My Original Post

I was wondering if there is anyone else out there in Thumper/EX500/Race digest, etc. land that has either built frames or is interested in the subject.

I've done road race frames for Honda 125 singles, 175 twins, Husky aircooled 510 and Laverda 750.

In the queue I've got RR/hot street frames for GS450/500 Suzuki, V50 & 1000 Guzzi, Rotax thumper, T150 Triumph, as well as dirt frames for Honda 125 thumpette single (MX), CB160 (Trials), B44 and B50 BSA (Trials and MX) and Rotax thumper (playbike). I've also been accumulating parts for and doing sketches on a hub-center steering front end. Please note that I haven't made any claims about these all being done anywhere close to the near future - planning is much more rapid and less injurious to vital body parts than cutting and shaping metal.

All my frames have been made of tubular steel, and I don't see a big need to do any aluminum spar concoctions.

I'd like to discuss chassis theory and practice, as well as seeing if anyone has a useful construction tip for making things go together faster. Yes, I know that if I'd just spend more time working in the garage instead of thinking about working in the garage my productivity would be greatly increased, but after procrastinating this many decades I don't see that situation changing anytime soon.

I'll look forward to your cogent responses, and I'll try to make coherent replys.


Michael Moore

My Reply to the Messages I Received

I'm going to consolidate my response to messages from Peter Hamilton, Randall Zempel, Bob Green and Stan Malyshev. Thanks for the interest and replys to my first post.

A disclaimer: I'm not an engineer, I just read a lot, talk to experienced people, and measure what others do. The following has worked for me - no warranty that you won't screw up and hurt yourself or someone else. Take a welding and machining course, or get a skilled person to give you lessons.

Frame materials: I use 1018 Cold Rolled Electric Welded (CREW) mild steel (MS) tubing. My understanding is that all of the steels have the same modulus of elasticity (Youngs Modulus) and are therefore equally stiff. This is why I chuckled at a Motorcyclist Magazine article where they raved how a chrome moly (CM) replica of a stock SRX600 swingarm was so much stiffer. CM steel is stronger, and will bend a bit more than mild steel before taking a permanent set. But you shouldn't be designing a frame with the tubes in bending, and as Kevin Cameron noted, not everyone is skilled enough to crash just hard enough to bend the MS frame, but not quite hard enough to bend the CM frame. CM is useful on dirt bikes as they are more prone to being dropped in rocks etc, and the CM won't dent as easily as MS. Carroll Smith avers that in many cases. unless you are going to be heat treating the part, CM isn't worth the additional cost. CM was originally developed to be gas welded together, and in the light gauges you will use probably wouldn't need normalizing if you do a reasonable pre/post heat with your torch.

Any airframe manual will give details on how to cut out damaged tube structure and splice in new material as needed. I don't think you would like renovating a smashed aluminum spar.

Allan Staniforth in "Race and Rally Car Source Book" says the first thing you should do when you start to build a vehicle is "write out on a piece of cardboard in large letters 'I will never, never, NEVER put bending loads into the middle of a tube or panel' and hang it up where you will read it every day."

If you put all your tubing in compression/tension then you can use a very thin gauge. My friend Craig Hanson, who got me started building frames, says that when he has actually calculated the gauge of steel to use, it is usually so thin it is scary. I mostly use 1" OD by .035 or .049" wall tubing. The thin wall stuff will dent easier, so don't throw hammers at the bike. Same thing with swing arm pivot and frame and shock mounts - use thin sheet with a box structure and weld on a washer or bush to give it enough section to resist ovaling from the bolts/pivot pin. I made the sheet steel triple clamps on my Laverda from .035" sheet with .065" wall tubing where the forks clamp up.

Joining the metal: I braze weld my frames together, as done by Seeley, Rickman, Cheney, Harris, etc. The high-strength bronze alloys I use (with an inline liquid flux bottle) claim a higher tensile strength than the tubing. I haven't bought any Sifbronze, an English brazing rod, because it is pretty expensive, but it does make beautiful beads. I've used Eutectic EutecRod #16, and have recently switched to MG 130 as it has similar specs and is much cheaper. Braze welding is much easier on the tubing since you don't get over a dull cherry red temperature. The drawback is the bronze contracts more as it cools, so you have to work out your welding sequence. When I can control my impatience to weld the whole thing up NOW, I try to do a bunch of little tacks, moving symmetrically around the frame, and then go back and do .5 to 1.0 " sections of bead in a similar pattern. Let the frame cool a bit between beads and things will hold their shape really well. Another nice thing about the brazing is you can do a reliable visual inspection of the joint. If too cold, the bronze won't "tin" the metal ahead of the fillet, and if too hot it flows out like water. I've found it easy to make zero penetration fusion welds that look OK but fail immediately.

