Fork Repair Help

Gbrebes · Mar 26, 2012

Hey Guys,

In the same vein of my "Frame Repair Help" thread, I need some ideas from you all on how to make a stronger springer fork. After finally getting my bike back together after reinforcing the bottom tube, I was excitedly zooming around my neighborhood late at night. In my over-excitedness I charged into a shallow/sharp dip just before a hill climb that I usually slow down for. This time I wanted to keep up my speed for the hill, but when I hit the dip my front tire bottomed out and I crashed. I did not get injured and I could not initially find the reason for the incident. After taking a long look at the fork, I realized that the fork arms actually bent. I have pictures.

I am reluctant to buy another fork that is similar due to this incident. I really want a home built leaf spring fork, so I'd rather not use the monark repops.

If anyone has any ideas of how to strengthen this design, let me know.

Gilbert

Mr.B. · Mar 26, 2012

Gbrebes said:
Hey Guys,

In the same vein of my "Frame Repair Help" thread, I need some ideas from you all on how to make a stronger springer fork. After finally getting my bike back together after reinforcing the bottom tube, I was excitedly zooming around my neighborhood late at night. In my over-excitedness I charged into a shallow/sharp dip just before a hill climb that I usually slow down for. This time I wanted to keep up my speed for the hill, but when I hit the dip my front tire bottomed out and I crashed. I did not get injured and I could not initially find the reason for the incident. After taking a long look at the fork, I realized that the fork arms actually bent. I have pictures.

I am reluctant to buy another fork that is similar due to this incident. I really want a home built leaf spring fork, so I'd rather not use the monark repops.

If anyone has any ideas of how to strengthen this design, let me know.

Gilbert

That stinks!

Maybe just look to the originals and use a trussed fork similar to what you have now.

But use larger/heavier or solid rods, keep them straight above the “king post” and use a heavier top plate?

Perhaps you can even straighten & modify the forks you have now if they’re not too badly damaged?

-Kirk

RicksRides · Mar 26, 2012

Glad to here youre un injured If you weld simply copy what ya have and use heavier chrome moly tubing, if not there are a number of good craftsman on here that would build you a pair for a price of coarse

Gbrebes · Mar 27, 2012

Mr.B. said:
That stinks!

Maybe just look to the originals and use a trussed fork similar to what you have now.

But use larger/heavier or solid rods, keep them straight above the “king post” and use a heavier top plate?

Perhaps you can even straighten & modify the forks you have now if they’re not too badly damaged?

-Kirk

Thanks Kirk,

You were the guy who came up with the main design for my frame repair, so I really appreciate it. I should have just pm'd you.

I would like to use a similar design. My truss rods were already solid, but thicker and straight after the "king posts" ( is that the two arms that hold the trusses at the top crown of the fork?) would be a great idea. I also think that I should braze the trusses to the fork dropouts as well as the "king posts". I guess brazing the trusses to the junction just before the top plate would help.

Also RicksRides, I have given thought to a complete scratch build of the fork. I weld, but for the moment, I only do brazing. I do not have power in my shed for a welder. I do not know if chromoly can be brazed, but making it from scratch makes sense. I can build it to suit, instead of modifiying parts that were not really meant for a heavy-ass bike with a motor reaching speeds past 30mph.

Thanks for the input,

Gilbert

rustycase · Mar 27, 2012

Gosh I hope your wife doesn't find out about this!

Glad to hear ur ok...

I'd give you some bicycle forks to truss up, for the cost of shipping, but I'd imagine you got plenty available down there in LA... Lots of everything down there!

Best
rc

rustycase · Mar 27, 2012

Gosh I hope your wife doesn't find out about this!

Glad to hear ur ok...

I'd give you some bicycle forks to truss up, for the cost of shipping, but I'd imagine you got plenty available down there in LA... Lots of everything down there!

Best
rc

cobrafreak · Mar 27, 2012

glad you were not seriously injured. All I can say is stack the leaves on top of each other for more spring rate.

Gbrebes · Mar 27, 2012

rustycase said:
Gosh I hope your wife doesn't find out about this!

Glad to hear ur ok...

I'd give you some bicycle forks to truss up, for the cost of shipping, but I'd imagine you got plenty available down there in LA... Lots of everything down there!

Best
rc

Yah, i just got a small abrasion near my knee. I sort of went down slowly. My front tire basically started rubbing the fender, I noticed the front fork wasn't springing back and I just sort of went over the right side of the handlebars and laid it down. So luckily, no crash evidence and now I just need to figure out a fix.

