# Giant STP chainline concerns



## JonathanGennick (Sep 15, 2006)

I've just built up a Giant STP frame, and I have some concern over chainline. I'm seeing more of a "crossed" effect than I had expected.

First, here's a gratuitous shot of the new build. Not all parts are jump-worthy, but that's ok for now. (I can't jump anyway).









Getting to the point, this next photo shows the chainline when I'm in the easiest gear:









The chain's at quite a large angle. Is that ok? Are the production STP bikes like that? Am I just seeing the result of short chainstays?

Would going to a different crankset improve things? I'm running a set of Shimano LX cranks at the moment.

Should I even worry about the chainline at this point?

It doesn't really straighten out until I reach the smallest cogs. I had just expected that the middle ring would be more centered with respect to the cassette.


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## clewttu (May 16, 2007)

generally, it should be lined up with the middle cog (5th)


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## ducktape (May 21, 2007)

All I can think off is that maybe the chainguide is BB mounted and for some reason there is the additional spacer as well? I don't know that much about chainline but I'm pretty sure if you run a BB mounted chainguide then you don't use the 2.5mm bb spacer you would normally.

Also don't look at the chainline from the top, look from the back of the bike so you're looking along the chain, does it seem to be inline when in the 4th or 5th gear?


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## JonathanGennick (Sep 15, 2006)

@ducktape, the guide is BB mounted. I did remove the spacer when installing the guide.

I'll take a look from the back of the bike when I have a chance. As I recall though, the chain only falls into line at around the 8th or 9th gear -- the smallest couple of cogs.

Not much I can do, really. The only solution I can think of is to try and find a different crankset + bottom-bracket setup that puts the rings closer in to the frame. There's not much room to move inward though. I don't want the chainring to hit the chainstay. 

I could single-speed the thing, but that won't work for my application. I need the gears. 

I'm tempted to adjust the derailleur stop so as not to allow the big cog in the back. That would help some.


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## clewttu (May 16, 2007)

whats the bottom bracket shell size? 
my trail bike has a 68mm shell, an MRP guide, with XT cranks/BB and it requires 1 spacer outside the guide adapter (essentially two spacers like youd have without the guide)...not that this means anything to your setup
if you dont have any spacers in there and its still that far out, try some washers on the chainring bolts to move the chainring inward some


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## JonathanGennick (Sep 15, 2006)

@clewttu: 73mm bottom-bracket shell. 

And that's a good idea on the washers. I may play around with that a bit.


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## JonathanGennick (Sep 15, 2006)

BTW, I had the neighbor kid test ride the bike yesterday. He rates it as an "Awesome!". 

I believe he'll be doing further research this afternoon  .


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## clewttu (May 16, 2007)

so only the guide is needed as a spacer, and thats what you have...weird


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## ducktape (May 21, 2007)

Well it's puzzling for sure. 
Do you find that it rubs the guide in extreme gears at all? If it doesn't maybe you're just not judging correctly when the chain line is straight (possibly harder to tell because of the chainguide?). That's pretty much all I can come up with.

I've got an STP as well, and I can't jump either (just built my first jump today actually to hopefully master on the bmx first). Anyway I've had a couple of different cranks on mine, Race Face Diabolus that it came with and FSA Afterburner or something at the moment. I've looked through the photos and no real useful once I can post, and the bike is in parts at the moment as I'm converting it to SS and Rigid so I can't go check how it looks in the large cog for ya!
Well there is only one photo where it's in the 2nd largest cog and the angle looks like yours might be pretty normal. I'm leaning towards thinking it's just a bad judgment call on when it's straight. Maybe you can measure it?
I'm not 100% on what the correct way to measure chain line is though! I'd be guessing it's measured from the bb to the middle of the chainring.


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## Mr. Blonde (May 18, 2008)

I've got the same crankset on an STP with a Gamut. My chainline is perfect. I had to run the guide as tight on the frame as I could and grind off the granny tabs to make it work. Which way is your adapter facing? Is the recessed side towards the frame or away?


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## JonathanGennick (Sep 15, 2006)

Mr. Blond. You run the MRP guide _and _a Gamut bashguard? Doesn't the bashguard get in the way of the guide?

I do have the guide installed correctly on my bike. And it is not the reason for the angle that you see in the chain.


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## JonathanGennick (Sep 15, 2006)

ducktape said:


> ...maybe you're just not judging correctly when the chain line is straight (possibly harder to tell because of the chainguide?).


I hate to admit it, but I think you've nailed it, and the problem is me! I just took another look with the chain on the middle cog. I made sure to sight from behind the wheel this time, and not looking down from above. The line in that position does not look to be perfectly straight, but it's close.

So I think my eyes may have been deceived when took those photos.


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## ducktape (May 21, 2007)

JonathanGennick said:


> I hate to admit it, but I think you've nailed it, and the problem is me! I just took another look with the chain on the middle cog. I made sure to sight from behind the wheel this time, and not looking down from above. The line in that position does not look to be perfectly straight, but it's close.
> 
> So I think my eyes may have been deceived when took those photos.


