Frame Reinforcement on '70 to '73 bikes ?

[Grewth]

I've noticed people welding extra gussets and "doubler" tubes welded over existing tubes on these early frames.
Is this being done because the frames are prone to fractures due to flexing ?
Or is the motivation to try to improve handling by reducing frame flex ?
I've only ever had one crack appear in one of the later '74 onwards type frames - near the bottom engine mounting - but these frames are much heavier than the original offering.
I'm thinking that maybe I should add some bracing to my second XS1B frame, as I've got some repair work to do on it anyway (lock stops cut off for some unknown reason), but I don't want to go OTT and do more harm than good.

 

[GLJ]

On my daily rider XS2 I had to fix the broken frame. It was broke clean through on each side of the down tube. Also had to fix some cracks in the frame by the swingarm mounts. Also had a crack in the engine cases in the top rear mount. The engine case crack is somewhat common same with the cracks by the swing arm mounts. Where mine was broke in 2 places in the cross tube is not common. I bought the bike in 1976 and do not know if the frame was broke when I got it or when I was riding it. The engine case was cracked when I bought it. It did not have a easy life.
https://www.xs650.com/threads/its-past-time-to-start-xs2-resurrection.53396/

The early frames are not known for their rigidity. When I decided to build another one I reinforced the frame not because it was broke I just wanted to see if I could make the frame stiffer. More pics here.
https://www.xs650.com/threads/just-another-xs2.65376/
Something else I did to both frames was add aluminum round stock between the front front engine mounts. If you look at the front mount from 74 on it's solid across there. It's just round stock sized for length and drilled and threaded. Figured it can't hurt and looks kinda cool.

 

[Grewth]

GLJ thanks for that.
That triangular box that appeared on the '74 front engine mounting was one of the modifications that Percy Tait developed I believe.

 

[Its been a long time]

GLJ thanks for that.
That triangular box that appeared on the '74 front engine mounting was one of the modifications that Percy Tait developed I believe.

I believe you're right. Percy was hired by Yamaha Europe to strengthen the bike. The XS2 had a dreadful reputation in the UK back then. But that's not surprising really given what it was selling against. The Triumphs and Nortons of the time handled really well even if they leaked oil. I recall the XS650B was the first bike with Percy's improvements built in. I had an XS650C new in 1976. I never met another XS650 owner in the time I owned that bike. I went back to two strokes after owning it. I think the XS650 is far more popular in the UK today than ever it was in the 1970's.

 

[650Skull]

To better understand how Yamaha reinforced the case at therear mount. Looking on the inside of the engine case. Can strengthen it by laying a bead of weld down on top of the weak looking brace

 

[DogBunny]

https://www.xs650.com/threads/win-some-lose-some-1971-xs1b.66004/
I have a couple pics of a cracked seat loop on a 1973 frame in the above thread, first post.
On a related note -- 2M strongly advises adding a bumper to the seat pan to prevent it from cracking. Go here:
https://www.xs650.com/threads/got-a-free-xs2-today.51713/#post-544554
and follow the links in post #15.