How does Hyperglide work?

What is hyperglide?

"Hyperglide" is the special shape of teeth on sprockets of a bicycle rear cluster. The latter can be either cassette or freewheel; I'll use "cassette" further here, although anything said here applies to freewheels, they are no different for the hyperglide principle. As Sheldon Brown says on his excellent bicycle tech page about Dura-Ace, it was Shimano who first introduced Hyperglide system in 1989.

Individual teeth are shaped differently, and neighbor sprockets are matched against one another so that the shift between them is much easier. Hyperglide mostly cares about downshift, i.e. shift from the smaller to the bigger sprocket. The opposite direction, upshift, is less troublesome; those of us who still remember (or use!) non-indexing shifters with non-HG rear clusters, certainly know that it is easier to "drop" the chain than to "climb it up". To upshift, it suffices to derail the chain from the bigger sprocket, and it falls on the smaller one. But when downshifting, the bigger sprocket may simply not allow the chain to engage, and thus to completely disengage from the smaller one. Often when downshifting with non-index systems, one has to overshift the lever to some extent, and then when the chain moves to the bigger sprocket, return the lever back to compensate.

This need is eliminated by the hyperglide invention. First, no overshifting is needed any more, and second, the chain almost does not jump, as it engages with the larger sprocket before it disengages from the smaller one.

The picture below demonstrates, how the chain is engaged with two sprockets simultaneously during a hyperglide downshift. Upper part lays down on the second cog, then two chain links are "hanging" between cogs, and then the link at about 8'o clock position is already caught by the first (largest) cog.

Hyperglide downshift from second to first gear

Hyperglide came at a cost. Before, weared-out sprocket could be flipped over and live a full second life. This is not true for newer hyperglide sprockets, and their teeth are noticeably lower. As a result, they wear out faster. But in fact probably the most significant contributor to faster wear-out is smaller thickness of sprockets, which is not caused by the hyperglide. If you want to have ten of them in the rear, they can not all be as thick as in 5-speed times. And shifting across 10 cogs without hyperlide is probably a no-option - I don't think anybody even tried that.

So, how does it work?

Another practical question is, how to know if two sprockets are "hyperglide-compatible"? Say you want to build a custom cassette or buy separate replacement sprockets. Which to chose for the best result?

When the chain shifts from one sprocket to another, chain pins will not necessarily fall exactly between teeth of the new sprocket. New teeth may appear just under the chain pins, or otherwise not exactly in the middle between them, as you see on the picture below.

The chain, forced to a new cog in just an arbitrary position.

The chain will, of course, finally engage with the new cog, but:

• You will feel a chain jump when it completely moves to the new place
• Most important: You'll most likely need to overpull your shifter a little bit - otherwise the chain may just scratch the larger cog and never engage with it.

To overshift would be difficult with indexed shifting! Hyperglide's principle is not to move the chain further than its future position. The key is sprockets having places specifically designed to derail the chain. There are one or more "key points" between each pair of sprockets, and the downshift happens only there. Sprockets are aligned against each other in such way, that in these key points their teeth make a perfect match for the chain. When the chain derails from one sprocket and moves towards the other, the new teeth are exactly situated in the position to accept the chain without any jump.

Look at the sprocket in the picture below. Call the side, which is (partially) visible on the cassette, "front" and the other side "rear". Most noticeable irregularity is one or more stamped thinned areas. There are five of them in the picture below. Call them hyperglide dents.

Five hyperglide dents

I intentionally selected a very widely spaced cassette: it is 11-32 8-speed. Here is how many of HG dents each sprocket has there:

It is possible to make one observation by looking at these numbers:

Investigate your cassette - does this hold true?

For the chain to engage with the bigger sprocket smoothly, the teeth of the smaller sprocket must be placed at the same distance before every HG dent of the bigger sprocket, so that they "pass" the chain prepared for the engagement. This goes well in line with our numerical observation! If the bigger sprocket has N teeth more than the smaller one, then there are exactly N places where distances between tooth pairs coincide. For example, if you take 18 and 21 tooth sprockets, select one tooth from each and match them, there will be 2 more (making altogether 21 - 18 = 3) pairs of teeth which will match. The scheme illustrates that:

18 and 21 sprockets: 3 places, where teeth align

(I coded this picture directly in PostScript, here is the source.)

For a smooth shift, we do not want teeth to "match", but to be spaced with some fixed distance, and sprockets are aligned to achieve that. This does not change the fact that the number of such places is equal to the tooth difference.

