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'18 Transition Sentinel
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1,015 Posts
Discussion Starter · #1 · (Edited)
HELP: front derailleur positioning on BottleRocket

Hi folks:

So, I'm practically done building up my new BottleRocket, and the remaining item is with positioning the front derailleur.

First, most importantly, when I put front drlr at the height that I'd normally put, the inner side of the cage hits/touches the chainstay right above the bearing. If I try to bring the cage out by limiting the "low" screw so that the cage doesn't touch the chainstay, then the chain will drag/rub inside the cage when in climbing gears. If I try to raise the front-drlr height up along the seattube (by about 5mm) to 'clear' the chainstay, then my shfting range gets severly limited (chain drags/rubs inside cage on lots more gear combo's than normally would).

Secondly, without shock, I tried to compress the frame, and - when fully compressed (bottomed-out position) for 2.25" stroke, the rear-end of the cage will hit/touch the chainstay (or is very close). This concerns me for going out there to hit some drops (possibly either the cage snaps, or the drlr clamp damages the seattube).

FYI, I'm using my Shimano XT FD-M751 (bottom-swing). Is this the cause (though I doubt it)? I know Transition website says "Top Swing Shimano or Low Mount SRAM". But isn't "top-swing" vs "bottom-swing" (or "high mount" vs "low mount" in SRAM lingo) just the difference in how the front drlr is /clamped/mounted on seattube to clear some certain tight suspension designs, yet still puts the cage in the same exact spot? BottleRocket has good clearance along the seattube, so that I think I can use either "swing" type front drlr. Or is this "swing" type really the key to make it or break it for the front drlr set up on BR?

Please help me on this. I'm almost ready to go ride, literally!
- PiroChu
 
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First and foremost, take the shock off the fram and get it set up so it doesn't hit the frame anywhere through the stroke of the swingarm. You should be able to get it so its within a mm or 2 which will be fine. Even if the chain drags on the bottom of the cage when it is on the stand, once you put your weight on the bike it will sag and the chain won't drag.

Then its about spacing out your cranks correctly to achieve a balance between the chain dragging on the cage and a proper chainline. Put the chain in the biggest ring, which is probably the middle if you're running a bash guard and then put the chain in the middle cog on the cassette, the chain should be as straight as possible when looking at it from the back.

Once those 2 things are taken care of you're good to go. You probably wont have all 18 gears, on my Dirtbag I got about 15, I cant go from the big ring up front to the 2 top gears on the cassette because the chainline is way off and I cant go from the granny ring up front to the small cog on the cassette because the chain drags on the front derailluer cage.
 

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Live to Ride.Ride to Live
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55 Posts
Same problem on my Dirtbag

I discovered too late that the front der. was touching the seatstay and i have a nice rubbing on it.
I use SRAM X-Gen with a 22 teeth plate and I thought of trying a shimano der., thinking that the cage might be smaller and prevent this situation.
What front der. are DB riders using out there?

Cheers.
 

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Live to Ride.Ride to Live
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55 Posts
touching the chainstay NOT seatstay

:madman:
the front der. touches the chainstay (NOT the seatstay as wrote) under compression
 

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'18 Transition Sentinel
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1,015 Posts
Discussion Starter · #5 ·
sixsixtysix said:
First and foremost, take the shock off the frame and get it set up so it doesn't hit the frame anywhere through the stroke of the swingarm. You should be able to get it so its within a mm or 2 which will be fine. Even if the chain drags on the bottom of the cage when it is on the stand, once you put your weight on the bike it will sag and the chain won't drag.
Thanks for that perfect advice! :thumbsup: I didn't even realized about the sag, when I initially posted. In fact, I was only working on it on the bike stand and had not even sat on it once yet. :blush:

Anyway, I first ended up putting the front drlr at 5mm higher than I would've (if I had a 44T ring) in order for frnt-drlr cage to "sit above & over" the chainstay (in the bearing area). I went test-riding it like that yesterday (sweet 1st ride, yey!) and it shifted perfectly when under sag (still horrible on bike stand, but that's OK).

Then today I also wanted to make sure that the 2.25" full bottoming will not have the chainstay copme up and smack the frnt-drlr cage, so I ended up putting it another 3mm higher from that position, which puts the front-drlr cage to sit 5mm above the chainstay (in the bearing area). I'll go test ride on Sunday - I'm sure it'll be fine (or, even if not, I know what/how to do/tweak now).

Below is a (poor) pic of it (size "small") from my 1st test ride on Wed. It's 41lbs 12oz, as seen in the pic.

Cheers,
- PiroChu
 

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I'm running a high-mount Sram X9 front on my Rocket, with a dual ring setup.
Took a bit of adjusting to get the cage to work around the pivot/swingarm while going through the travel, but it works perfect now with no chainrub or clearance issues.

Pic when new:
 
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