I will use TIG when doing things like the triple clamps where there is a lot of easily distorted sheet. They can be brazed, it just takes more time and care to avoid warpage. TIG is also handy when sticking thin parts to thick parts. The equipment to do braze welding is MUCH cheaper than TIG. Avoid MIG welding - it should be reserved for your trailer, etc, and other things with thick wall steel unless you are a wiz at it. Plus it requires more cleanup of weld spatter, and the shape of the bead isn't as good as Tig or O/A gas welding..

If you are careful I don't think you need to worry about changing heat treatment etc when modifying an existing frame. Most of them aren't normalized or heat treated anyway. Find an old junk frame and take a hacksaw to it. When you get all the way through the first major frame tube you'll probably find it springs together/apart an easy 1/4" or more from the locked in stresses.

Buy, modify or build: If you have limited fabrication skills/equipment, by all means pick up a used TZ/NS chassis - the parts all fit together right from the start. If you can trim back an existing frame and then readily modify it, go for it. BUT, I found that on my Alazurra that I could have built a lighter, better frame in the time it took to triangulate, reangle the steering head, move the motor forward 1 1/2 inches, and lengthen the swingarm. Then again, I knew the 650 was going to go back on the street with the stock tank, seat etc. You will spend more time making all the little brackets and spacers than you will putting the main frame together.

To be continued: frame fixtures, frame design, useful books, etc

The other guys' posts follow:

I'm an engineering student and I'm working out (still mentally) a plan for my senior design project, which will, I think, involve a simple, single cylinder bike with an experimental engine (thinking about all sorts of neat stuff for it, from alternative fuels to continuous nitrous injection or something neat like that). Also, it'll have a non-hub-center front end that I've thought out and designed on ANSYS finite element analysis.

I've been debating whether to make an entirely new frame, which would fully take advantage of my front end, or modify the hell out of an existing one, which might be considerably easier. Pretty big time constraints here, as I've got other classes as well! I hear of people welding stuff onto stock frames.

Does anyone worry about the welds ruining any heat-treating/cold working the frame material has undergone?

>All my frames have been made of tubular steel, and I don't see a big need to do any aluminum spar concoctions.

Wholeheartedly agree with you here. For one-offs and experimentals, it's silly to use aluminum, which is only really used to its advantage in spar frames (flames here???). Tube frames are the way to go, and steel is the material for tube frames. Do you use chrome moly? What alloy? Do you have to shot-peen it or heat treat it post-weld? (pretty sure you have to heat-treat the moly).


Michael, I am planning to build a single in a TZ250 frame. But I don't yet have the frame, and welcome alternatives. What advantages do you see to a homebrew frame over a factory chassis?

randall zempel


Making / modifying frames seems to be a major pastime of singly folk.

I've just finished the Husaberg as you probalbly figured out when I went bonkers over the proposed Supermono weight limit. The thing is, I reckon another 15lbs+ could be lost by making a tiny frame for the tiny motor, rather than modifying what is basically a copy of a TZ chassis designed to take a big Rotax or Honda single. This would make it weigh in at less than 220lbs - wet.

I think you're right about steel. Easier to fix/modify. I'm a bit worried about how the Alloy of the Harris will stand up to the vibration of the Husaberg. I'll have to keep an extra look out for cracks. Tigcraft and MHD are starting to make steel frames for a number of people this year so it will be interesting. I doubt there will be any difference in performance, but I'd bet the steel frames will be lighter 'cos the tendancy to over engineer them won't be so great.

Re the FZR400 & SRX. Can you guys get any of the 250 two strokes over there like the RGV or KR1 or even the TZR ?



Your post brought back some buried desires of mine to learn welding, machining, and design (and hopefully to produce a frame for a Formula Singles (AFM) Rotax or XR600 racer).

I was planning to buy a welding apparatus (I wonder how expensive TIG units are), get some books on it and on design, look for a finite element analysis CAD package or somesuch, and have at it. However, it looks like all disposable income (and then some) is going toward racing costs, so I've been delaying on that whole deal.


Stan Malyshev
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