[crashing_sux]

Or maybe not. Might be a stupid idea, or maybe it's been done before but I've never heard of it so here goes.

I was thinking about the whole deraileur issue today, along with the fact that short cages tend to hold up a lot better than long cages and wondered if maybe separating the chain tensioning and gear shifting functions wouldn't be the solution.

You could use a chain tensioner mounted behind the front chainrings, similar to a roox rollercoaster, but better designed so it didn't break all of the time. Since it wont' be controlling gear changes you could probably make it pretty flexible to take hits without breaking.

That would just leave you with needing to push the chain back and forth on the rear and, basically you'd just need to bars on teh side of the chain (like a front deraileur) to push it back and forth. Since the size difference of the rear cogs is much less than a front deraileur it wouldn't have to be nearly as large, seems like it could be very low profile, not much thicker than the chain itself.

I'd like to hear what you 's think about it, stupid, pointless, just might work?

 

[Matt D]

If I understand you correctly after reading it 4 times, you want to eliminate the rear der. as we know it, right? And replace it with a front-der-ish type shifter? If so, it won't work. The whole point of having a cage on a der. is to compensate for the increase/decrease in chain length when you shift, and to keep tension on the chain. W/O that cage, your chain can't grow and shrink enough to keep the chain running on the cassette teeth.

 

[thaflyinfatman]

Originally posted by Matt D
If I understand you correctly after reading it 4 times, you want to eliminate the rear der. as we know it, right? And replace it with a front-der-ish type shifter? If so, it won't work. The whole point of having a cage on a der. is to compensate for the increase/decrease in chain length when you shift, and to keep tension on the chain. W/O that cage, your chain can't grow and shrink enough to keep the chain running on the cassette teeth.

Which is why he mentioned that you have the chain tensioner up front, near the chainrings.

Makes sense to me, but I don't think you'd be able to keep such a thing rigid enough; a conventional derailleur can swing backwards and forwards but one like this would need to NOT do that.

 

[Rik]

I don't know if guideplates would be good enough for moving the chain side to side... I'd see a guide sprocket almost definately needed... and then, you'd need to make sure the chain was tensioned enough and had enough chain wrap to not jump off said guide sprocket, so you'd need another guide sprocket... bringing us to the current derailleur

 

[Matt D]

Originally posted by thaflyinfatman
Which is why he mentioned that you have the chain tensioner up front, near the chainrings.

Makes sense to me, but I don't think you'd be able to keep such a thing rigid enough; a conventional derailleur can swing backwards and forwards but one like this would need to NOT do that.

That would have to be one heck of a tensioner to take up enough slack in the chain.

 

[binary visions]

Originally posted by Matt D
That would have to be one heck of a tensioner to take up enough slack in the chain.

Yeah, I can't see how you'd be able to take up enough chain slack with a device that's at all smaller than, say, a roadie derailleur. That would cause clearance issues with the BB/chainring area, so you'd want to move it back to the rear axle.. And while you're at it, you may as well not have two seperate mechanisms, it's just one more thing to break.

The derailleur may not be the best answer, but it's a pretty highly evolved device. There's not much you can do to make it better. The answer, of course, is to move to a completely different type of drivetrain.. Enter the g-boxx..

 

[D_D]

Originally posted by Matt D
That would have to be one heck of a tensioner to take up enough slack in the chain.

You would pretty much have to bolt on a non shifting mech. It would also not have great clearance an may end up getting smashed even more often than a conventional mech.

It would also be pretty expensive compared to a normal mech. You can by a decent road mech for ~$40. Thats only possible because road mechs are used on road bikes. A mech that is only used on DH bikes isn't going to sell anywhere near as much and could end up a lot more expensive.

 

[BMXman]

I was actually thinking about this and wondered how come the rear derailluer is not mounted further back on bike so it's more inline with the chainstays. It wouldn't hang so low and you would pretty much bang the chainstays instead of the derailluer...D

 

[Jm_]

awe crap, i just got a brilliant idea.

 

[crashing_sux]

As far as chain tension I don't think that would be an issue. I've heard of a few people using Roox Rollercoasters for chain tension on single speed full suspension bikes so it alone was responsible for maintaining chain tension while the chain length was changing.

The downside is that they aren't too reliable but the design could probably be improved quite a bit. Maybe it's just not a good idea but I figured I'd see if it had been done before.

I'm all over the internal gearbox idea, I've been riding Rohloff's the past two years and love them but they still need to come a long way if they are to compete with deraileurs for a lot of people.

 

[Matt D]

Originally posted by crashing_sux
...using Roox Rollercoasters for chain tension on single speed full suspension bikes ...

 

[binary visions]

There is a WHOLE LOT more chaingrowth during the course of shifting through 3 chainrings and 9 cassette gears then any suspension design in the history of mountain biking.

 

[Jm_]

The only problem is that it only goes backwards now.....

