PDA

View Full Version : Tubing for Bike Forks



Duffy
06-07-2012, 12:08 AM
My grandson has an off-road bike, (WIIIDE tires, little engine.) On the weekend he had a significant Oops and bent the forks. (Fortunately the bike missed him after he somersaulted over the handle bars-hurt his back, leg and pride!)
He has signed up for a welding course this fall and intends to build a new set of forks. I offered to find out what the best alloy tube was to use for these things. Of course, he had no particulars other than a guesstimated 1" to 1 1/4" diameter.
So I am asking any knowledgeable souls out there "what type of tubing does he get?"

winchman
06-07-2012, 12:28 AM
Some 4130 seamless tubing would work.

Arthur.Marks
06-07-2012, 01:34 AM
http://www.cycle-frames.com/bicycle-frame-tubing/UNICROWN-BLADES/

Black_Moons
06-07-2012, 05:27 AM
I would say that depends entirely on how he plans to attach the tubing, How many pounds your grandson is (Grandsons tend to come in the 10lb baby size up to the 350lb football player size), and how likey he is to try and bend the forks again :)

Also how much cash he has..

Forrest Addy
06-07-2012, 07:43 AM
Look at airfraft tubefrom Wikes or Spenser. Careful woth the welding. It is 4130 and if weled will air-harden and crack if not post-heated properly.

gbritnell
06-07-2012, 08:45 AM
Depending on the type of forks that are on the bike, expensive or not, they could be chrome plated if they have oil in them. If they are just one tube inside another with a spring then some DOM tubing should work.
gbritnell

MotorradMike
06-07-2012, 08:48 AM
Are they shock absorber type forks or more like bicycle forks?
The former can sometimes be straightened.

justanengineer
06-07-2012, 08:51 AM
If he rides like I used to, he doesnt need tubing, he needs solid round. :D

You could then tell him its "billet" so he could feel "cool" about his "custom" bike.

jep24601
06-07-2012, 09:53 AM
If the fork tubes are only a little bent they can be straightened quite easily. If they are creased at the bottom yoke then they will need to be replaced.
Dedicated fork tubes are probably the only real answer as replacements because of internal diameter and threadings and polishing and plating.

easymike29
06-07-2012, 12:22 PM
I'm surprised that no one has brought up the topic of liability in giving such advice.

Gene

Arthur.Marks
06-07-2012, 12:58 PM
If it is a suspension fork, quite simply you're not going to be building a new one. There are simple spring suspension forks on the Wallyworld bike level, but most any bike will have either air or hydraulic suspension today. These are not the simplest of construction. There are many that are surprisingly complex---especially for "downhill" use with long travels. The simplest to build yourself is arguably the Unicrown construction. ANY fork you will be building yourself will be from CroMoly (i.e. 4130) tubing. Aluminum has the wrong characteristics for a one-piece fork (i.e. immediate failure vs. bending).

I would stick solidly to bicycle specific tubing. Thickest wall you can get. Not heat-treated. That requires more advanced joining considerations. A fork is the most catastrophic of failures on a bicycle. If you don't break your head and loose your teeth, you're next likely to fracture your clavicle. Either is extremely painful and obviously needs to be protected strongly from structural failure being the root cause.

Joining for a bicycle comes in two varieties for a unicrown construction: brass fillet brazing or TIG welding. MIG is generally not suitable here. Accurate mitering is paramount, but that doesn't mean you can't achieve it with a simple half-round file and paper printed template. Many have done it with such simple tools to excellent result.

The hidden difficulty will be alignment. You need to work out an appropriate trail (i.e. caster angle), length to axle, get both dropouts in the same plane, and match the width to the axle spacing of your wheel. Even more care is needed to achieve this if your son has a clamping through-axle type hub. Then it becomes essentially a line-boring operation.

I can say from experience that some answers to my questions on this forum cause me pause. I hope my response only does that---and not for you to abandon it entirely. If in doubt think... Thicker is better as to material selection, use the minimum heat necessary to get it joined, and go slow and methodical with set up. It will be a good project. If you feel you need more help along the way, you can receive expert advise from:

http://groups.google.com/group/framebuilders
http://forums.mtbr.com/frame-building/
http://www.velocipedesalon.com/forum/f10/

Willy
06-07-2012, 01:08 PM
X 2 to what Artur Marks just stated.
Some very good advice.

We do need more details from the OP about the bike and forks in order to offer proper advice though.

Not saying it can't be done, but if it were me I don't think I would choose to make a set of forks as my first welding project...motorcycle or bicycle!

darryl
06-07-2012, 05:08 PM
Agreeing with Willy- making your own forks is not the place to be experimenting or learning to weld. I would say the same for any chassis frame building or load-bearing structure where a failure would be 'inopportune' at best, and dangerous to lethal as a real possibility.

Duffy
06-07-2012, 05:19 PM
Thanks VERY much for all your advice guys. I have now seen the culprit, (or victim, depending upon viewpoint.)
These forks are just about sledgehammer simple. They consist of a pair of arms joined at an angle slightly less than 180 degrees to the steering post, about 12" apart. The arms are a bit over 1.5" OD. There is a short stub of 1.0" ID tube at each fork end for the front axle assembly. There are also a couple of "bolting ears" welded to each fork either side of the steering post.
There is absolutely NO front "suspension;" the rider's comfort is provided by the big fat low-pressure tire.
I suspect that these bikes were never very popular. They look like the first try at an ATV, which was quickly followed by a tricycle, (another failure,) and finally by a quad unit. All they seem to have going for them is cheapness, with a little Tecumseh engine, centrifugal vari-speed clutch and a couple of FAT tires. The sad thing is, they go fast enough to maim or kill, yet they hardly provide the thrill of a REAL bike.
Personally, I hope that he cant fix it, but THAT is just an over-protective Grandad. Hell, the man is 21!:rolleyes:

Peter.
06-07-2012, 06:56 PM
Work on a motorbike - guaranteed to bring out the 'oh no' brigade. Sir John built a complete motorbike when he was younger, imagine the disasters that could have befallen him if just one of those welds wasn't quite right.

It's a wonder he lived long enough to sling a dodgy load.

gnm109
06-07-2012, 10:47 PM
If the fork tubes are only a little bent they can be straightened quite easily. If they are creased at the bottom yoke then they will need to be replaced.
Dedicated fork tubes are probably the only real answer as replacements because of internal diameter and threadings and polishing and plating.

Yes, they could certainly be straightened, unless the angle is quite pronounced. I've straightened a few sets of fork tubes in the past. Some alloy V-blocks and some press time would do and they could be checked in a lathe or simply rolled on a level floor to see how they look.


.