"Gusset" under downtube

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reohn2
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Re: "Gusset" under downtube

Postby reohn2 » 6 Oct 2019, 10:26pm

slowster wrote:
jimlews wrote: the gusset stays and I go down the Waxoyl and hairdryer route

When you do it, be aware that there is/may be a gap in the join between the down tube and head tube, such that if you intend to fill the void up with Waxoyl, that will not happen. Instead of seeing the Waxoyl eventually start to overflow from the gusset opening, what you will instead find is that there is a lot of Waxoyl that has flowed into the down tube and thence into the head tube.

DAMHIKT.

I'd very much doubt that to be the case,as IMO the weld of the full circumferance of the downtube to headtube would be completed,before the gusset is welded in place.
Last edited by reohn2 on 6 Oct 2019, 10:32pm, edited 1 time in total.
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jimlews
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Re: "Gusset" under downtube

Postby jimlews » 6 Oct 2019, 10:28pm

horizon wrote:
jimlews wrote:OK, probably time to put this one to bed.



Though like all naughty children it is quite likely to want to stay up a lot longer. :mrgreen:

Even if you hadn't wanted to cut it off, it was still a good question. I have a Spa tourer frame which I am currently building up. I like the idea of the strengthener but now that it has been queried I would like to know more. I cannot imagine that many bike frames get such a forensic examination as the Spa frame has had on this forum (on multiple threads) but until we hear from Spa or the frame's originator we are really none the wiser. In fact, I am surprised it hadn't been raised long ago.



Thanks, "horizon" I was beginning to think that I was the naughty kid. Usually IS the case.

slowster
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Re: "Gusset" under downtube

Postby slowster » 6 Oct 2019, 10:33pm

reohn2 wrote:
slowster wrote:
jimlews wrote: the gusset stays and I go down the Waxoyl and hairdryer route

When you do it, be aware that there is/may be a gap in the join between the down tube and head tube, such that if you intend to fill the void up with Waxoyl, that will not happen. Instead of seeing the Waxoyl eventually start to overflow from the gusset opening, what you will instead find is that there is a lot of Waxoyl that has flowed into the down tube and thence into the head tube.

DAMHIKT.

I'd very much doubt that to be the case.

I draw my esteemed fellow forum member's attention to the second paragraph of my post.

reohn2
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Re: "Gusset" under downtube

Postby reohn2 » 6 Oct 2019, 10:35pm

slowster wrote:
reohn2 wrote:
slowster wrote:When you do it, be aware that there is/may be a gap in the join between the down tube and head tube, such that if you intend to fill the void up with Waxoyl, that will not happen. Instead of seeing the Waxoyl eventually start to overflow from the gusset opening, what you will instead find is that there is a lot of Waxoyl that has flowed into the down tube and thence into the head tube.

DAMHIKT.

I'd very much doubt that to be the case.

I draw my esteemed fellow forum member's attention to the second paragraph of my post.


Not a clue what that means :?
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slowster
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Re: "Gusset" under downtube

Postby slowster » 6 Oct 2019, 10:39pm

reohn2 wrote:Not a clue what that means :?

'Don't ask me how I know this.'

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531colin
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Re: "Gusset" under downtube

Postby 531colin » 6 Oct 2019, 10:55pm

Well, I'm just too late to this, then.
Some snippets....
As Brucey says, this is one of the most highly stressed bits of your frame. A fork which is designed to take a load in low-rider panniers and/or a disc brake can put a fair bit of force through here.
If you are pushing the design/specification in terms of loads applied, then you can demonstrate that this gusset adds stiffness. That doesn't mean you need this gusset for a gentle pootle around the lanes, but it might mean that you can ride home after an "off" instead of getting a taxi, or that your load-lugging bike will last as long as your "Sunday best" before the frame fails.
Frame failures on Spa bikes are uncommon, and I can't call to mind any failures at this joint (crosses fingers!).
Our biggest generator of failures was one builder whose braze-ons had a habit of coming off, and their welds occasionally started fatigue cracks; they don't build for us any more. Somebody thinks broken frames won't get billed back to the builder in the Far East, I can assure you they do! It certainly isn't in the builder's interest to have frames fail.
Theres not a lot of choice in 725 tubes, so I don't think that's a factor in different manufacturers choosing to use a gusset or not, see https://www.torchandfile.com/assets/images/PARTS%20LIST%202019.pdf
Why is the bottom of the gusset un-welded? because the last thing you want is a line of weld across the tube at that point. Have a look at a few lugged frames which have had a front shunt, the downtube is creased just at the end of the lug. Lugs are thinned and pointed to eliminate stress raisers, and gussets are open-ended and scalloped for the same reason.
My bottom line is that a gusset adds strength and stiffness for negligible weight and cost. Whats not to like?

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horizon
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Re: "Gusset" under downtube

Postby horizon » 6 Oct 2019, 11:05pm

531colin wrote:Well, I'm just too late to this, then.



