rear triangle seat tube

General Discussion about electric bicycles.
cheapcookie   100 W

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rear triangle seat tube

Post by cheapcookie » Sep 14 2019 12:57am

Hey folks wondering why there is a "seat tube" inside the rear triangle, it is welded to the rear triangle.
I want to fit a 6374 on it or cut it off entirely.
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ferret   1 kW

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Re: rear triangle seat tube

Post by ferret » Sep 14 2019 6:07am

Probably it's for mounting a front derailleur.

Avner.
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MadRhino   100 GW

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Re: rear triangle seat tube

Post by MadRhino » Sep 14 2019 7:54am

It is welded to the bottom bracket, free from the swing arm. You can use it to mount anything that you wish. If you cut it off, leave a short part of it to avoid weakening the BB weld to the down tube.
Make it fool-proof, and I will make a better fool.
Current bikes
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Trek Session 10 mod. Variable geometry. 70mph
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cheapcookie   100 W

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Re: rear triangle seat tube

Post by cheapcookie » Sep 14 2019 12:49pm

MadRhino wrote:
Sep 14 2019 7:54am
It is welded to the bottom bracket, free from the swing arm. You can use it to mount anything that you wish. If you cut it off, leave a short part of it to avoid weakening the BB weld to the down tube.
I swear it's welded to the rear triangle
you can kinda see it here

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cheapcookie   100 W

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Re: rear triangle seat tube

Post by cheapcookie » Sep 14 2019 12:51pm

Image

cheapcookie   100 W

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Re: rear triangle seat tube

Post by cheapcookie » Sep 14 2019 12:57pm

Image

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MadRhino   100 GW

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Re: rear triangle seat tube

Post by MadRhino » Sep 14 2019 7:05pm

Then it is not much use, if it is moving with the swing arm. Can’t use it to mount anything related to the BB and crank because the spacing does vary with suspension action. It could be used to mount a mud guard, or a disc side drive maybe.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

Voltron   10 MW

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Re: rear triangle seat tube

Post by Voltron » Sep 14 2019 8:31pm

I think it is on the swingarm on this design, which does make it move up and down a little relative to the chainring, but otherwise the swingarm would rise up and hit the deraileur.

markz   100 GW

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Re: rear triangle seat tube

Post by markz » Sep 15 2019 1:08pm

Probably a limit stop to the suspension travel, or its just a rear triangle used also on a different bicycle all together.

What is the brand and model of the bicycle?

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dogman dan   100 GW

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Re: rear triangle seat tube

Post by dogman dan » Sep 16 2019 6:24am

Its for hanging a front derailleur on, if you want a triple ring front gear. Can't use it to mount your seat, nor would you want to sit on it if you did. It would be like a hardtail, but moving all the time so it would yank your balls off.

cheapcookie   100 W

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Re: rear triangle seat tube

Post by cheapcookie » Sep 16 2019 12:50pm

dogman dan wrote:
Sep 16 2019 6:24am
Its for hanging a front derailleur on, if you want a triple ring front gear. Can't use it to mount your seat, nor would you want to sit on it if you did. It would be like a hardtail, but moving all the time so it would yank your balls off.
wow I would have never guessed it's for a front derailleur because motion dependence of rear triangle, thanks dogman and ferret!

markz   100 GW

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Re: rear triangle seat tube

Post by markz » Sep 16 2019 1:06pm

Very odd place for a front derailleur mount. Its mounted to the rear triangle, which moves up and down over every bump, while the crank itself is stationary and apart of the main front half of the bike. Very odd location for a f.d. I was almost thinking it could have been for chain tension or mud guard mount, rear rack mount.

Here is a nice side shot
https://p.vitalmtb.com/photos/users/289 ... 1389593127

Another
https://www.sicklines.com/gallery/showp ... bs/cat/851
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commencal supreme2.jpg
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MadRhino   100 GW

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Re: rear triangle seat tube

Post by MadRhino » Sep 16 2019 10:52pm

Come on. It is a DH racing frame. Absolutely no one using a double chain ring on a DH bike since the last century. I would guess this part of the swing arm was just designed common for two or more models of the brand.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

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dogman dan   100 GW

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Re: rear triangle seat tube

Post by dogman dan » Sep 17 2019 6:15am

Did I say it was a smart design? Maybe it's for a derailleur, that is used only as a chain guide.

