28T TA chain ring question?

[AlanW]

I have decided to replace my inner chain ring on my triple for a 28T from a 30T, this is just to give me a slightly lower gear while keeping my 10sp 12/23 cassette.

On the OE Shimano chain ring there is a very tiny pip/lug that denotes the orientation that it should be installed at, ie the tiny lug is in line with the crank. Now this tiny pip/lug is situated slap bang in-between two of the mounting holes. I'm guessing that this is quite critical in order for the chain to mesh with the adjacent chain ring teeth, as you shift either up or down.

On this new TA chain ring it has tiny arrow stamped on it, which would suggest to me that this is serves the exact same purpose as the tiny pip/lug on the OE Shimano ring? However, one slight problem though, this arrow is directly adjacent to a mounting hole, and not in-between two mounting holes as the OE one is?

Therefore it is impossible to mount it with the arrow in line with the crank?

So in an effort to match the teeth on the two chain rings to give me a better understanding as to how it should be mounted, I then tried laying the TA ring on top of the OE Shimano one. While the teeth are spaced the same as you would expect, they are not machined in the same relationship to the mounting holes as the OE Shimano one, and I have tried moving the TA around to the five mounting hole options (if that makes sense?) but the teeth will not align with the mounting holes exactly the same as the OE chain ring?

Never the less, I have mounted it and have done a short ride, while it changes okay, it is no where near as sweet and precise as when the OE is fitted. That said, I have also tried rotating the chain ring on the bolts and you can certainly make it change better or worse just by mounting it in a different orientation.

Any thoughts or suggestion as to why they would put this arrow on when it is physically not possible to mount it this way?

Plus, on the TA ring the five mounting holes have a recess machined on one side, so does this recess fit over the lugs on the chain set, or are they recessed to allow for the pan head Allen screws to sit flush against the chain ring?

[gaz]

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[AlanW]

Making a few guesses but here goes....

The TA ring in question is going to be a 74BCD Zephyr. The TA Zephyr chainset is no longer manufactured. When it was made it was similar in style to the Spa XD2, i.e. the crank arm was also a spider arm. The arrow would have pointed at the crank arm.



As for the recessed side, definetely recessed for the allen bolts.

I'm surprised it doesn't shift cleanly.

That's interesting info, thanks.

This is the item in question - HERE If I have read your post correctly, then this item is probably old stock then?

[meic]

It wouldnt have to be old stock as these rings are used on lots of other makes of cranksets as replacements for the originals.

The recess is for the bolt heads, as you say, on my cranksets as it is not large enough for the lugs to fit into but the TA may be different.

I find no difference when rotating my inner ring, logic says that a 30t will be identical in all 5 positions but a 28t can be different.
If you believe it makes a difference the best thing is to put the arrow at 180 degrees to where you would like it to be (but it will not go) as the rings are probably symmetrical in that way.

[AlanW]

This is my set up, the chain ring is from the Zelito range according to the packet?



The black permanent marker pen line is the best position for the chain ring after trying all the other options. There is clearly one position that is the optimum position and this is supported by the fact that when then bike is in the work stand and you rotate the cranks slowly while up shifting the chain drops directly on to the adjacent teeth, the same applies when dropping down. In the other positions the chain can be up to a whole tooth out out of mesh, so while it will finally located onto the teeth after the change, the chain has to adjust itself first.

I would therefore imagine that down shifting while still having a load (even a slight load) to the chain would result in a delay in the gear change and possible chain derailment?

[BigG]

The simple answer to your question is that the pip and the arrow are both designed to be in line with the crank, either directly under it or on a line extending from it. If you think about it, the profiling of teeth for good changing is repeated twice each revolution as the power input almost stops at both top and bottom dead centre. Whether the pip or arrow is on the same side as the crank or 180 degrees from it depends on the disposition of the crank mounting holes; that is whether one of them is actually on the crank or whether they are placed either side of the crank.

[andrew_s]

The simple answer to your question is that the pip and the arrow are both designed to be in line with the crank, either directly under it or on a line extending from it.

This.

However, for the inner ring on a triple, it doesn't matter at all where the mark goes as all the teeth are full size with no ramps or pins (which are there to aid shifting onto or from a smaller chainring).

On the subject of TA chainrings, the individual teeth generally don't line up. If you take a dozen 52T rings and thread them onto 5 rods, the teeth will not all be in the same place (according to George Longstaff, so possibly no longer the case).

[CREPELLO]

The simple answer to your question is that the pip and the arrow are both designed to be in line with the crank, either directly under it or on a line extending from it.

This.

However, for the inner ring on a triple, it doesn't matter at all where the mark goes as all the teeth are full size with no ramps or pins (which are there to aid shifting onto or from a smaller chainring).

I'm glad you said this. I was beginning to think I'd mis-understood how chainsets and their rings work. I would suggest that chainsets aren't really timed in any real sense at all, like cassettes supposedly are.

On the subject of TA chainrings, the individual teeth generally don't line up. If you take a dozen 52T rings and thread them onto 5 rods, the teeth will not all be in the same place (according to George Longstaff, so possibly no longer the case

And even if they were all in line, I don't think the chain could be designed to fall perfectly onto the teeth below as inferred above. You could design it for one size set of rings, but what about if one ring is swapped out for one slightly smaller. It's teeth are now going to be out of line . But never been a problem for me.

[AlanW]

Sorry but following on from the tests that I conducted I have to disagree, there is certainly a sweet spot for the chain ring to be mounted in. You can only test this with the bike in work stand and slowly rotate the cranks while shifting up, dependant on what position the chain is picked up, dictates the position that it is then dropped onto the adjacent chain ring.

It certain positions, when the chain was picked up it dropped it right on top of the teeth on the adjacent chain ring, therefore the chain had to rotate slightly before it mesh correctly. In the best position the chain dropped right in mesh with no additional rotation required.

The position of the teeth on the TA ring are not in the same position relative to a OE one with regards to the mounting holes.

[CREPELLO]

You may well be right and I have noticed something like a tiny delayed engagement whening dropping down a ring sometimes. However, this sweet spot cannot be easily designed into the chainset timing if you're using custom rings. In some combinations perhaps. But it would never put me off choosing custom rings.

Good front shifting must be more than just down to the timing of the rings. I've just swapped front mechs on a bike I was having trouble with front shifting. The new mech has fixed it for me, timed teeth or none. I also have one bike that uses a middle ring without ramps and pins, yet it changes sweetly. Now why is that? Perhaps it is ring timing...I don't know.