Chainstay angle

[mikeymo]

Front derailleurs (or at least the MTB ones I've looked at) quote a "chainstay angle". Which in this context seems to be the angle between the chainstay and the seat tube. For Shimano the two choices, each of which is a range, are 63-66 degrees and 66-69 degrees. 63-66 seems to be "road" or "trekking", 66-69 seems to be mountain bike.

Mountain bike front derailleurs with 63-66 are rare, at least in the used/NOS market. Even though many of them were available new (talking 9 speed here).

I've calculate a chainstay angle of 65 degrees for my bike, using manufacturers published seat tube angle, chainstay length and BB drop. I'm one degree, outside the "spec" for a MTB FD which has a chainstay angle spec of 66-69.

The tail of my current front derailleur is 115mm from the centre of the bottom bracket. At that distance, one degree of difference is equal to 2 mm.

It's a steel touring frame, with 450mm chainstays. The chainstays, at the derailleur end, are 32mm deep, top to bottom.

As far as I can tell, the problems of mismatched front derailler and chainstay angles are:

1. The trailing edge of the derailleur cage may strike the chainstay. If the frame's chainstay angle is too small (as in my case). But this would depend on how high the FD is mounted, which depends on largest chainring. And also perhaps on chainstay thickness (top to bottom). My steel frame may has thinner chainstays than MTB aluminium ones?

2. The chain rubs on the horizontal trailing edge of the chainstay cage. If the frame's chainstay angle is too big. This wouldn't happen in my scenario, presumably.

Does that about sum it up?

I'm guessing that I might be able to "get away" with this setup, mainly depending on what my largest chainring is.

Or, as they are actually available now, in triple nine speed 63-66 degrees, what is Alivio like quality wise? I'm not really hip to MTB groupsets.

Thanks.

[Brucey]

I've calculate a chainstay angle of 65 degrees for my bike, using manufacturers published seat tube angle, chainstay length and BB drop. I'm one degree, outside the "spec" for a MTB FD which has a chainstay angle spec of 66-69.

you might be better off measuring the angle eg from a photo taken square on with a long focal length lens. The reason for this is twofold

a) there will be an error because the chainstay may 'point' higher than the axle (or lower) and
b) chainstays are tapered and the angle of interest is arguably between the seat tube and the top edge of the chainstay.

In addition to the issues mentioned the chain is meant to run at a specific range of angles through the FD and this can affect shifting too.

Bottom line is that you most often don't know for sure until you try.

cheers

[mikeymo]

I've calculate a chainstay angle of 65 degrees for my bike, using manufacturers published seat tube angle, chainstay length and BB drop. I'm one degree, outside the "spec" for a MTB FD which has a chainstay angle spec of 66-69.

you might be better off measuring the angle eg from a photo taken square on with a long focal length lens. The reason for this is twofold

a) there will be an error because the chainstay may 'point' higher than the axle (or lower) and

You mean the centre of the axle might not necessarily be on a line drawn along the centre of the chainstay? Let me have a look....

Yes, you're right. Sighting down the chainstay from the back of the bike I can see that the end of the QR is a little lower than the centre of the chainstay. Which means, let me think about it... that my chainstay angle is less than I calculated it. Damn.

b) chainstays are tapered and the angle of interest is arguably between the seat tube and the top edge of the chainstay.

Yes, that thought crossed my mind too. I was sort of hoping that slimmer chainstays on a steel bike compared to big fat aluminium mountain bike (is that true?) might make it less crucial.

In addition to the issues mentioned the chain is meant to run at a specific range of angles through the FD and this can affect shifting too.

Yes, I'd sort of imagined that. But couldn't work out in my mind what the effect of chainstay angle different to the spec would have, hence...

Bottom line is that you most often don't know for sure until you try.

Sounds about right.

Or buy that Alivio for not-very-much-money.

Thanks.

[Valbrona]

And I thought I was a bit of a perfectionist who obsesses over the minor details of his bike ...

[mikeymo]

And I thought I was a bit of a perfectionist who obsesses over the minor details of his bike ...

You're right. I don't suppose it matters if the front derailleur collides with the chainstay.

[Brucey]

on reflection I would suppose that the most significant effect of the chainstay angle is to affect the rated capacity of the mech before the thing hits the chainstay. The angle the chain runs at through the mech will vary (with sprocket size) by much more than three degrees anyway.

