. With the 2.3 Exiwolf.

 

[sportcult]

What will happen is the sliders will alloy a few different tire sizes without rubbing on the seat tube. I think we will end up using an e-type front dérailleur to keep clearance. As we go on I'll talk a bit about B.B. drop and it's effect on stability. I think 2.25" drop with give about a 12.3 B.B. height with tire squish. With the 2.3 Exiwolf.

What length fork is that assuming? (a-c)

 

[Ted Wojcik]

I haven't set the fork length yet, but I'm voting for 80mm travel. I would draw the frame around an a-c length of 485mm

 

[MMcG]

An 80mm fork sounds good to me for this type of singletrack carver bike. If someone prefers a slightly slacker front end - well they'd still have that type of option to go 100mm and slacken things out a tad. It seems to make more sense than the opposite - setting it at 100mm and then if someone wanted less travel they'd have a wicked steep HT angle.

Man I can't wait to see this frame and thread evolve.

 

[Ted Wojcik]

Good morning. If you would like to take a look, here is the drawing we are working on. I will put in the details later today

 

[MMcG]

I like the slope of that Top Tube.

 

[Ted Wojcik]

Here is the drawing,. More detail will be added as things go on. Photos coming soon

 

[pinkshirtphotos]

if i understand correctly your toe will be in the path of the tyre when you are at 3 and 9? is this with all 29ers or just your frame (never really cared to notice before)

 

[sportcult]

Just built up a new frame with some interesting numbers that relate to this project. Here are some pics:

Frame: Origin-8 Scout29 (size 18")
4130 chro-moly

chainstays as shown are 16.75"
BB height is 12.75", shell is 68mm
rear tire is Kenda Karma 1.9
Fork is 490mm a-c

side view to give an idea of the geometry:

tire clearance:

As you can see this is nearing the practical limit on stay length. With a little wider bottom bracket and manipulated or machined stays and bridges, larger tires could be accommodated at this length. With this size tire there is also clearance for a standard XT front derailler.

 

[MMcG]

That's a cool frame - looks choppered a bit with that Niner fork though (still bent is it?)

 

[MMcG]

Ted - that bb drop on the frame = what in terms of BB height?

Your drawings have helped me understand what bb drop is - thanks for that.

Is that now a 17" seat tube instead of an 18?

The Monkey Butt - I like it.

Best,

Mark

 

[Ted Wojcik]

It looks like there is toe clip overlap, but because the crank arms are away from the centerline, and the wheel will travel in an arc about the steering axis, I don't think there will be any interference. With a tire diameter of 29.375 and a B.B. drop of 2.25, that will give us a B.B. height of 12.4375 not considering tire squish. This design can easily be changed to an 18" frame if we want, It seems that we already have someone willingl to buy a 17", so I thought I would save time and draw it as a 17.

Ted