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26" bike polo frame project - wall thickness tubing advice needed

Hi guys so I am starting to design a bike polo frame and I am needing some advice in the wall thickness of the tubing. I am planning to use SAE 4130 Chromoly Steel for the tubing and I will have someone do the welds with TIG. I am not a frame builder and the tubing I choose it is from what I thought it would make a sturdy bike but maybe the wall thickness in the tubes are not the best or right ones. I used the American bottom bracket spec cause here in Argentina it is easy to buy 3 piece bmx cranks for a reasonable price and having the option to access to a bunch of options when it comes to chain wheels.
My aim do something like the geo of the joust, I hope some of you guys could lead in the right direction with this project. Thanks in advance!

Head tube: ------- (ID 34mm) - (OD 36.5mm) - (Wall 1.25mm)
Top Tube: -------- (ID 31.8mm) - (OD 33.5mm) - (Wall 0.85mm)
Down Tube: ----- (ID 31.8mm) - (OD 33.5mm) - (Wall 0.85mm)
Seat Tube: ------ (ID 31.8mm) - (OD 33.5mm) - (Wall 0.85mm)
Bottom Bracket: (ID 51.3mm) - (OD 54.3mm) - (Wall 1.5mm)
Seat Stays: ------ (ID 18mm) - (OD 20mm) - (Wall 1mm)
Chain Stays: ---- (ID 18mm) - (OD 20mm) - (Wall 1mm)

*All the tubing is straight gauge of course.
ID = Inside Diameter
OD = Outside Diameter

I would bump up the wall thickness on the down tube. You could go with something closer to 16mm with a .85mm wall for the seat stays. Consider that straight gauge tubing and bmx cranks are going to maybe make your bike heavier than you anticipate, though. Try to shave weight where you can. I think the seat stays are one of those places.

If I were you, I would shoot for a euro bottom bracket and MTB cranks as well.

(I have no experience building bicycles.)

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fixcraft.net

Nick Kruse wrote:

I would bump up the wall thickness on the down tube. You could go with something closer to 16mm with a .85mm wall for the seat stays. Consider that straight gauge tubing and bmx cranks are going to maybe make your bike heavier than you anticipate, though. Try to shave weight where you can. I think the seat stays are one of those places.

If I were you, I would shoot for a euro bottom bracket and MTB cranks as well.

(I have no experience building bicycles.)

Thanks for the input Nick.

When you say Euro BB, what do you mean? You refer to the english / ISO thread 1.37" x 24 TPI (34.7mm x 24TPI) with a shell width of 68mm or 73mm? Basically the commonly used in mountain bikes but I just want to clarify cause here in Argentina we don't call them Euro.

It is a good idea to shave some weight and downsize the seatstays but I will have to do it also with the chainstays, the thing is that if I bump up the wall of the down tube to lets say from 0.85 to 1 mm I will be increasing the wall of also the seat tube and the top tube, I intended this that way to cut costs cause otherwise I will have to order 5 different size of tubing instead of 4 (head, bb, front triangle, seatstays and chainstays). Chromoly at least here it is not that cheap so I am seeing if it is even viable to do it.

Only place i havent broken a frame is the seat tube/down tube, so i dont reckon you need to make it stronger.
But ive never built a bike before, ask a framebuilders advise preferably one with a good few years experience before you make any big decisions.

Tubing diameter and wall thickness is good with straight gauge 4130.
18mm seat- and chainstays can be done in 0.8-0.9mm.

(have built a few frames, 2 poloframes, too.)