 

[crashing_sux]

Originally posted by BMXman
I was actually thinking about this and wondered how come the rear derailluer is not mounted further back on bike so it's more inline with the chainstays. It wouldn't hang so low and you would pretty much bang the chainstays instead of the derailluer...D

Just a wild guess here but I think it's so you have enough chain wrap on the cogs to prevent the chain skipping.

 

[crashing_sux]

Originally posted by Jm_
The only problem is that it only goes backwards now.....

It may be a problem for some but I love the fact that when I'm rolling backwards out of abubacas I can still pedal forward. That whole pedaling backwards deal get's me all cornfused.

I wonder how much efficiency you'd lose putting a "reverse" in your front chainring and rear hub, then you'd keep your fancy top mounted deraileur, normal rotation pedaling, and go forward!

 

[stoner_303]

Originally posted by Jm_
The only problem is that it only goes backwards now.....

why are you crossing the chain? also, your not gonna hit your der. on anything if its on top of the cassette...i like it. are there any reasons why this idea wouldn't work?

 

[binary visions]

Originally posted by stoner_303
why are you crossing the chain? also, your not gonna hit your der. on anything if its on top of the cassette...i like it. are there any reasons why this idea wouldn't work?

Think about what happens to the tensioning mechanism of the derailleur if you mounted it above the rear axle and tried to pedal forward. SNAP goes the derailleur...

 

[stoner_303]

Originally posted by binary visions
Think about what happens to the tensioning mechanism of the derailleur if you mounted it above the rear axle and tried to pedal forward. SNAP goes the derailleur...

ahhh, ic, that makes sense, im just kinda slow.

 

[crashing_sux]

Originally posted by binary visions
Yeah, I can't see how you'd be able to take up enough chain slack with a device that's at all smaller than, say, a roadie derailleur. That would cause clearance issues with the BB/chainring area, so you'd want to move it back to the rear axle.. And while you're at it, you may as well not have two seperate mechanisms, it's just one more thing to break.

The derailleur may not be the best answer, but it's a pretty highly evolved device. There's not much you can do to make it better. The answer, of course, is to move to a completely different type of drivetrain.. Enter the g-boxx..

I missed this, I see now that it wasn't the amount of tension but being able to take up enough slack. I think you're right, that the device couldn't be much smaller than a conventional roadie deraileur, and looking at a Roox Rollercoaster it's not. It's still a double pulley tensioner, just mounted up front, although depending on the bike design you might be able to hide quite a bit of it's height up and behind the front chainring, instead of just hanging it directly below.

 

[dw]

Sunn used to do this in the late 90s- partly because the bikes had so much chain growth that they needed it.

 

[binary visions]

Originally posted by dw
Sunn used to do this in the late 90s- partly because the bikes had so much chain growth that they needed it.

Ahhahaha. That's great!

 

[Fulton]

Originally posted by stoner_303
ahhh, ic, that makes sense, im just kinda slow.

also, it wouldn't shift. you need to guide the chain before it goes on to the cogs, moving it after it has left the cogs isn't going to do anything (well, nothing productive )

 

[Incubus]

Running 29" wheels will give you an additional 1.5" of derailleur clearance. Any DH" frames out there that'll accept a 29" wheel? :devil:

 

[Jm_]

Originally posted by Incubus
Running 29" wheels will give you an additional 1.5" of derailleur clearance. Any DH" frames out there that'll accept a 29" wheel? :devil:

bad solution, that will kick travel back down 2" or more. Not many people are going to give up their 8/8" frames and forks to run 6/6".

Not a horrible idea, but in application those big wheels will limit several things, unless they raise the bottom brackets to like 15"....i hope not though.

 

[binary visions]

Originally posted by Jm_
bad solution, that will kick travel back down 2" or more. Not many people are going to give up their 8/8" frames and forks to run 6/6".

I think you missed the :devil: smilie in his post...

 

[Jm_]

Originally posted by binary visions
I think you missed the :devil: smilie in his post...

I was thinking of it in the sense that "if a bike was designed to use 29"ers", instead of just putting 29" wheels on any existing bike.

 

[J5ive]

Whats wrong with the saint idea?

All we need is someone to make aftermarket though axels?

How hard can it be?

Its just a big bolt that replaces your 12 mm axel with a washer on the end perhaps to give the saint mech somewhere to sit?

 

[binary visions]

Originally posted by J5ive
Whats wrong with the saint idea?

All we need is someone to make aftermarket though axels?

How hard can it be?

Its just a big bolt that replaces your 12 mm axel with a washer on the end perhaps to give the saint mech somewhere to sit?

Well, besides the fact that it's proprietary (which I understand - it is, after all, a Shimano product), it's only a band-aid to the problem - it doesn't address the root cause which is that we have a relatively flimsy piece of metal hanging down where it can be snagged and bashed on debris. So while there's nothing really wrong with the Saint idea, something better is needed.