LOL and you had everyone else convinced too 

Honestly the times I've been trying to ride my bike up steepish hill in the easiest gear the thought has crossed my mind weather it's good for the bike to spend a lot of time in that gear because in a normal situation you would have changed to a granny up front. Mind you the only times it's really been a concern is if my derailleur wasn't properly tuned and was making noise! 
I guess that sort of angle is expected when you're running 1x9 it's only the more extreme angles when you've got the three rings up front that are a big no no.

If you're really not happy though something you could consider doing is removing some cogs and putting spacers in, like if you don't need the 3 samllest (hardest) ones you could take them out, center the rest on the hub for good chainline and put spacers each side. Then whatever they do to stop the derailleur over shifting to those non existing gears (H & L screws). I would essentially turn you bike into say a 6 speed (depending how many you took off) and the same would apply if you didn't need the easiest gears for example. It's probably the kind of mods that some 4x riders might do I guess, less gears but not single speed. Something you could consider if you need gears but you don't think you need the whole spread.


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## Mr. Blonde (May 18, 2008)

JonathanGennick said:


> Mr. Blond. You run the MRP guide _and _a Gamut bashguard? Doesn't the bashguard get in the way of the guide?
> 
> I do have the guide installed correctly on my bike. And it is not the reason for the angle that you see in the chain.


:skep:

Gamut makes chainguides too you know. I know for a fact you don't have it installed correctly as my chainline is perfect. The guide has no bearing on the chainline. If you answer my questions I can help you.


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## JonathanGennick (Sep 15, 2006)

Mr. Blonde said:


> Gamut makes chainguides too you know.


I do know that. I had misread your earlier post. Sorry for that.



Mr. Blonde said:


> I know for a fact you don't have it installed correctly as my chainline is perfect.


The MRP guide on my bike is installed correctly. It replaces one spacer, and because of that does not affect chainline at all.

If I get time during lunch, I'll try and take a photo or two.


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## JonathanGennick (Sep 15, 2006)

*More photos*

Ok. I did grab some photos during lunch. You can seem them below. I can gain 3mm by moving the spacers that replace the big ring inboard. Otherwise though, I don't see much room for improvement. If I've made a mistake, I'm not seeing it.

Mr. Blonde, you asked about orientation of the guide. The inboard side of the 1.X arm is flat. That's the side that has the knurled texture where it contacts the frame's bottom-bracket.

First photo. LX cranks. You can see the 3mm spacers on the outboard side of the crank arms. The 3mm spacers are installed exactly as shown in the instructions from MRP. I'm thinking though, that I can move them inboard to improve one the chainline. However, I would then need to bend the guide arm to avoid having the guide rub. Not sure yet that I want to go there.









Second photo. Shows the MRP 1.X arm where it replaces the bottom-bracket spacer.









Third photo. Shows the chainline when in the middle cog. You can see that the line is not perfectly straight.









Possibly a different crankset would fit the frame differently, allowing for a better chainline. I'm open to suggestions there.


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## Mr. Blonde (May 18, 2008)

I was under the impression that guide was mounted to an adapter. I didn't realize it was a bb mount. I think it's a problem with that guide. Stupid question: Are you absolutely sure the non drive arm is all the way on the splines? Your drive side arm is definitely riding further out than mine. What kind of chainring are you running? Do you have some nice dial calipers around that you can use to measure the thickness of the guide boomerang?


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## JonathanGennick (Sep 15, 2006)

Mr. Blonde said:


> I was under the impression that guide was mounted to an adapter. I didn't realize it was a bb mount. I think it's a problem with that guide.


Yeah, it's a bb mount. Not sure how else to mount anything down there. There aren't any ISCG tabs on the bike.

Hey, your comment about your drive side arm not riding so far got my attention. What brand and model of crank are you running? What bottom bracket?



Mr. Blonde said:


> Stupid question: Are you absolutely sure the non drive arm is all the way on the splines?


I just went out and double-checked. Yes. The non-drive arm looks properly engaged. The little safety tab is completely tucked in, which it wouldn't be if the arm was out of position.



Mr. Blonde said:


> Your drive side arm is definitely riding further out than mine. What kind of chainring are you running?


I'm running a Race Face downhill ring.



Mr. Blonde said:


> Do you have some nice dial calipers around that you can use to measure the thickness of the guide boomerang?


I have some no non-dial calipers. I can't work them down in there well enough to take a good measurement. I'd need to remove the crank, and I don't have time tonight to do that.


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## ducktape (May 21, 2007)

What about the next smallest sprocket down (ie 4th gear in), is the chainline straight then? I've found on my stp the chainline looked perfect on the 4th gear in rather than the 5th.


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## JonathanGennick (Sep 15, 2006)

ducktape said:


> What about the next smallest sprocket down (ie 4th gear in), is the chainline straight then? I've found on my stp the chainline looked perfect on the 4th gear in rather than the 5th.


Yes. The line looks awfully darn straight when on the fourth smallest cog. Here are some photos showing the chain in that position:


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## ducktape (May 21, 2007)

In that case I'd think it's normal, or at least the same as mine.


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## JonathanGennick (Sep 15, 2006)

ducktape said:


> In that case I'd think it's normal, or at least the same as mine.


That's good to know, actually. At least it suggests that I didn't overlook something or make a poor choice of crankset.


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