Utility Cycling - Building my first longbike, got a couple questions.

I'm using a late 90's aluminum 18" MTB for the main frame. I have both rigid and suspension forks for it and haven't decided which to use just yet. The new rear triangle will come from a 15" chromoly MTB frame that I just eBayed. I went with a small frame size so that the top of the seat tube is about even with the top of the tire. This will let me keep the rack and center of gravity low. I used a steel donor frame so that I can braze on it. I intend to bend down and shorten the top tube of the donor frame and attach it to the seatstay bridge on my main frame. I currently intend to attach it by cutting the headtube in half after brazing short sections of tubing to it. I will insert bolts through those tubes as a clamp around the seatstay bridge and fill it in with polyurethane window sealant to allow some small "give." I intend to cut a hole into the top of my rack to allow inserting a seatpost into the new rear triangle for a makeshift tandem (on which I'll be doing all the work). I'll also be installing foot pegs on the bolt that will be passing through the donor BB shell.

1) For those of you that built up a DIY xtracycleish frame extension, did you change the frame geometry at all? Since I have control over the length of the member that connects from the top of the new rear triangle to the main frame, I could increase it or decrease it to change the head tube angle. John Allen suggests that decreasing the trail could improve the handling of a tandem. I could do this to my longbike by giving it a humpbacked geometry. Aside from the handling, this would also provide more bottom bracket clearance. http://www.bikexprt.com/bicycle/tandem.htm

2) I'm a little worried about the strength of the seatstay bridge. I take comfort in knowing that the xtracycle mounts to the chainstay bridge so my seatstay bridge should be able to handle it.

On my longtails (almost finished three) I do not depend on only the chainstay bridge but also resting some weight on the chainstays and "seat tube" just abowe the BB.

Iit is not only about strenght but if the strain is put the way it is designed to do. Strain comming from- or going in a different direction than intended, it could break from wery little.

I do not think I`we seen the solution you describe, so I do not know if it has been done. I`we seen some where the top tube from the rear bike is welded to the "seat tube" (cannot remember the name. Part of the bike frame, not the loose one).

I trust that triangles is strong, so I use that system. My guttfeeling is not so sure about the long time strength of your solution. Take a close look at the "big dummy" and "kona ute" to see if there is anything you could copy.

I do know that triangles are ideal. I suspect, however, that the seatstays on the main frame will work just fine to close up that gap. In an ideal world, the rear triangle will rotate about the bolt through the BB shell. That should cause the force, F, through the former top tube to be nearly perpendicular to the moment arm from the bottom bracket. This will allow the former top tube to be in nearly perfect compression with little risk of buckling. If I remember correctly, an ideal truss uses members in tension and compression with little to no shear or moment being transferred. In my case, I'm just adding on another section to the truss.

(at least, I hope so)

Looks better with your drawing. Did not think so much about other ways than "mine" since I decided on using triangles from full suspension bikes and "no weld". I`we seen some diifferent ways that looked odd to me, but who knows. Need to find my old schoolbooks to find out. Still not sure about seatstay bracket instead of seat tube, but you could reinforce it maybe?

Waiting to hear how it works. Lucky those guys who find a tandem to convert.

I'm with you about the seatstay bracket/bridge. I'd rather mount it to the top of the seat tube, but I'm worried about the donor top tube interfering with the rack. Speaking of racks, I plan to build up one like this, but with connections to both sets of rear dropouts and 3/4" x 1/8" stainless steel. http://www.bicyclinglife.com/HowTo/HeavyDutyRacks.htm Hmmm, thinking of that makes me realize that I should strive to make as solid a connection as possible between the donor top tube and the main frame rather than using polyurethane for a flexible connection.

I have thought about mounting to the top of the seat tube. It would look something like this.

badmother, thanks for replying, by the way. Your enthusiasm for these DIY longbikes is probably responsible for kickstarting me into making up one of my own.

badmother, thanks for replying, by the way. Your enthusiasm for these DIY longbikes is probably responsible for kickstarting me into making up one of my own.

"Blush":D

Make sure the weight is kept down as much as possible. My first dogbike is also from a lightweight (rigid fork) MTB, but then I added too much to make it strong enough..

Thinking of making another one this winter for that reason. Just got hold of a great cromo hybrid that I am looking (staring) at at the moment. Also has got a alu triangle that is going to be used.

Could also redo the first one, not sure.

I`ll do some loud "thinking" about your rack.

The rack in the link is great, but it depends on what you are building the bike for. I realised one of the reasons my "dogbike" is heavy is I made it too strong for my use. Old steel racks + alu ekstra "legs". I ride with the dog + chlotes, food, campingchair and so on. Sometimes a grocery run. I am not car- free due to health, so it is mainly bulky items, dog is 25 kg.

My sons "longtail" (he complained this morning about it not being finished) I used two new alu racks. Planning on some ekstra legs.

Those who use it for heavy loads say it handle better if you put some of the weight in front. I put a basket there also becouse I like easy accsess to my things.

One of the other homebuilders (hsean I think he is called) just decided to make two, one for speed and one for heavy loads. I am thinking the same.

I figure i'd mention some stuff I did to my Black Xtra I built from recycled bikes. one thing is, it's not welded togeather, I'm not sure if you could call the back of mine a triangle since it's just two arms howing back wheel, however, even without all the hard work, aside from painting, this bike easly holds over 400lbs I hit curbs, jumps, offroad, and it's fun to ride, was very easy to make to.

Only "No construction is stronger than its weakest link", so wheels, tyres and all need to be in good shape. We both had our crashes :D:D

Badmother's work also inspired me to build my own... :D

I used a Norco Katmandu and the rear triangle from a CCM FS... the Norco frame is single gauge chromoly. I did not even consider using a suspension fork, and I used two Al racks to mount the deck.

The bike has seen a lot of use and besides carrying a bunch of stuff in the bags and on the deck has also been used a tow vehicle for the 6 and 8 foot BAW trailers we have at the shop.

The bb height is a little low but that has cause no issues and I do like the way the frames join and maintain a straight line along the chain stays which lets me use one idler (a Suntour open cage derailer) and no upper chain guide.

The bike has been solid as a rock and the only change I have been considering is using some moustache bars... I have installed some bar ends that are not shown in these pics.

http://www.ravingbikefiend.com/bikepics/longbike5.jpg

http://www.ravingbikefiend.com/bikepics/stretch1.jpg

Sixty-Fiver, I agree with the consesus of the forum that your DIY longbike is pretty much the most elegant of the ones that I've seen. This is primarily due to the fact that you actually have racks on yours. So many of the longbikes in the DIY sticky are just longer without the added cargo capacity that they were made for.

I don't have any extra racks laying around that I could mount on this ride, so I figure that I can fashion some up from stainless and bolts for less than I could buy two racks that combined would support my 130 lb girlfriend. I plan to build with that as my goal, and figure that I'll rarely need much more than that.

The skate board works well to, his gave me the idea of doing the same, but the skateboard isn't all flat or as long as needed, so I noticed something while I was on break, Banana board, long boards, that would be perfect and looks neat to, that or you could always make something of wood, Eventualy mine will have chain saddle bags, yes heavy, and a larger bed to put things on with nice decritive wood fence around, like old trucks had. lol that and I get alumume flashing free so the back of the bike and most the wheel will be hidden, eventualy lots of dents will come though lol, but yes, tis a fine bike he has,

And so, longbikes as we all know, king of all bikes, around here people call mine monster bike. lol