new and improuved giant cruzbike
- Thread starter MrSteve
- Start date
trapdoor2
Zen MBB Master
That missing bit of top tube seriously weakens the frame! Compression stresses from your weight (not to mention bumps in the road) gotta go somewhere. If you had let the plywood insert go all the way back to the seat-tube (top to bottom), I'd be a bit happier. As it sits, the bottom tube is now subject to bending loads it was not designed for. When it fails, it will fail at the point where the plywood ends (down by the original BB). Basically, it is an accident waiting for a place to happen.
In a fully triangulated frame, all the stresses are essentially tension and compression (to some extent the fork/headtube produces bending stress in the top and bottom tube). The tubes are designed to accomodate tension/compression forces.
In a serious accident, those remaining bits of seat-tube are going to do some nasty damage to boot.
Suffice to say, it looks very dangerous to me.
In a fully triangulated frame, all the stresses are essentially tension and compression (to some extent the fork/headtube produces bending stress in the top and bottom tube). The tubes are designed to accomodate tension/compression forces.
In a serious accident, those remaining bits of seat-tube are going to do some nasty damage to boot.
Suffice to say, it looks very dangerous to me.
trapdoor2
Zen MBB Master
Jack, fiberglass isn't going to be enough. The lower tube will bend where the plywood ends and that little bit of plywood you've left at the seat back (where it is cut @ 90 degrees - another stress no-no!) is going to snap...putting that back bit of top tube thru your lower spine (a good bump and that thin aluminum seat won't provide much resistance). It won't be pretty.
At the very least, it will fold in half, pinching you between the seat and steering tube. Just looking at it from here, the handlebar ought to slam right under your chin when the BB hits the ground. It may indeed support you for a while...and then it will turn on you at the worst possible moment.
You're an adult and I'm all for hot-rodding...but this thing has all the earmarks of being deadly.
John Tolhurst
Zen MBB Master
Jack, what is the frame made of?
Doug Burton
Zen MBB Master
Hi Jack,
I'll not be calling the ambulance just yet lol.
However, you have a good analysis tool for your Giant parked right next to it: the Silvio!
I was looking at your plywood seat brace. Compare it to the frame structure in the Silvio, where the seat pan and the large-diameter short tube connect the seat tube and down tube. In the Silvio, those parts keep the seat and down tubes spread apart by forming a rigid triangle, just like your Giant top tube.
Now, if you extend your wood seat brace all the way to the seat tube, you could conceivably use it to keep the tubes spread, but then the tubes would want to push the brace out the top, like squeezing a grape.
If the frame is steel, this will happen slow (bending), if it's aluminum, it'll happen suddenly (snapping). If its a steel frame, I'd just have a welder weld in a brace tube at the desired seat height, and take a carefull look at how you spread the loads at the downtube/brace meeting point.
Don't wanna dampen your innovation or pioneer spirit; it's just that I got paid to break things for 12 years and vibration is a subtle destructive force that will eat a frame, even if you get all the static loads correct.
Give her a good once over. Shoot me an e-mail if you wanna talk.
Best,
Doug
I'll not be calling the ambulance just yet lol.
However, you have a good analysis tool for your Giant parked right next to it: the Silvio!
I was looking at your plywood seat brace. Compare it to the frame structure in the Silvio, where the seat pan and the large-diameter short tube connect the seat tube and down tube. In the Silvio, those parts keep the seat and down tubes spread apart by forming a rigid triangle, just like your Giant top tube.
Now, if you extend your wood seat brace all the way to the seat tube, you could conceivably use it to keep the tubes spread, but then the tubes would want to push the brace out the top, like squeezing a grape.
If the frame is steel, this will happen slow (bending), if it's aluminum, it'll happen suddenly (snapping). If its a steel frame, I'd just have a welder weld in a brace tube at the desired seat height, and take a carefull look at how you spread the loads at the downtube/brace meeting point.
Don't wanna dampen your innovation or pioneer spirit; it's just that I got paid to break things for 12 years and vibration is a subtle destructive force that will eat a frame, even if you get all the static loads correct.
Give her a good once over. Shoot me an e-mail if you wanna talk.
Best,
Doug
trapdoor2
Zen MBB Master
That should take care of the potiential lower tube collapse, Jack, but it won't solve the other problem area...the headtube (which I neglected previously). You've cut away about 50% of its ability to handle braking loads by removing the top tube. Now, the only thing reacting braking loads (which, I suspect, are the largest) is the welds where the bottom tube attach to the headtube. Again, it may not fail the first time...but with aluminum, you'll be very lucky to get any warning when the whole front end parts company with the bike.
If this were steel, I would recommend welding in a replacement top tube that follows the contour of your proposed seat pattern and perhaps with a gusset or two in critical areas. The same can be done in aluminum but will likely require heat treatment afterwards...which is expensive. Plus, you have to know what alloy of aluminum you're working with (6061? 7005?) as some alloys cannot be welded to each other. There may still be some "woodworking" options but because of the limited space required by your seating arrangement, it will be tough to sort out.
Sorry, I should just let this go. I've been a fabricator/welder/aircraft mechanic/engineer/analyst for too long and have seen too many stupid lawsuits ruin people's lives...win or lose, the lawyers get fed.
I HATE being the guy that says "don't do it". :x I was raised by California hot-rodders who would try anything...and I've done much of it myself. That I survived...well, perhaps someone's dogma ate my karma.
Dave may think it funny but I certainly would have 911 on speed-dial before attempting to ride this any faster than, well...how fast do you want to take a header when that headtube joint fails under braking?
edit: PS I'm done. I promise not to post further on this subject.
Peder Torgersen
Guru
I think you made a very good technical report.
What happens if a modified bike falls apart and the rider is injured? Will the insurance company not pay or reduce their payment to a patient in the USA?
Peder
