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Triumph Unit Frame Differences pre/post 1964.

4319 Views 12 Replies 5 Participants Last post by  missinglink
Does anyone happen to know the differences between the early and late unit construction 350/500 frames? I think they changed in some way for the 1964 models.

I really wanted a David Bird hardtail (4" stretch; 2.5" drop) for my 1962 3TA/5TA frame as I wanted the bottom tubes without the flat area for the battery, but unfortunately their hardtails won't fit the early unit frames for some reason.

Factory Metalworks look as if they sell a bolt-on hardtail for my frame, which they list as fitting all 1958 - 1974 unit frame loops, which is good news, but I'm curious to know what prevents the David Bird hardtail fitting the earlier unit frames when the Factory Metalworks one does fit.


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No radical changes are highlighted in Bacon's Resto book. Just the usual minor detail changes through the years.

Okay, thanks :) It's odd about the hardtail situation then.
Don't take this as gospel but I didn't think the frame change in 65/66 made any difference in the tail end. I have a 65 and the top rail of the front section of frame looks like a long, thin dog bone and is bolted on, in 66 they went to a welded in heavier top rail with a thin lower top (forget what they call the piece) rail, like the 650's had. Not sure I'm saying that correctly, pictures would be a big help if I had any.

Just went to the Lowbrow web site, looked up the unit hardtail section for 650/500 and went through the order part question and it doesn't ask for a year it only asks if it's a 650 or a 500, stretch, drop and ?? but not year. I am under the impression that the same hardtail that bolts to a 65 or earlier will fit on a 66 up. Did you call them to ask.

Yes, I sent a message to Todd via their contact details. He replied saying they "will not fit on the early 500 front section". Obviously I don't doubt his word, but I'm curious to find out the differences between the early/late unit construction frames that are causing the problem with my one!

I seem to remember somewhere that 1964 was mentioned in some significant way with regard to hardtail fitting, but I can't find it now.

My frame sounds very similar to yours. Early in the production the petrol tanks were fitted as stressed frame members, bolting to the same lugs that your bolt-on top rail is attached to, but the tanks were prone to splitting, so Triumph changed the mounting to underneath the tanks via brackets/rubber mounts and fitted the bolt-on top brace between the old tank mounts, like with your frame.

I've been thinking about this and I wonder if they won't sell the hardtails for the early frames in case someone is still using the original stressed member petrol tank, which I presume would receive considerably more stress via the hardtail compared with the factory swinging arm set-up. If they used to split with a swinging arm rear suspension then a hardtail would surely make things worse! I suppose I could ask them directly, but this makes a good subject for the forum in case anyone else is researching it, so I've brought the subject here.

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65 is the year for the new tank , same frame and a bolt in toptube .
Weld in toptube for 66 .
I`ve got a 61 unit frame with a DB hardtail fitted , had a very small issue with the upper bolt on mounts hitting the rear tank mounts , took me 5 minutes to sort out with a file .
I don`t know if the stock tank would be under more stress with a hardtail than a swinging arm , but a toptube is a pretty simple thing to make.
Anything that makes that frame a bit stiffer is a good thing .

Apart from having to file a tiny amount to make the upper mounts fit , the hardtail is top notch .
Thanks for that. Very interesting to hear!

My reasoning with the stressed member-plus-hardtail versus factory swinging arm subframe is that the forces fed to the subframe attachment point (near the tank's rear mounting point) would be greater due to there being no change for energy to be dissipated (hardtail) compared with the swinging arm's suspension set-up.

If their hardtails do actually fit then I'd like to get one of theirs because I want the option of avoiding the flat section in the bottom tubes for a battery tray. I don't intend to use the stressed member tank as I want to make my own, and use a purpose-built bolt-on top tube of a different shape from standard.

Sounds like a plan ! You may be right about the top tube stresses , I`m no engineer .

Can`t help wondering if they don`t officially sell hardtails for those years , in case someone does something stupid and sues them . David Bird is a stand up guy , I`d be interested to hear what he has to say about it .

Good luck with the build , lets see some pictures when you get going .
That was the only thing I could think of. If yours fits then mine should too, so I think I'll go with that and see what happens. Worst case scenario is that I need to modify it or find a front frame loop that does fit.

Thanks again; I'll post some photographs once I get started!

Any updates on this? Im in the same situation, looking to get a bolt on david bird hardtail but I have a '62 5ta with the large tank and no toptube. Are you running the original tank missinglink? Or have you made a toptube to bolt in the tank mounts? Cheers
I haven't got mine yet, but I'm certainly going to as Missinglink has confirmed that they will physically fit.

My plans involve using a bolt-in top tube anyway, but if they didn't then I think I would alter my plans to incorporate one as I think that is probably the reason why they say the hardtails won't fit that year range.

If there was enough flex there to split the petrol tanks with a factory swinging arm set-up then the stresses there are bound to be greater with a rigid rear end.
Any updates on this? Im in the same situation, looking to get a bolt on david bird hardtail but I have a '62 5ta with the large tank and no toptube. Are you running the original tank missinglink? Or have you made a toptube to bolt in the tank mounts? Cheers
I made a bolt in top tube , but the way I did the seat mount means I can swap between the stock tank and my banana tank.

Some pics...

I think it`s wise to err on the side of caution regarding the stress on the tank , but remember , these bikes were real utility machines in the days when most families couldn`t afford a car . There`s a lot more frame stress on a bike that`s ridden two up with a weeks shopping on the tank rack than on a lightweight custom ridden solo .
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