Scratch built DT5 Frame

[opl505e]

We have been asked by a good friend to built a douglas DT5 frame.
He has previously had a frame and jig built but the guy made it out of thin wall tube!!! no high tensile!!
The frame subsequently bent out of shape on its first run.

We have been loaned a DT5 bare frame and have the jig that was made.

We are going to put the original doug. frame into the jig and check its accuracy.
Then we will be reusing all the lugs that have been made for the new frame and retubing that one in the jig.

My question is has anyone else made a new douglas frame and if so what tube/pipe technology did you use?
Douglas's were made from 10 gauge steel...I don't even know if it was high tensile or just BMS, if going over to high tensile what have people used eg wall thickness? reynolds? etc

The difficult part is going to be the swan neck and the front tubes that are bent.

We have some original douglas dt5 drawings but none of them show what radiuses are used for the swan neck front and back.

My suggestion is we make fibre glass moulds of the swan neck end to end and then send that as the profile for the pipe benders, does anyone have any dt5 frame drawings that include the radius dimensions?

Any help would be much appreciated

[Doug]

Matthew,

If you have the frame drawing, you will note Douglas used a variety of tubing gauges from 8 to 14, and some of the tubes like the front down tubes and top tank tube were taper wall; the end with the thicker wall being towards the headstock. Taper wall tubing is practically a lost art and unlikely to be replicated in this day and age, and you will have to settle for a constant wall thickness. It would behoove you to make the engine rails one gauge heavier than original, as these are prone to be bent on the originals when the machine is used for sprinting. The load on the primary chain tends to pull the engine up towards the transmission. There are few componet drawings for frame tubes. Some like the DT fork strut tube are just listed as 'MS' for Mild Steel. Others like the front down tubes on another model state 'MS [Med. Carb.', sort of a contradiction! Also is states 'A-Quality', what ever that means. No heat treatment spec, so it was used as received, or they had a process convention for all such tubing that they did not bother to document on individual drawings. Drafting standards and conventions were quite variable from firm to firm in those days!

As for the top tube, if you have a bare frame it should be a relatively simple matter to trace the outline on a sheet of cardboard, then using a large compass or trammel do some experimentation and figure out what the radius is. I have done this on other frames, it works quite well. It would seem far less bother than fiberglass molds. I have heard of one replica DT frame that was made by simply welding fitted tubes, no attempt being made to replicate the lugs. But I do not know who had it done, or nor have I seen it to determine how they dealt with the various details.

-Doug

[Neil Warner]

Hi Mathew,
               Grade "A" quality probably refers to low carbon steel tube ASTM  A595. Tapered tube is still manufactured but I don't know the cost. Try the internet you never know your luck

Regards

Neil

[opl505e]

Thanks chaps, My friend Brian will be doing most of the build up but he does not have internet so this is a valuable resource with all the info you all have.

Yes was aware of tapered tube but think the price will be a shock, I'll enquire

If anyone has the swan neck dimensions that would really help

[Doug]

Matthew,

I did a little research on tapered tube. Seems it is still manufactured in China, minimum order two metric tons...   

-Doug

[opl505e]

Cheers Doug, might as well go for 4 tonnes just to be safe lol

[chris mac]

Butted tubing , both single and double is often available from bicycle frame makers.  Dont know about UK but here in the US try Nova Cycle Supplies
Hope this helps

[opl505e]

Cheers guys, thinking about it the swan neck was not heat treated just bloody thick at the top, so even if we went to butted tubing it would be thinner than the original.

Therefore we are considering a correct O/D Hight tensile tubing with substantial thickness to provide the strength at the neck.

On a Douglas the neck on that tube is the weak point

[Doug]

Matthew,

There  is some suggestion the DT headstock was intentionally kept limber to allow a little 'give' when heeled over. Even so, unless you plan to evoke the traditional leg trailing broad slide on the tarmac, it is probably a trait you can do without. DTs in sprinting guise are also said to be prone to speed wobble if you shut off the throttle too abruptly at the end of the run. This is probably more a factor of the steering geometry than rigidity of the headstock.

The flexibility of the headstock being a competitive advantage may be an old wives tale. The real reason may have more to do with using up frame components from the RA model, with which the DT shared headstock lugs and steering stem, as well as other bits. RAs were already being converted for dirt track racing and were successful, so why tamper any more than necessary with a good thing.

-Doug

[opl505e]

I'll get some pictures of the parts, headstock and bits and bobs that were made for the first repro frame, these bits are all going to be reused on our retube.

Will have to take a close look as it looks like the rear wheel carrier has been machined (maybe CNC) and thus is not a proper lug, more a lugless attachment to the frame

Its difficult to hit the middle ground in deciding what the frame is trying to do. I believe it is not going to be an exact replica as tapered tubing, correct lugs etc would make the cost ridiculous. Its going to be an external dimensionally correct DT5 frame which will be able to accept all the DT5 parts and be raced the way it was meant to be.