Frame design

Ted Wojcik · 11-15-2007, 04:32 AM

How about a discussion of some of the challenges in designing a 29er frame. Let's talk about making a frame with an 18" seat tube and using a front dérailleur. When done, I will build the frame and we will have a local 29 fan test it. We'll call it Monkey Butt.

tozovr · 11-15-2007, 04:40 AM

with the 18" ST and a front derailleur the main issue is the operating angles of the actual derailleur right? So the tube either needs to have a crazy angle (steep?) or you need to manipulate it right? In steepening the angle you effectively shorten the Top Tube...then you get Toe overlap issues and possible fork crown interference.

If we call it monkey butt, the front should be brown and the back, Blue.

Ted Wojcik · 11-15-2007, 04:48 AM

Actually, the biggest problem is if we make the chainstays short enough for good climbing/traction, there is an issue with the front dérailleur hitting the rear tire when on the small ring. If the chainstays are long enough to minimize this issue, the wheel base gets long and steering starts to slow down in addition to the decrease in traction. As this discussion goes on, I will present some drawings with exact dimensions as well as recommendation on how we might address these challenges.

tozovr · 11-15-2007, 04:56 AM

I'm cheating...FTW, Darrell and I just chatted about this on the Sinister 29er a few months back

to keep a sub 17" seat tube angle the seat tube got a nice dogleg in it.

I spoke with Chris a week or two ago, sounds like you guys are kicking ass!

RJ

Angus · 11-15-2007, 05:08 AM

Ted,
Great to have you aboard an on RM! I have admired your quality for years.
I am not a frame designer (nor do I play one on TV). But I will be watching this thread to see what develops, as I am an avid 29er fan. Cheers...

**MMcG** · 11-15-2007, 05:18 AM

I'd say put a little bend in it - get the chainstays to around 17" and find an older Shimano rear derailleur (say an old XT or LX or something - one of the models that Surly used to recommend on the Karate Monkey) and then have at it.

Also now that so many new tires are coming out - design considerations should also include the ability to run as wide a tire as possible should a rider prefer to go that route, but with a design that would allow for ample chainring clearance for at least a two ring set up.

Another thought would be a seat tube to top tube brace on the frame to provide for a slightly more sloping top tube which could provide a little more clearance for the rider.

Just a few thoughts on my end.

**MMcG** · 11-15-2007, 05:20 AM

I had another thought here.

What about some type of sliding rear dropout setting or slotted dropouts at different chainstay lenghts. So for example, an east coaster or a short chainstay advocate could run the frame in a short chainstay mode, but a west coaster or a fireroad type of guy could lengthen the chainstays a tad for his or her preference? Could this be done via Paragon Sliders or perhaps some type of system like they utilize on the Evil Sovereign?

Ted Wojcik · 11-15-2007, 05:26 AM

This is the dialog I'm looking for. I'm on the way to the shop now, so I will continue this chat later in the day. Let's see what grows out of this.

Stray_cat · 11-15-2007, 05:34 AM

I'd certainly like to see a 29er using a shorter wheel base. Most of the ones I've tired I've though to myself "hey I'm on a big bike." I'd say do what ever you have to do to get those chainstays short.

**MMcG** · 11-15-2007, 06:41 AM

Quote:

Originally Posted by Ted Wojcik

This is the dialog I'm looking for. I'm on the way to the shop now, so I will continue this chat later in the day. Let's see what grows out of this.

Ted - thanks for starting up this thread. Should make for a real solid conversation here. I made it a sticky so it will stay on top for people to see easily.

All the best,

Mark

Toddre · 11-15-2007, 07:08 AM

This should be interesting to follow... Can't wait to see how it turns out..Maybe it will help me with some tweaking of some frame designs I'm working on for future projects...of course, the frame here is too small again...lol

**MMcG** · 11-15-2007, 07:46 AM

Quote:

Originally Posted by Toddre

This should be interesting to follow... Can't wait to see how it turns out..Maybe it will help me with some tweaking of some frame designs I'm working on for future projects...of course, the frame here is too small again...lol

We'll just get you a really Long seatpost!

Possum · 11-15-2007, 08:48 AM

How about a 17.75" chainstay? 73.5deg STA?

When you say 18" seat tube, do you mean "traditional" 18" seat tube (i.e. for a rider that would normally ride an 18" (c-c) frame in a 26" bike)? This would be roughly a "medium" sized frame, with an ETT of 23.75-24"?

At this size, I don't see toe overlap being a problem, unless you get into like a 71deg STA, unless your rider has Sasquatch feet.

I see your primary drivers here being a)clearance with the front der and the rear tire and b)sufficient mud clearance for the rear tire while still being able to fit 3 chainrings and not use a 120mm wide bb.

My suggestion would be a 73mm shell, so that you can put the stays a little farther outboard. You're probably still going to have to do some of your own custom bending to get enough clearance for a Rampage and be able to use all 3 rings.

I honestly think the front derailleur location takes a back seat to this issue.

Thoughts?

sportcult · 11-15-2007, 09:11 AM

How about a braze on style front derailler? You could just have a little arm to place it where you need it.

eMcK · 11-15-2007, 10:43 AM

Great thread.

Rather than the 73mm shell, perhaps go with a press-in bering design as used on quite a few road bikes and the new Pivots. This should give plenty of space to get those chainstays wide enough for 2.3's and some mud.

I would give up my front derailer for some really short chainstays, but I would guess to be in the minority there. I like the Wolfhound style split seat tube, but it would seem to preclude the use of a front derailer, although I wonder if an E-type or braze on would work.

What I would like to see other than short chainstays: Longish top tube (~24") slackish head angle (~70) to go with a higher rake fork, and slackish (~71) seat angle. High BB, at least 12", maybe a bit more. Some extra standover would be great also, with either an extended seattube or a small brace, ala 1st gen El Mariachis.

I happened to be looking at a custom 20" wheeled kid's bike today with the Wojcik name on it.