I think we were all just sitting around waiting for you to appear . . . :)

My bottom line is that a gusset adds strength and stiffness for negligible weight and cost. Whats not to like?


So far that's been my take on it too. And then of course there are those full-size panniers that I put on the front low-riders . . . :mrgreen:
Let's just get Brexit done so that we can get on with the important job of re-joining the EU!

slowster
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Re: "Gusset" under downtube

Postby slowster » 6 Oct 2019, 11:14pm

reohn2 wrote:IMO the weld of the full circumferance of the downtube to headtube would be completed,before the gusset is welded in place.

That is what I had assumed. It has since occurred to me that leaving a gap in the weld circumference might be deliberate and serve a purpose, since otherwise there is likely to remain an air pocket in the void between the gusset and downtube when the frame is lowered into the bath for the electrophoretic paint deposition prior to the main coat of spray paint. A small gap in the weld should allow the liquid to flow through the void and ensure that the bare metal surface inside receives a thin coating of paint.

531colin wrote:Somebody thinks broken frames won't get billed back to the builder in the Far East, I can assure you they do!

Even if the failure was down to the designer? I can think of a UK brand which experienced what seemed to me an excessive number of reports of downtube cracks in one of its premium frames. That resulted in the design being changed to include a beefier downtube, from which I inferred that the problem was not the fault of the manufacturer but due to the designer specifying an insufficiently strong downtube.

PDQ Mobile
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Re: "Gusset" under downtube

Postby PDQ Mobile » 6 Oct 2019, 11:18pm

While I understand that a heavily loaded bike will put more stress here at the down tube join especially under heavy braking.
I am unsure why front low riders/panniers load the joint significantly more?

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horizon
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Re: "Gusset" under downtube

Postby horizon » 6 Oct 2019, 11:27pm

PDQ Mobile wrote:While I understand that a heavily loaded bike will put more stress here at the down tube join especially under heavy braking.
I am unsure why front low riders/panniers load the joint significantly more?


What Colin wrote:
A fork which is designed to take a load in low-rider panniers and/or a disc brake can put a fair bit of force through here.
Let's just get Brexit done so that we can get on with the important job of re-joining the EU!

Brucey
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Re: "Gusset" under downtube

Postby Brucey » 6 Oct 2019, 11:49pm

the tail wags the dog, too.

If the tail is made heavier, it wags the dog even harder.

cheers
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reohn2
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Re: "Gusset" under downtube

Postby reohn2 » 7 Oct 2019, 7:50am

slowster wrote:
reohn2 wrote:Not a clue what that means :?

'Don't ask me how I know this.'

Point taken.
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PDQ Mobile
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Re: "Gusset" under downtube

Postby PDQ Mobile » 7 Oct 2019, 8:36am

horizon wrote:
PDQ Mobile wrote:While I understand that a heavily loaded bike will put more stress here at the down tube join especially under heavy braking.
I am unsure why front low riders/panniers load the joint significantly more?


What Colin wrote:
A fork which is designed to take a load in low-rider panniers and/or a disc brake can put a fair bit of force through here.


So it's the extra stiffness of the fork and the anti/counter rotational effect at the disc hub that are the cause, not the weight of the panniers themselves?
A rim braked bike does not stress the joint in the same way?

Brucey wrote:the tail wags the dog, too.

If the tail is made heavier, it wags the dog even harder.

cheers


That I do fully understand and I tried to say that.

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531colin
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Re: "Gusset" under downtube

Postby 531colin » 7 Oct 2019, 11:05am

Fork stiffness is what loads the frame, and its there whether you have panniers and disc brake in use or not.
Stiffer to handle the loads of the brake and the panniers, so these forks are generally less tapered than (say) 631 blades. Also the brake and low-rider bosses need to be brazed into a bit of tube that isn't flexing like mad, you can put the bosses through the middle of 631but not all frame builders like to do it. The front and back of the fork blade can be either in tension or compression depending on whats happening, and you don't want a stress-raiser there on a thin, flexy tube. Its best practice to put it through the middle in any case.

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531colin
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Re: "Gusset" under downtube

Postby 531colin » 7 Oct 2019, 11:27am

slowster wrote:
531colin wrote:Somebody thinks broken frames won't get billed back to the builder in the Far East, I can assure you they do!

Even if the failure was down to the designer? ………..

Designer and builder should be on the same side?
If I sent a drawing and the builder said they would build them but not warranty them, then I would re-consider. (It hasn't happened!)
If they built them without question and then refused to warranty them, I don't think they would be building any more.
In any case, if its 725 we are talking about, refer to the tube list I linked earlier. There is only one tube diameter that boasts a choice in gauges, so that you can have an inch and eighth top tube in 0.7/0.4/0.7 gauge or the standard 0.8/0.5/0.8. I suppose you could also use the light gauge inch and eighth tube for a downtube, but theres nothing to match it for a top tube, other than itself or inch tubing in the standard gauge, neither of which make much sense to me.