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Rush skips a gear
Just talking about cables and stuff in the other thread, reminded me of a prob I have with my Rush. When ever I go off a gutter and pedaling, it seems to want to skip a gear.
A run down on the bike, it's only recently been built, It's got all new outer and inner cables, XTR shifters, XTR shadow long cage derailleur, XT cassette, XTR chain. Now, I'm pretty confident it's not a component problem as it was happening when I had m SLX shifters on there and the SLX rear derailleur on there. I thought that this problem was from them, so I went the upgrade.
I later did some fiddling with the outer cable, I pulled a bit of slack for the rear mech where it comes out of the rear frame and super-glued some cable-ties to it, so it wouldn't go back into the rear frame. This helped quite a bit and now only does it half the time that it used to. It won't do it everytime, but it seems that it's only when the suspension comresses a fair bit and there is tension on the chain, ie pedaling hard off a gutter say from stationary waiting for a light to go green, then you go and go hard. Or say giving it A-holes up a hill and you are really cranking on the pedals and reefing on the bars.
Have any other Rush owners encountered this?
Was there a fix?
My suspicion is it's cable related, but it's routed exactly where it's meant to be. There's plenty of slack in the cable, at least I think there is. lol
Cheers
Tark 
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Given you say the bike is pretty new, would our old friend cable stretch possibly be the culprit? As the inner settles down you will just have to give the barrel adjuster a little tweak to take up any slack.
Perhaps the outer is also a bit short if it disappears into the frame (rear suspension arm I presume) as you describe?
I am pretty sure it's not cable stretch for a couple of reasons. Firstly, it has happened on two different derailleurs, so I've undone the cable off the old mech and done it up again to fit the new mech. Secondly, it shifts perfectly everytime on demand.
The outer being too short is my guess... It seems to be about the right length, it looks even off the bars, there's plenty of slack, but the rear is where I question it all.
Here's a couple of pics to see what I've done.
Here's the way it was just after swapping the SLX rear mech for an XTR rear mech. The problem continued exactly the same as it did with the SLX.
Here's what it looks like now, notice the extra cable sticking out the back. Unfortunately I don't have a pic of the cable-ties, but you can see that there is more cable sticking out before it terminates in the rear mech. It's weird, it's natural positions seems to want to be like the first pic. I had to forcibly hold it out to cable-tie/superglue it. Just to clarify, I've only glued the cable-ties to the cable, and not prevented the cable from moving outward from the swing arm.
I'm a bit at my wits end. I know it's not meant to do it, but just can't figure out what's going on with it. It's been an awesome bike other than this problem. Not bad for under 2k build.
Hmmm... it looks OK from those pics. There's a hi-res image here which looks very similar too:
http://de.wikipedia.org/wiki/Datei:Cannondale_Ru...
Is the hanger OK?
It doesnt look like you have much chain wrap around the cassette?
Try adjusting the screw at the rear of the derailleur to bring the jockey wheel in closer to the cassette to get more chain wrap.
The more chain wrapped around the gear the more distributed the load is on the individual gear cogs.
My Cannondale Prophet has the same rear end construction as the Rush, and it used to skip a sprocket every now and then over fast rutted terrain, especially when braking.
It was fixed by changing the spacers/shims on the crank - moving the chainrings inwards by half a millimetre or so - which changed the chain line.
That also (eventually) cured a major chain suck problem on the chainrings (actually I was trying to fix the chain suck and the rear cassette skip remedy was just a by-product).
The hanger was slightly out of wack when i got the frame, which we discovered when i initially found this problem.. But I got that straightened. That might be something to consider.
I set the derailleur position so that the jockey wheel is pretty close to the 34 tooth sprocket. I will however, double check this.
Just to clarify, the problem isn't like the chain skipping on the same gear, it feels like the derailleur is actually shifting to a lower gear, but then doesn't, if that makes sence. As I can't see it happening when it happens, I am guessing the chain links could be riding up on the teeth of the rear sprocket like it was about to change gear, then it drops back in.
What's the best way to set up/check the chain line?
The way I set it up was making sure the cranks were even on the frame. The crank chain rings look pretty central to the rear cassette, but if half a mm makes a difference, then it might also be worth a look.
Cheers guys!
If anyone thinks of something else..
Tark
Can you temporarily remove the shock and watch what happens to the dérailleur, cable and chain when you simulate full compression of the suspension?
Is any particular gear combination more prone to the problem than others? eg Granny and #3 at the back. If so try middle ring and #1 instead. Might help pinpoint the cause.
Is the jarring from jumping off the gutter causing the dérailleur to 'bounce' or move in any way?
Optimal chain line may vary between bikes but I recall reading that middle ring and 5th gear at the back should be lined up.