If the tail of the FD is set back about 15cm (~6") behind the BB, three degrees difference in chainstay angle potentially makes ~8mm difference in the minimum height. This is equivalent to ~4T capacity/chainring size. Obviously there will be a margin of error built into the specs to allow for chainstay thickness, amount of taper etc. Anyway the likely outcome is that if you use a "-6" mech on a touring frame with 700C wheels, you will quite possibly run into trouble if you try and run the smallest rated chainring sizes. Unfortunately this is exactly what touring cyclists often want to do.

An estimate of the angle change with different sprocket sizes (12-32T, say) suggests that the effect of this on the angle the chain runs through the mech is about twice as great as the effect of 3 degree difference in chainstay angle with frame design.

cheers

[PH]

And I thought I was a bit of a perfectionist who obsesses over the minor details of his bike ...

I inadvertently ran the wrong front mech when I moved the entire drivetrain from an MTB to hybrid, I'd assumed they were all the same. It never shifted well, LBS has several goes , then someone on a club run pointed it out, changed it and it was instantly perfect.
Sometimes the minor details are important...

[MartinC]

My experience is that if you're using an MTB mech on a frame with 700c wheels the version for the narrower chainstay angle is better 'cos there's less chance of the cage fouling the chainstay when on the small ring. The problem is that MTB front mechs come in a variety of options but retailers will tell you which cable pull or clamp version you can order but not which chainstay angle version they're supplying.

[mikeymo]

And I thought I was a bit of a perfectionist who obsesses over the minor details of his bike ...

I inadvertently ran the wrong front mech when I moved the entire drivetrain from an MTB to hybrid, I'd assumed they were all the same. It never shifted well, LBS has several goes , then someone on a club run pointed it out, changed it and it was instantly perfect.
Sometimes the minor details are important...

Thanks. Good to hear that these things do sometimes matter, and that I'm not being a "perfectionist" by considering factors which might stop the bike working well.

[mikeymo]

Anyway the likely outcome is that if you use a "-6" mech on a touring frame with 700C wheels, you will quite possibly run into trouble if you try and run the smallest rated chainring sizes. Unfortunately this is exactly what touring cyclists often want to do.
cheers

Yes, that's what I thought - "this will only matter at the extremes". But the extremes is what I'm looking at.

Though Shimano usually did a 63-66 version of each MTB FD, it's just getting one that's difficult, old.

But Alivio FDs are available with 63-66 chainstay angle. What's the quality like of them? You said once that you thought FDs was the part of Shimano's line up where quality varied least. So that's probably the way to go.

Though 50% of this isn't to do with ultra low touring gears. But flat bar/trigger shifter conversion.

Thanks.

[axel_knutt]

And I thought I was a bit of a perfectionist who obsesses over the minor details of his bike ...

I inadvertently ran the wrong front mech when I moved the entire drivetrain from an MTB to hybrid, I'd assumed they were all the same. It never shifted well, LBS has several goes , then someone on a club run pointed it out, changed it and it was instantly perfect.
Sometimes the minor details are important...

Dawes built my Horizon with a wrong chainstay angle front derailleur. It worked (or the bikes wouldn't have sold, obviously), but it wasn't as smooth and slick as a correct one. I think most wrong derailleurs get fitted by people who don't know there's a difference, when I last bought one the guy at Spa Cycles didn't know there were different versions until I pointed it out.

[axel_knutt]

Though Shimano usually did a 63-66 version of each MTB FD, it's just getting one that's difficult

When I bought my first one in 2007 I got it from Roseversand, because CJ told me that Madison weren't importing them into the UK.

[mikeymo]

And I thought I was a bit of a perfectionist who obsesses over the minor details of his bike ...

I inadvertently ran the wrong front mech when I moved the entire drivetrain from an MTB to hybrid, I'd assumed they were all the same. It never shifted well, LBS has several goes , then someone on a club run pointed it out, changed it and it was instantly perfect.
Sometimes the minor details are important...

Dawes built my Horizon with a wrong chainstay angle front derailleur. It worked (or the bikes wouldn't have sold, obviously), but it wasn't as smooth and slick as a correct one. I think most wrong derailleurs get fitted by people who don't know there's a difference, when I last bought one the guy at Spa Cycles didn't know there were different versions until I pointed it out.

Interesting. Thanks for this revealing information. It's reassuring that, far from being a "perfectionist", I'm paying attention to something that might actually make a difference.

[mikeymo]

Though Shimano usually did a 63-66 version of each MTB FD, it's just getting one that's difficult

When I bought my first one in 2007 I got it from Roseversand, because CJ told me that Madison weren't importing them into the UK.

Right. If I buy a new Alivio 63-66, I'll look abroad if unobtainable in the UK.