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MadRhino   100 GW

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Re: rear triangle seat tube

Post by MadRhino » Sep 17 2019 6:07pm

Well, to design a part that is common to many models in production is not unusual. Car manufacturers are doing it all the time. This same part on another bike would accommodate a derailer mount, on the DH bike it does save the cost of making a part that is specific to a bike that is made in small series. A chain guide/tensioner could be mounted there, but they are normally part of the chain guide/ bash guard combo that mounts on the BB.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

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E-HP   100 kW

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Re: rear triangle seat tube

Post by E-HP » Sep 17 2019 7:18pm

Here's an example of the derailleur mounted on a swingarm. This looks even odder since it would move back and forth too, not just up and down.
full susp.jpg
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markz   100 GW

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Re: rear triangle seat tube

Post by markz » Sep 17 2019 7:21pm

Nice find, I couldnt find a picture of a f.d. on the rear half of sny bike.
Then again I did not even really try to find a picture either.

I wonder if its a special front derailleur, one with lots of space from the inner nose to the tail. Im going by the terms used here. http://www.abundantadventures.com/mtfaq ... .mods.html but I am sure Sheldon Brown has it somewhere on his webpage. Need lots of height for an up and down movement of the r. tri.
E-HP wrote:
Sep 17 2019 7:18pm
Here's an example of the derailleur mounted on a swingarm. This looks even odder since it would move back and forth too, not just up and down.

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MadRhino   100 GW

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Re: rear triangle seat tube

Post by MadRhino » Sep 18 2019 12:10pm

Not so much of an issue with such a short travel FS frame. I mean, it doesn’t move much. It is common for enduro and AM bikes to have 2 chain rings. Long travel bikes are made for freeriding. Not only a 2nd chain ring is useless, but the swing arm action does make a big difference in chainline length between compresses and extended positions. That is why they are equipped with the opposite of a derailer: some anti-derailing DH chain guides and bash guard.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

Voltron   10 MW

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Re: rear triangle seat tube

Post by Voltron » Sep 18 2019 4:58pm

It really isn't much motion like MadRhino wrote.
Re chain length change from the suspension, on a single pivot like that, it def does change. But the good suspensions these days pretty much eliminate that, so pedaling doesn't interfere with the motion, or yank your pedals around on big hits.

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Re: rear triangle seat tube

Post by Grantmac » Sep 18 2019 5:19pm

How large is that space and what frame is it?

It's definitely not a DH frame with that shock.

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E-HP   100 kW

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Re: rear triangle seat tube

Post by E-HP » Sep 18 2019 6:06pm

Grantmac wrote:
Sep 18 2019 5:19pm
How large is that space and what frame is it?

It's definitely not a DH frame with that shock.
They still refer to it as a downhill bike, but it only has 150mm of rear travel (but that was decent a decade ago).
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cheapcookie   100 W

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Re: rear triangle seat tube

Post by cheapcookie » Sep 18 2019 6:25pm

ended up buying the bike, here's what I want to do
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Grantmac   10 kW

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Re: rear triangle seat tube

Post by Grantmac » Sep 18 2019 7:54pm

You're going for a left hand drive? What is the dropout spacing?
Not planning to put the motor in the swingarm?

cheapcookie   100 W

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Re: rear triangle seat tube

Post by cheapcookie » Sep 18 2019 8:46pm

Grantmac wrote:
Sep 18 2019 7:54pm
You're going for a left hand drive? What is the dropout spacing?
Not planning to put the motor in the swingarm?
Dropout 135mm, don't want to cut the nipple in triangle

Grantmac   10 kW

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Re: rear triangle seat tube

Post by Grantmac » Sep 18 2019 9:19pm

How do you plan to mount both the brake rotor and the sprocket on that drop out width (this is something I'm working on too.

Also if you are planning a jackshaft I'd mount it in that space behind the stub or else possibly on the swingarm pivot if there is clearance to the crank arm. Then your primary chain isn't going around two 90 degree bends.

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