TromboneAl

I'm reinstalling a 39T Shimano chainring on a 1992 Trek 2300 double. There is no triangle mark or other mark on it (just the brand name). How do I align this?

Thanks.

FBinNY 

If the outer ring isn't gated (cut teeth) no specific alignment is needed, since shifting will happen anywhere.

OTOH if the outer ring has a shift gate, wrap a length of chain around the ring and lead it over through the gate (as if shifting), and down to the inner ring. Of the 5 possible positions, you want the one where the teeth line up so the chain drops in best (ie. the rollers line up with the troughs between the teeth).

Before Hyperglide, I used to set team cranks up so a chain shifted with the right pedal coming over the top lined up for smooth engagement. Then when riders shifted at the right time they wouldn't get that momentary slippage as the chain looked for engagement.

TromboneAl

Thanks, Fibby.

What are "gated" chainrings, and "shift gates?"

The new Shimano 39T chainring that I put on my bike has a triangle and includes instructions for lining it up. All the teeth are rounded and the same.

FBinNY 

Shift gates are those 1-3 cut down teeth you see on outer chainrings. The gate makes it easier for th chain to derail and move sideways without having to climb up over the top of the teeth. Gates make it easier to shift under load, and usually ensure that shifts happen at that specific location, so that the timing where the chain meets the inner ring is more predictable.

Gates are what puts the Hyper in Hyperglide -- or maybe it's the glide.

Anyway, innermost rings never have nor need gates because the chain is always lifting up off the ring (or dropping from the top) during shifts.

HillRider

A 39T chainring intended for a double crank won't have the triangle alignment mark. It's rotational orientation relative to the big chainring isn't important as there are no shaped teeth or pins. For a 9 or 10-speed ring, the Shimano name, tooth count, etc. engraving should face inward toward the frame. Current double cranks have the shaped teeth, etc. on the big ring to improve shifting to and from the smaller ring. The smaller ring doesn't have or need them.

For a 39T chainring intended for triple use, the triangle should be aligned with the drive side crank arm as the shaped teeth are there to improve shifting to and from the granny.

FBinNY 

Really? I'm not challenging, just asking. My Shimano specific knowledge wouldn't fill a thimble, but in the Campy world, not only do inners have timing mark (a diamond) but the outer is specific to the inner it's made to work with. Timing is important so the chain transitioning meets the new chainring phased so it engages smoothly. Kind of the derailleur version of synchromesh.

ThermionicScott 

If the inner ring doesn't have a triangle or bump that indicates alignment (and the teeth are all uniform) then you can mount it any way you please, and have the benefit that you can shift the ring around periodically to even out the wear.

FastJake

+1

All the Shimano inner double rings I've come across have been the same all the way around, although some did have the timing mark.

FBinNY 

Yes, there are never gates on a ring intended to be the innermost. Gates aren't needed because the chain doesn't shift sideways (no place to go) but lifts off radially. However, timing is still important for best shift performance (hence the timing marks (if they have them).

As I said, I don't have Shimano specific knowledge, but I can't see that Shimano wouldn't provide for correct timing with companion outers.

The timing issue is one reason that cassette systems replaced the threaded outer sprocket with a splined sprocket and lockring, which was more expensive to produce. There's no way to orient a threaded sprocket, the old cassettes were like old synchromesh in cars before 1st gear had sychro too.

Al1943

HillRider is correct. The lettering and tooth count number on Shimano inner rings face left, toward the frame.

FBinNY 

This had me bugged. Shimano introduced Hyperglide front, and that would call for timed rings. So I decided to check the tech docs, and sure enough, the inner ring does have a timing mark. (at least in Ultegra, which is the one I checked).

I don't know if others have timing marks, but the lack doesn't mean that timing doesn't matter. Lacking a guide, I'd either time them with the chain as I described earlier. Or if Shimano is consistent about marking near a bolt hole, put the mark near the arm opposite the crank (not the timing mark which goes to the crank arm).

HillRider

Shimano sells their double chainrings in matched pairs, for example 52/42, 53/39, etc. The outer ring's shaped teeth, pins, etc are supposed to be specific to the specific inner ring. The inner ring's teeth are all the same and I can't see how a rotational orientation would matter. Aligning them a specific way can't hurt but I don't see the benefit.

The inner rings, particularly 9 and 10-speed, have to have the engraving face inward to keep the gap between the chainrings proper as the inner ring's teeth are offset. My son-in-law once installed a new 39T inner ring "backwards" (engraving to the outside) and the shifting was dreadful. Turning it around solved the problem completely.

Looigi

The difference is in the relative phase of the inner ring teeth with respect to the big ring teeth and any shifting enhancements the big ring might have such as shaped teeth, lift pins, ramps, etc..

For example, when shifting to the big ring in a given position of crank rotation, the the links may line up properly with the teeth on the big ring. At a different position of rotation, the pins rather that the links will line up with the teeth of the big ring and the chain will ride on top of the teeth instead of in them. One way ramps and pins improve shifting is by lifting and aligning the chain to minimize this happening.

