My Bike Build

One nice part of small engine bikes is that the parts are so much less expensive!
Imagine what those new parts would cost for a 450 or a 650.
Any frame design drawings you can post for us?

Right, lets make a frame from scratch. I have probably bitten of more than I can chew here but I am a pretty stubborn so I will see if I can chew my way through this process.

I designed the frame to consist of ¾” tube with .035” wall with some ½” tube tying the main members together.

I bought a heap of 4130 tubing, sundry solid bars and plate for the frame and some mild steel RHS for the jig



I needed to make a jig to hold the three important interface points in the correct locations. Head tube, shock upper mount and swing arm pivot point.





The jig also has some assembly aids to help confirm frame members are in the correct locations (but less accurately positioned than the other three items)

Several items in both the frame and the jig require turning. I was going to pay someone to do this for me but then I love buying tools so I bought a lathe. I don’t have a shed to put it in so I set it up at a friends place.



Here is my workspace. I’m not going to let the lack of a garage stop this build. (Although rain normally puts a damper on things)



So I turned up the frame components that needed turning. This is the first part that I messed up.



The head tube starts life with ¼” wall and finishes up with a 1.5mm wall except where the bearing races seat. My boring bar will only reach half way through the head tube so I have to do one half then the other. After finishing the first half, I then managed to make the bearing race to big. Doh! Start again. I will fix this one at a later date.

Now you can hate me. I didn’t take any other pics between here and the next ones. Sorry.

So, just like magic, a frame comes off of the jig.













I am somewhat happy with it. Made some mistakes that I can live with but my welding leaves a lot to be desired. I found it really difficult welding in and around all the tight confined spaces.

I have taken up a quick welding class to see if I can do better on the swing arm.

To make up for the lack of pics with the frame, I will take a few more of the swing arm.

Next post, a brake caliper mount.

Hey Don,

With your adjustable head angle, what angle did you end up with? What is the range of adjustment? Did you try many different settings? Did the different angles make more or less difference than you thought it would?

The frame looks great!

I have been a 25 degree guy for some time. I haven't done many tests of angle, but I did accidently do one. I was in a race in Idaho with my 1975 350/400 Honda and I adjusted the head angle to somewhat less than I used for the California desert. During the race it slipped to full chopper. It was hard to turn.

Not being an expurt (pun intended), I am not as sensitive to the changes as I would be on a short paved track.

I would suggest 2-3 degrees on each side of what you think you expect to be right. How much you can put in depends on the design you use.

My design was a pivot at the bottom of the head crown and a slot at the top. I used a bolt adjustment to provide support to keep the angle from increasing (over jumps). An insert in the headcrown would be more rigid, but provide less adjustment.

Good luck

Don

How well did everything stay straight after the welding? Are you going to have the frame stress-relieved or go with it as is?

Super cool idea, I'll be following this one

Thanks Don. My frame, as you can see, is not designed to be adjustable within the frame and without adjusting any other variable. I can move my forks up and down in the tripple clamps and I ensured that the bike is designed with the forks in the middle of that adjustment so that gives about a degree either way

It all stayed pretty good but 90% of the welding was done on the jig. I had to weld in the cross bracing after I removed the jig and that ment I had to open up the swinarm mounts and hold them open while i welded the bracing in. It still clossed up a couple off mm though so I will have to make an allowance in the swing arm

whats the weight of the frame?
Great work with the reveal btw

Please continue, this is going to be awesome

The design weight without weld is just under 5kg. I weigh the actual frame at 5.2kg.

Does anyone know what a frame from an MX bike weighs?

Very clever, I'm in.

Do you gave a budget in mind?

Not really. Just what ver it takes to make it how I want it

Now, I have brakes and forks for my build but there are some problems.

The brakes made by Formula and are from a mini KTM. An SX65 if a recall correctly. They are designed for axial mounts.

The forks are Marzocchi Monster T’s designed for downhill mountain bikes. They use a bicycle standard radial post mount.

The front hub is also a bicycle component using a bicycle standard IS 6 bolt disk mount.

The formula disk is a 5 bolt design.

Obviously none of these components simply bolt together and due to the mix of bicycle and motorcycle parts, there are no off the shelf adaptors available.

I guess if I can make a bike, I should be able to make a caliper mount so that is what I did.

The first problem I solved was the disk. This problem was easy and has given an opportunity to add some cool bling. An English bicycle component manufacture called Hope Tech make an ultra bling vented disk rotor. It is 200mm in diameter, which is a little smaller than I would like but with such a light design, it should work.







Next I modeled up the forks, hub, rotor and caliper in the CAD program and then designed a mount to interface them all.



Using the dimensions from the CAD model, I marked out the flat shape, drilled all the required mounting holes and the proceeded to fuck up the first on when I tried to bend it up into shape. Damn!



OK, lets make a second one and this time, I bought some 20mm x 20mm solid square bar to use as a former during the bending process. The former idea was much better as it meant I didn’t have to mark out bends and try to line them up. Plus, now I can use the same formers to make others if needed.

Here is the finished part and again all assembled on the forks.







Now back to the swing arm

I solved my problem this way.

Double Discs



Fabbing my own Hubs



Don

After the frame, the swingarm becomes the next time to build.
The swingarm is built in largely the same way that the main frame was built and I promised a few more photos to document how I did it.

Like the main frame, a CAD model was made to get the size and shape correct. I then designed a jig that would locate and hold the three critical interface points (pivot, shock mount and axle).



Here is the jig assembled.



Then I got on my lathe and turned up the bearing housings, the tube connecting the two bearing housings and the shock mount items. Then I didn’t take any photos of them!

I bought some 5mm plate for the rear dropouts. I want these to be reasonably heavy duty as these items take all of the rear wheel input loads without any spring softening the input.

Ideally, I would send out a CAD file to some business that does NC profiling. NC profiling is expensive for small one off jobs so, like all the other plate components on my built, I will cut them out myself. I transfer all the key items (corners, holes etc) onto the plate, centre punch all the hole and internal corner centres. Drill out the internal corners and holes. Then use an angle grinder to cut out the shape.



You can see that only one dropout has the swingarm axle holes. That is because I want to drill both at the same time so that both plates match. Once the swingarm is welded, I will cut the material out to connect the holes making an adjustable slot.

The bearing housing and connecting tube are welded together with some temporary “bearings” in place and the swingarm axle to hold everything concentric. The bearing housing, shock pivot and dropouts are then fastened to the jig.





Next I make the tubes that connect all the critical item together. The first rule for a good weld is a good fit prior to welding so I am really trying to get better at this.







I continue fitting tubes working from most important to least.





That’s all for this post. Next time the swing arm should be completed.

Great job on that swingarm. If you need stuff nc cut shoot me a pm and we can talk, I have access to a waterjet machine and postage would be reasonable.

Do you have access to Melbourne, Australia? Having a waterjet would be awsome

Subscribed. Ultralight bike is on my list of wants.