I tightened the you-know-what out of my rear belt before the ride, and the bike was pulling pretty hard up hills (its all hills behind my house) with a lot of torque/acceleration power in the flats, so the bike was running better than ever before the crash. I can't help but thinking that if I had taken that dip at the speed I normally do, I would be riding it again tonight instead of writing about how to fix it. But at least there was no injury, and I will go on to make the bike stronger in the end.

Gilbert

harry76 · Mar 27, 2012

Sorry to hear about your misfortune, i love this bike. And i cant help with any advice other then whats been said, but can only suggest doing practice runs during the day. I know your wife doesnt want you riding it so you're riding it at night, so if shes knows about this crash use this as your chance to suggest day time tests for safetys sake.

Hope you get it back on the road very soon.

Mr.B. · Mar 27, 2012

Yes I did mean the two arms that hold the trusses at the top crown of the fork when I wrote the term king post.

With the “Z” bend you have now it may allow for a little flexing.

But since your current fork bend so easily perhaps starting from scratch would be a good option!

I am pretty sure you can braze chromoly but someone here should know that for sure?

Other wise maybe starting with a older rigid mountain bike fork could save fab time, some of those were pretty heavy duty...

-Kirk

curtisfox · Mar 28, 2012

I have been looking and like Mr.B says the top mount looks like it failled. The forks can be strieghtend in a press,if the top tube is bent you can put a 7/8 rod or shaft in and do that in a press a little at a time. Take a look at some of the oridgonals and judge form there,like Monark.I know the top is at least 3/16" to a 1/4" inch thick.

That must have been a big hole or dip.........Glad you are in one piece.......Curt

curtisfox · Mar 28, 2012

I am in the proses of building a leaf spring fork and I used older snowmobile springs that have three leaf, I am only useing two. I know it would be hard to find them out there and it getting hard here but I can find them $20 a pair. I run a shaft up inside the fork for strength. If you pm me you email I can send you some pictures.Got to learn to post them......Curt

Gbrebes · Apr 19, 2012

Hey Everyone,

Still working on building a new set of forks. I purchased this set of forks on ebay recently.

They look pretty strong and I started fabricating a truss king pin as well as a leaf spring clamp. I then took the forks to my local bike shop to get the steerer tube shortened an inch and add some threads.

Well the shop guy said he could not thread it because the tube is made of bronze and his die won't cut it.

Has anyone had any experience with bronze steerer tubes? And have you heard of anyone cutting threads in one?

I guess I will hunt around for a new pair. Kind of a drag though as i already fabricated a burly king pin out of 1/4" steel plate.

Gilbert

curtisfox · Apr 19, 2012

Put a magnet on it if it sticks it is not bronze. Bronze or brass is soft I think and should tap easy. Try another shop I am sure head tubes are not bronze,every one I have seen are steel. If the magnet sticks take it back and show him maybe...........Curt

rustycase · Apr 19, 2012

I think I'd find someone else to cut that thread.
AFAIK, bronze can be threaded. I have installed threaded bronze parts!
and they were cut threads, not cast.

Best
rc

Gbrebes · Apr 30, 2012

Thanks for the encouragement curtisfox and rustycase,

I did end up taking the forks to another bike shop and they cut the threads and shortened the head tube with no issues or considerations of how it could not be done.

Here are some pics of my new beefier fork that I built from the ebay forks.

I give a big thanks to forum member Russel, whom I stole this idea from, of a clamped home made king pin set up.

I fabricated and brazed a truss with a horizontal brace, home made king pin and brazed in 1/2" bolt for the head tube brace. I made the head tube brace at the top out of 3/8" steel plate.

It seems pretty strong, I rode the bike last night and it didn't break yet.

I will take a picture of it all together and post later.

Gilbert

culvercityclassic · Apr 30, 2012

Those look real nice, show a pic of em on the bike.

curtisfox · Apr 30, 2012

Thats a hole new meaning to weres the beef. That should do the trick. Love it.........Curt

rustycase · Apr 30, 2012

lol

"and it didn't break yet."

They are really not supposed to break!

lol

Very nice job on the truss, Gilbert.
It will help distribute the loading where the legs attach to the lower crown.
MUCH stronger!

And you have done a fine looking job, too!
rc

DareDevil · May 1, 2012

Hi Gilbert... Good to see that your brazing...with all the modern technology in welding .. the old brazing is very strong and was the preferred way to assemble bike frames right up into the 60's ...I will be brazing my project ..even though I have a mig and tig...

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