FBinNY 

I'll try to explain, though I wish I had some decent photo's which would help.

During a shift the timing of the chain is set by the chainring it's still attached to. So as you shift the chain leads out through the shift gate, and down toward the smaller ring, When it reaches the ring the rollers will always be in a very specific place since it's still attached to the larger ring.

The timing mark ensures that the inner ring's teeth will be oriented to slide into the chain between the rollers for fast smooth engagement, rather than having the teeth hit the rollers and not engage immediately.

Timing only matters if shifts occur at the same place, since with differing numbers of teeth the relative position of the teeth changes andonly repeats in certain places. But with gated outer rings, a properly timed inner puts the "glide" in xxxxxxglide.

BTW- The same thing happens on the cassette, where the shift gates are timed to be in the right place so the shift happens smoothly, rather than skipping before engaging. That's why all cassettes with gated shifting have a "king" spline to ensure all the sprockets are timed correctly to their neighbors.

lennyk

Do all modern chainrings require this trial of 5 possible times?

FBinNY 

No most have timing marks of some kind. In any case, we shifted just fine for decades before hyperglide (or whatever name) so it's not an issue of necessity as much as optimization.

If you mount a ring randomly and are happy with the smoothness and fast engagement through the shift, you're fine. OTOH, if you find issues of slippage on engaging, or similar issues, then consider that poor timing may be a factor.

HillRider

OK, now I'll try to explain.

For the big chainring you are correct, the shift gate location, etc is important but the rotational orientation is easy to determine since the chain catching pin is always placed right behind the crank arm. No ambiguity there.

For the inner chainring of a Shimano double, all of the teeth are identical in height and shape and there are no pins, ramps, gates, etc. on these rings. None. Zero, Nada. So, except for having the offset in the proper direction (i.e. engraving facing inward) the rotational orientation doesn't matter.

The middle chainring of a triple crank, definitely does have shaped teeth, pins, etc to assist shifting to and from the granny in the same manner a double has to shift to and from the small ring. These chainring certainly have an alignment index mark.

The granny ring of a triple is also "flat" with no shaped teeth, pins, etc. for the same reason the small ring of a double has none. There is no need to shift down to or back from a smaller ring.

FBinNY 

Yes, you are 100% right ------ as far as you go. HOWEVER consider that the ring has 39 teeth and 5 bolt holes. Since 39 doesn't divide by 5 it means that the teeth are in different rotational orientation at each bolt hole. That means that the phase (or timing) relationship between the teeth of the inner ring and those of the outer ring depends on which bolt hole is at 12 o'clock.

With me so far. Now as the chain leads through the shift gate and off the outer ring it's still attached, so the rotational position of the roller as it drops to the inner ring is predictable (with respect to crank angle). So the inner ring should be timed to meet the chain properly oriented (rotationally) for the chain to slip right on (the glide in hyperglide).

If you drive a car with standard transmission, it's comparable to dropping engine speed to match revs on upshifts, so the gear can slip right in.

I wish I had a good photo showing this, but you can see it on your own bike. Shift to high, then slowly downshift the front, advancing the crank until the chain (on it's secant) is just about to engage the inner ring, and STOP. Now look at whether the teeth line up ready to slide into the chain, or if they're about to bump into the rollers. Imagine if the teeth were rotated forward or back a fractional pitch (1/10") and think about whether they'd engage more or less smoothly.

As I've said a few times. This isn't critical, it just makes shifts smoother.

BTW- I took a photo showing what I'm describing. Do I need to belong to a photo sharing service to upload? Or someone might tell me how it's done.

HillRider

OK, I follow your reasoning and it's likely correct. However, as I mentioned, the inner double chainrings I have do not have any rotational indicator (i.e. the little triangle) on them so it's a guess how they to be installed. Maybe the bolt hole with the tooth count and Shimano name engraved around it goes 180° from the crank arm the way it is on the one triple I have.

BTW, Shimano's installation sheet for the new 105 cranks (FC-5700 and 5703) does show a triangle mark on the inner ring of the double so the older chainring ones I have may be outliers.

Now, a question. What about a 40T or 45T chainrings where the tooth count is evenly divisible by 5? Are they exempt from rotational position effects?

FBinNY 

Timing only comes into play with gated shifting whereby the exact place where the shift starts is predictable. Otherwise there's no basis onwhich to time anything. It's the same on cassettes or freewheels where the gate on each sprocket has to be in the right place with respect to the neighboring sprocket for a smooth shift. Before gated shifting (hyperglide) it was random, so there was nothing to time.

No they're not immune to the effects, except that if they're not timed right there's noting you can do about it. And if they are timed right, they would be in any orientation.

Now, can I upload a photo directly from my computer to BF, or do I need a photo sharing service?

FastJake

Gee, I never thought about the "timing" of my inner chainring, relative to the outer ring. Now I'll take a look at the inner ring and make sure it's correctly positioned.