Good luck!
Jon
lol.. I actually did that as soon as I got home tonight (couldn't stop thinking about it all day today), to see if the cable was being pulled. But the cable barely moves, and because of the cable-ties I put at the rear, the part between the rear of the suspension arm and the derailleur is solid as. The chain seems to pull ever so slightly on the rear derailleur, but it's a few mm.
Particular gear?.... Umm... I don't use granny on this bike (well haven't yet, just chuck it down there to relieve cable tension), I'd have to say it's in the middle ring and say 5th or 6th on the cassette. I have a feeling it's done it in the big-dog on the front and around the middle of the cassette also. So there should be a fair amount of tension on the chain.
Could it be bike flex?
I should also add, this has only ever happened on the road, I've only ridden it off road twice now (damn rain!), and I don't think I'd ever crank as hard on the dirt as I do on the road.
I'll check that chain line (middle-5th) I had a look at it tonight whilst I was going over it, and I did notice that the chain rings look like they are reasonably centralized to the rear cassette, but I didn't straight edge it. I'll find out exactly what it's meant to be and I'll get my big straight edge that I've got at work and get it spot on.
Yeah, the jarring could be doing something. But it happens when I climb as well, so probably negates that.
It seems to only happen when I really crank on the pedals, The two examples I recall very clearly is;
A) Sitting waiting for the little man to go green so I could cross. It went green, (I was holding on to the pole as I was clipped in), so I let go and cranked on the pedals hard, lifting with one foot and pushing down with the other, whilst reefing on the bars for leverage. As I'm doing this I dropped down the gutter, half popping up the front wheel so both wheels land at the same time.
B) Climbing up a short bit of hill on the road, from The narrabeen lakes loop (where it peels off across the G/course) to the round about at Wheeler Heights shops. It's not a real big hill and I crank up it hard some times in the big chain ring/and 5th or 6th, or in the middle and 7th or 8th pending how I'm feeling. Again, if I'm really cranking and pulling hard on the bars I can generally make it do it. But I try and avoid this gear thing happening so I don't go so hard.
When the rain eases off, I'll get it out for a ride and see if I can concentrate on making it happen consistently to better describe what's happening.
There are folks around that are paid to fix issues like this. Or at least take the mystery out of the diagnosis if it can not be resolved, which in some cases is the way it is.
Just a thought...
I was looking at a Rush SL, and the cable loop at the FRONT of the swing arm was too long. When the suspension compressed, it exaggerated the loop and put tension on the cable, causing it to try and change to a lower gear.
I set it a bit shorter, then adjusted the cable tension slightly on the loose side when unloaded so it would be 'just right' when compressed.
Give that a try, or post a shot of the cable as it exits the swingarm to the cable stop on the down tube and I can have a look and see if it looks like the same issue.
And I agree with the previous post about the 'b' knuckle adjustment. Back it off so the rear der jockey wheel is sitting a good bit closer to the cluster. Check that it's got clearance when you are in the biggest gear though.
Whisperer
Yeah Fankles, I know there's folks around... lol, It's just one of those things. I was hoping that it was a common thing for Rush's if say the setup isn't right or if it's just that I've done something wrong. 99.9% of the time it's perfect. In fact, I rode it to work today and back, and believe it or not, I couldn't get it to do it.
The thing I did change is the derailleur distance (the B knuckle is it?) to the cassette this morning before I rode(thanks for the tip!). I put it into 1st and I moved it about 2mm closer to where I thought it was close enough to 1st gear cog, I didn't want to go too close so that it wouldn't interfere with the changing in and out of 1st. It wasn't a whole lot, but it was something. As it turns out that may have been the problem... I tried my hardest to get it to do it, pedalled like a mofo in a tall gear really cranking on it, reafing on the bars. But just couldn't get it to slip... Stopped at a few lights from a track stand, to again, cranking like a mofo. No gear issues. So it looks like that was it.
Thanks heaps for all the help and ideas guys!
I will post a pic up of the cable routing though and the closeness of the derailleur to the cassette.
I used to have the same issue with my Proflex's. the issue was related to suspension movement and the last section of outer cable.
Once you ran the outer-cable through the swningarm in one piece the issue of ghost shifting disappeared. So my take here would be that is is the same issue and related purely to the last section of outer cable.
Here's the pics;
Front gear cables,
Cable between front frame and swing-arm,
Cable coming out of swing-arm,
Clearance between deraileur and cassette,
It looks good to me, but anyone see anything?
my Prophet derailer sits a lot more lower and forward giving more chain wrap?
Got a pic of it to compare?
Couldn't work out how to link to image
http://nobmob.com/node/15392
[Mod: linked in image]