FEATHERBED RAKE ANGLE

by **Rohan** » Sun Oct 30, 2011 3:56 am

Carbonfibre wrote:Certainly a frame made out of thin gauge steel tube would distort to the extent it would not be usable if built using the hearth brazing process. However as the ERW tubes used are approximately double the wall thickness of a welded frame, problems with distortion are easily avoided.

You just keep making these pronouncements that are just plain not true.

I've had an early Model 7 Norton twin dommie frame (lugged, with plunger rear suspension) that was made in thin wall tubing. If you flick the tubing with your finger, it went 'ting' - not the thunk of plain old pipe. How or why this may have been used for I can't imagine, but it had to be factory, no-one would go to all that trouble to totally retube it ?.

It was said that Manx plunger frames had hi-tensile tubing, anyone confirm ?
Renowned for cracking/breaking and needing constant repairs for the privateers, the featherbed couldn't come soon enough....

by **Carbonfibre** » Sun Oct 30, 2011 4:41 am

Not sure whether or not you have ever done any work on an old lugged British frame? I suspect not, as if you had then the heavy gauge tubes used would be pretty familiar to you!

Going back to basics, discounting the problems related to tubes distorting if thin high tensile tube was used in a lugged frame (as you suggest), there would certainly be problems with cracking as the HAZ affecting the tube adjacent to the rather substantial investment cast frame lugs, would mean problems with cracking were almost inevitable. This being another very good reason for heavy gauge ERW being used, rather than the chrom moly (?) tubes you claim.

If you would like to confirm any of the above contact any professional frame builder, and I am sure they will agree than other than on push bikes, very thin high tensile tubes are simply not used to build lug type hearth brazed frames!

by **Rohan** » Sun Oct 30, 2011 12:18 pm

Carbonfibre, I have plenty of Norton / lugged frames, some (most !) of them have needed work.

http://i1113.photobucket.com/albums/k50 ... CN0890.jpg
This is a plain 1950 Model 7 frame (500cc dommie twin), freshly blasted and phosphated prior to painting. Nortons painted some frames, in colours, in the early 1950s, so its being painted not enamelled. If you look VERY closely, you can see where the rear suspension towers have been (professionally) welded, in nickel bronze. Common fault in old Norton frames if they had a hard life...

The Model 7 frame in hi-tensile tubing seems to be pretty special stuff, but have yet to find out why. Obviously it can be done....

by **Cheesy** » Sun Oct 30, 2011 1:16 pm

Rohan wrote:
Cheesy wrote: the hearth brazed frames would not have distorted much or even at all as every part of the frame undergoes the same heating and cooling cycle at the same time <snip>

Ever seen a steel framed building thats been in a fire - or anything with a complex shape for that matter. Curves tend to straighten, and straight bits tend to curl, especially with welded bits added to them in irregular patterns.

Having said that, the old lugged type frames had been oven-brazed for the best part of 100 years, so they probably knew what they were doing , and knew how to get them to work.
As was said previously, be interesting to see a good rundown of how it was done though - folks who can successfully rework the old lugged style frames (and forks) are a little thin on the ground these days.

As a perhaps an example, there are these replica girder forks coming out of India, for things like military M20s and 16H Nortons and 3HW Triumphs. Someone said the M20 version they received was about 1/2" out of square (why does my front wheel want to go sideways ?), and for such a simple shape that is a lot. Without seeing it, the reasons for that could be numerous, but not staying in shape while it was brazed up is certainly one of them, and sagging while hot is another.
It is probably quite important to support or hang the lugged frame in some way when it goes in for brazing, but knowing that method (without experimenting ?) is anyones guess ?
Obviously you want them all to come out with the wheelbases all the same, and the space for the engine/gearbox fitment all identical.

Cheers.

Yes I have but the steel in the building has significant external loads on it, the bike frame only has to support its own weight, Im not saying it wont 'move' it will but it will not be very significant (again due to the uniform heating and cooling).

Carbon, can you please explain to me why a hearth brazed frame could not be made with thin wall tubing and also where the heat affected zones are and what causes them?

by **Carbonfibre** » Sun Oct 30, 2011 2:17 pm

The main reason for hearth brazed frames was to reduce production costs, as its something that could be carried out by semi skilled workers. The process necessarily involves uneven heating, as the lugs themselves have to be hot enough for the braze to flow into the joints properly, while other parts of the structure dont get anywhere near as hot.

Heavy investment cast lugs used by makers such as BSA and Triumph, dictated the use of thick wall ERW tubes, but the light looking lugs on the frame in Rohans pic would work very well with high tensile tube, as the levels of heat would be far lower and HT tubes would not be adversely affected. The lugged frame seems to have originated from cycle manufacturers, and the production process was over the years altered to reduce costs, which resulted in heavy but reliable frames, with handling and feel which often wasnt that great.

by **Rohan** » Sun Oct 30, 2011 3:14 pm

3 Things.

Norton frame lugs weren't investment cast - that is a much later development, from the recent bicycle industry ? They weren't steel either. Cast steel was a rare commodity, until quite recently. No other maker used investment cast (lost wax) frame lugs either, where cometh this information ?

The WHOLE FRAME was placed in the oven and heated, not just the individual joints and lugs (of which there were many) so the whole frame got pretty hot.

With all the lugs, and drilling and tube fitting and gas furnaces, this wasn't done to be "cheap", it was because no other method of frame making had yet been made reliable.
With a patternmaker, mould maker, furnace operator, and all the casting cleaning and drilling operations, calling this "unskilled" is just...
WW2 welding developments changed that, of course, (and simplified and cheapened it ?) but not in the Norton world until the welded featherbed came along...

P.S. At 440 lbs dry, the Model 7 wasn't exactly light.
Although not just the ultra heavy frame was responsible, that iron head weighs a ton...

by **Jeandr** » Sun Oct 30, 2011 6:17 pm

Rohan wrote:Reading the very first post may help... ?

Yep, that clears up things but does not account for the replies, you may as well pick a number out of thin air and go with it :wink:

Jean

by **Rohan** » Sun Oct 30, 2011 6:33 pm

True.
But 26 degrees and 24 degrees for a slimline are the numbers quoted in various sources, including in Roy Bacons books and the sketch on the NOC - that have around for some years.
If these are correct, or not in fact the case. then how these came into being is of interest ?
If the frames were not built to the blueprints, that is getting just......?
Kens book is going to be interesting - if it throws any light on this.
All just history of course, and there are now more featherbed copies than ever, built to who knows what spec , to add to the story.

by **Jeandr** » Sun Oct 30, 2011 7:16 pm

So why not go with 25 degrees :?:

right in the middle between 24 and 26 degrees :wink:

after all, who will know except you and your last will if you get it notarized :roll:

It will most likely handle just fine.

Jean

by **Rohan** » Mon Oct 31, 2011 3:21 pm

Says somewhere back a few posts he has decided to go with 26 degrees.
Be interesting to hear how it rides.
Might now have the slowest steering slimline in creation ?!

by **Matt Spencer** » Mon Oct 31, 2011 4:36 pm

Or the least unstable daragging a peg though the S'ss at Ton Downt Bipass .

FEATHERBED RAKE ANGLE

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