Noodle-like XL frame problem

[ltwiens38]

Hi everyone,

Bit of background: I've been building for 4+ years, 26 frames so far, mtb, cx, rd, cargo, fat, etc. I started building because I needed a frame that would fit my 220mm cranks (I'm a Zinnonite and 6'8", 240 lbs). My CX and Mtn bikes have 38.1mm True Temper OX Platinum DTs, 31.8mm top tubes - oversize and thick to handle the huge torque of long cranks and a big rider. Both those frames work well for me. Because of the long cranks, the BB is quite high and therefore the ST is relatively short compared to the TT: 525mm c-t ST, 649mm ETT on my CX frame, 465mm c-t ST, 663mm ETT on my Mtn bike frame. I know, that's weird but, it works and it works well. (I don't really want anyone to get sidetracked with the long cranks and weird geo here. I've studied, researched and ridden a lot and it works very well for me)

Last summer, I decided to do a little experiment and go with smaller diameter and thinner walled tubes for a road+ bike. You can see it here: Lyle's Road Plus Light Tourer | LTWiens This has similar geo to my CX frame however, the chainstays are longer, and the ST is 605mm instead of 525mm. I also went with really small seatstays - my nod to Rob English.

The problem: it's kind of noodley. IOWs, it's a little too flexy for my weight. It's rideable but, doesn't really go where I want. What I'm wondering is if another top tube would solve the issue? Mr. Brodie, my instructor has advised that I just start over: - "experiment complete, results: small tubing doesn't work on those lengths. Start over." That may be the best course of action but, I want to know IF adding a second TT would solve the problem.

Lyle Wiens

[Curt Goodrich]

It might help but I agree with Mr. Brodie. Mistakes are always more informative than successes.

[ltwiens38]

Thanks Curt. Yes, the mistake has taught me a lot. It's just hard to come to terms with shelving a frame that I spent so much time and money on. I'll try and look at it as an investment in R&D. If I knew with certainty that adding the second TT would solve the issue, that might change things but, I may end up putting more time and money into a project that produces sub-optimal results.

Thanks


Lyle Wiens

[steve garro]

ST: dropper post tube for 30.9 post just use a rigid post
TT: 35mm DT cut down to best use of the butting profiles
DT: 38mm 9/6/9
SS's: 19mm / 3/4"
SS's: MTB or Tandem

I have built quite a few Giant Bikes

- Garro.

[Rody]

Lyle,

I've built a fair share of double top tube cruisers, as has Steve above...the additional tube alone will not define the performance of the bike.

The issue you have is the under spec'd rear end.

While the main tubes may provide sufficient torsional resistance for their diameter and length, the rear end is so diminutive that it undermines the strength of the assembly. If the chainstays/seatstays do not do their part, the main tubes will be stressed beyond their ability to compensate.

Though you wanted to give a nod to another's design, understand that you need to build holistically for the intended riders size and performance goals.

You just got more meat than those tiny tubes can handle :)

Hang it on the wall and begin again.

cheers,

rody

[ltwiens38]

Thanks Rody. I had my suspicions that the rear triangle was related. It's compounded by the length of the stays too.

I will start over and build something better.

[Rody]

Good choice, can’t wait to what you come up with :)

[jon_norstog]

My approach on the stiffness/compliance thing is to build stiffness into the heart of the bike - specifically the seat tube and chainstays, and let the front triangle flex just enough for comfort and to act as a kind of spring, storing and releasing energy when you get on the pedals. Seatstays, I like a little cush there if it is a bike that is going to be ridden sitting down in the saddle.

I like the 1 1/4" and 1 3/8" TT/DT combination on BMX bikes and mountain bikes that were being jumped and routinely thrahed, with 3/4" and 7/8" x .035" straight gauge seat and chain stays. For real monsters and on dirt jumping bikes like the "Acid Rain" and "Black Rain" I would go up 1/8" for the big tubes and go to .049" on the chainstays.

HTH

jn

[ltwiens38]

Here's what I came up with:

41.3 x 1.0 DT, 34.1/34.9 ST, 34.9 x 969 TT. 29x55mm Compass tires.

I wanted to try the dropped chainstay design - because I think it looks cool but also because it legitimately helps fit everything into that space - A bigger-than-normal problem with my own bikes because of the high BB (because of 215mm cranks). It's hard to tell in the picture but, the DT runs past the bb by 60mm and then I brazed the CS's right onto the DT.

IMG_20190919_133304 (1).jpg

I'm happy to report that the ride is fantastic. A little too stiff but, I don't mind.

More pictures to come soon.

Lyle Wiens