subframe setup help please

[themanoy]

i have an old XR250 which i turned into supermoto and ever since i got the sumo wheels on i have been hooliganing around town and popping wheelies everywhere! i blame ryan, marat and everyone in this forum for getting me into this!

its been a month already since i last rode my oldie 250 cuz i had a major wipeout in the nearby mountain twisties. i was hospitalized for a day and couldn't move without causing pain to the right side of my torso. but now im ok and its time to go back ride! unfortunately my bike is not in good shape cuz the one piece subframe broke in several places caused by the crash last month.

im planning to make a new subframe and i want it to be super sturdy and incorporate a scrape bar into it! i read somewhere that round bar would be perfect but some say that thicker hollow tubing is better. im confused now and i need help. what should i go for and is there any specific type of metal that i should use?

[Dirtyd851]

Heavy wall tubing is more structurally sound than solid bar, is more resilient to bend, as well as lighter...
On a per pound basis, steel pipe is stronger in bending but weaker in shear because bending distribution curve is triangular with minimum stress at center and shear stess distribution curve is parabolic with maximum shear stress at center

In addition, steel pipe is better (per pound) in torsion because most of the torsional load is carried near the perimeter of the cross section. This is why most drive shafts are hollow.

We know troque is product of tangential force and radial distance so in that way as radius increases torsional stress increases whereas at centre of shaft we have very less torsional stress so we can afford to keep shaft hollow

[themanoy]

whoa! thanks for the very scientific explanation! will go get me some thick walled tubes tomorrow and start working on the subframe!

[Ryan Moore]

Originally Posted by themanoy

whoa! thanks for the very scientific explanation! will go get me some thick walled tubes tomorrow and start working on the subframe!

I use .125 in. wall tubing on my subframes and it holds up good. I usually make them a little short though, about 3 inches shorter than the rear fender so its not sticking out super far, the longer the bar is, the easier it will bend on impact. Stunt pegs are a must also if you really want to have some fun. I run mine about 24 inches up and back fromt the regular pegs.

here is are some pics of my lastest setup, running plastic, stock exhaust, and stunt pegs, it works awsome on the honda. It took several prototypes to get all the geometry just right so the bike spins like a top when I'm busting figure eights on it. It looks simple, but it took along time to get the bar length, hieght, stunt peg position all dialed in.

http://www.50stunt.com/product_p/crf450%20subframe.htm

[themanoy]

nice new bike ryan! are you using square bar for your subframe?

i bought 1/2 diameter x 6mm galvanized steel pipes. will these do well for my subframe?

[Ryan Moore]

Originally Posted by themanoy

nice new bike ryan! are you using square bar for your subframe?

i bought 1/2 diameter x 6mm galvanized steel pipes. will these do well for my subframe?

Yes, I used .75 x .125 square tubing.

[themanoy]

thanks ryan! i will post pics of my progress soon and let you guys critique my work on the subframe.

[Splash4195]

What year honda and is it a 450 Ryan? Maybe a X model?

[fast507]

DONT WELD GALVANIZED!!!!! HIGHLY TOXIC!!!!! You should get yourself a piece of 4130 from your local steel shop and use it instead!!!

[themanoy]

oh boy. too late buddy the subframe is already finished but i still have to do some finishing touches. will post pics as soon as possible.

[themanoy]

finally got some pics of my bike. its not yet done though and im having a hard time putting back the stock airbox. forgot to measure the tubes without including the airbox.

is there anything else that i should do to make my subframe better?

[Ryan Moore]

that should work for a foot bar, but if you scrape it, its gonna bend. that bar is very long, and the tubes are pretty small. it looks about 4-5 inches to long, and the longer it is, the more it will bend at the point were the tubes come together. that round tube at the end will really dig into the street also.

[themanoy]

thanks for the comment ryan. i was wondering bout the tubes so much more when i looked at pictures of your old CRF. im going to reinforce the subframe this sunday and find a way to get the stock airbox in.

[Ryan Moore]

Originally Posted by themanoy

thanks for the comment ryan. i was wondering bout the tubes so much more when i looked at pictures of your old CRF. im going to reinforce the subframe this sunday and find a way to get the stock airbox in.

The round tubing subframe I made for my crf 450 is 1 inch, .125 wall chromoly tubing, super strong, and it is 5 inches shorter than the stock rear fender so the bike is past verticle when it touches the ground. It holds up like a champ, never bent, but I couldn't get the airbox and plastics on the bike.