Sturmcrow

I want to build a rack on the back of my bicycle that will be strong enough to hold my girlfriend. The rack will be similar to the configuration of an Xtracycle's rack i.e. with a horizontal and vertical portion that would look like an L to either side from the rear. The current design would subject one of the rack members to a maximum foreseeable compressive force around 135 lbs (in her defense, my girlfriend is only 130 lbs, but the rack is designed to be multipurpose).

Is there any reason to be concerned that the dropout rack braze-on may fail? The bike is an older steel MTB with two rear rack braze-ons on the dropouts. I intend to try connecting the suspect member to both braze-ons.

NoReg

On the one hand we have all seen folks ferry folks on the back of bikes like that. And the extracycle seems designed for heavy use.

On the other hand a lot of tubing charts top out at 180 pounds, and you can go heavier, but if you weigh the same as her you are at 270 from the get go. And the weight is biased towards the back end, and possibly one side. Also a few (as many as I have been able to get to) long range tourist coment the one bad undersize thing on a touring bikes are the rack mounts which are basically fender mounts. A lot of folks get breakages. There isn't any real rating on the cast parts, and they often aren't all that evenly formed. So it is hard to feel that confident.

Recently the FF guys have been reaming on us backyard guys for not having 35 year experience to back up our projects. An example of something like that here would be BMX pegs, maybe not 35 years, but it's a big market. I once studied a series of fancy front forks they use for those bikes, and was impressed with the titanic way they mount some of the those threaded load take-off. Very beefy. So that might be the kind of thing to study. I was just looking for ideas for ultimate touring racks, and there the loads are probably at most 60 pounds.

calikid2006

First off, if your girlfriend weighs 130lbs the force would be much, much more than 135 max force on that rack. If you hit even the slightest bump or dip the force will be probably closer to 185 lbs. I think if the braze on is tig welded or even brass brazed it will hold up (for a while at least). What I think would not hold up would be the rack. Even a strong rack does not have the lateral strength of that weight, if you take a hard turn that thing would break.
And lastly, I think your girlfriend should be towing you around on the back. That would be way better and funnier.
But just like peterpan, I dont have 35 years of building experience so who knows.

DannoXYZ 

Or more, design for at least 2x the static weight loads. Not uncommon to experience 4-Gs of compressive loads when hitting bumps. So plan on 500lbs or so.

Rack-eyelets won't work because of the bolts loaded in single-shear. You'd want to use seat-stay sized steel-tubing at 2mm thickness at the smallest end. Then use U-shape ends to wrap around both sides of the rack-eyelets and load the bolt in double-shear.

Sturmcrow

I appreciate the replies. I'll try to answer some concerns with a little more information.

I left out some info the first time in the interest of brevity. The rack is intended to simulate an Xtracycle's rear rack with horizontal and vertical load supports (i.e. it will look like _| |_ from the rear). I've already made the bike into a longbike by extending the rear wheel back 16.5 inches with a small MTB frame. Now I intend to construct the rack from 6061-T6 aluminum bar stock. I have included a safety factor of at least two, not only to take into account dynamic loads, but also to reduce fatigue cracking. The 135-lb member load that I quoted earlier is a maximum design load based on what that member would see if I stepped on the lower rack crossbeam, a load 2.5 times the designed transport load. The material I am using should have no problem supporting that load without buckling, but I wanted to make sure that the rack eye would also support it.

From your comments, I believe that I will make up a little sign saying "No Step" and tape it on that rack crossbeam to protect the rack eyelets. I believe that the heaviest load I could ever foresee myself carrying would be an 80-lb bag of concrete mix on each side or my 130 lb girlfriend on the top. The 80lb loads would result in static member loads of 54.4 lbs. The 130 lb load would result in static member loads of 32.5 lbs and can handle 250 lbs without buckling. For safety reasons, I will not post the actual material dimensions I am using because I would not want someone to take this as professionally designed and hurt themselves. Loads were determined using MATruss. Allowable stress was determined using BeamCALC.