Now it is clear that HG dents are exactly the places where the chain is caught by the bigger sprocket. Look at the shape of the HG dent: it repeats the shape of a chain link. So, the job of a HG dent is to let the chain, which is already pressed to the bigger sprocket by the derailer, to move a bit more there. The link which presses the larger sprocket is not engaged with either and hangs in between.

The next link after the pressed one will engage with the bigger sprocket... in 50% of cases.

There are "external" and "internal", or "wide" and "narrow" links in the chain. Either of the two can fall on the HG dent. Let's see what happens:

1) "Narrow" link falls on the HG dent. Then the next wide link engages with the sprocket.

2) "Wide" link falls on the HG dent. The next link is narrow. Narrow link never engages first (previous wide link is still too wide to let it to). It still hangs in the air, and only the second next, again a wide one, engages.

Case 1: narrow link falls on the HD,
next wide link (with "PC48" mark) engages. Case 2: wide link (with "PC48" mark) falls on the HD,
second next link (also wide, with "15" mark) engages.

Look now at the rear side of the sprocket. HG dents are not visible now, but you know where they are: there are no holes drilled in teeth with dents (probably because of reliability issues). Notice something special in teeth, which follow them?

Back side of the cog.
Hyperglide dents are under teeth with no hole drilled. Enlarged view: teeth after HD.
3 next teeth are special.

Sure: after each "HG tooth", there are three thinned teeth. The first one is shaped wedge-like, the next two are just some 0.5mm thinner. (By the way, I do not understand, why there are three of them: in the light of the pictures, only two thinned teeth are needed.)

Now you know what is required for the hyperglide shifts to happen:

Some questions

There are findings which I can not explain:

1) Some teeth are symmetric, others are somewhat slanted opposite to the rotation of the cassette. In particular, all HD teeth are slanted. What does this accomplish?

Different teeth of two identical cogs are aligned. Same teeth from another perspective.

2) Each tooth before HG dent is wedge-shaped from the front side. I can only imagine that this helps upshift.

3) Are there any provisions on the sprocket to facilitate chain disengaging? I do not immediately see any.

Brand compatibility (does such thing exist?)

Shape of HG dents is different between Shimano and Sram sprockets. (There is no sence to compare Shimano and Campagnolo sprockets, as they can not be mixed by fitting on the same hub.) They differ in shape of other, non-HD teeth as well. These differences are not critical for shifting though. There could be no cross-brand hyperglide at all, if they had different sprocket alignment with respect to the hub - we know that the teeth of the smaller sprocket have to be positioned on a defined distance from the HG dents of the bigger sprocket. Although the two manufacturers do not claim official compatibility, HDs are placed at similar spots in the two Shimano and Sram cassettes I hold in my hands now. If you have two similar-toothed cassettes from the two mainstream manufacturers, please tell me whether it holds in your case.

Speeds compatibility (no, it does not)

Sprocket thickness does not vary terribly much between different speed versions, at least not between adjacent ones. So from the spacing viewpoint, you can safely replace N-speed sprocket with (N +/- 1)-speed sprocket - your derailer will not feel the difference. But sprockets will not necessarily be hyperglide-compatible. There are some which are, but that is a matter of luck. Below are 8-speed and 9-speed 18T sprockets, both from the same sequence 16-18-21, but their teeth do not align at all.

8 speed and 9 speed sprockets: teeth do not align

Conclusion (what to look for)

Suppose you want to replace an individual sprocket - because it is weared out and you have a replacement, or because you are building a custom cassette (in which case the new sprocket will have different number of teeth). Now you know what to look for to make the upshift to this new sprocket "full-featured" hyperglide:

• If you are replacing a sprocket with another one of the same size: Align old and new sprocket as though they are both installed on the hub. To make for a perfect hyperglide, their teeth must aligh, and their HG dents must align.

• If you are changing for a different size: Suppose, for example, that you have a 12-25 cassette and you want to replace 23 and 25 with 24 and 28 to have more of a "bailout" gear. Your new 24 will take the chain passed from the 21. Fortunately, 24/28 pair comes from sequence (21, 24, 28), so the new 24 will catch the chain from 21, as it did on the original cassette. Compare your 21 and the 21 from the cassette, which 24 comes from (and with which the 24 "hyperglides" by design of the manufacturer). Installed on the hub, teeth of the two 21-s must align.

  If your new pair of sprockets will not be present in any industry-made cassette - well, you do not have any reference to check whether the shift will be hyperglide or not.

Final word (take it all easy)

Remember that people shifted gears without hyperglide for about half a century. So even if your sprockets are not HG-compatible, you'll still be able to shift, although not as smoothly. Maybe you'll need to "overshift" across the non-HG jump; in fact, almost all shifters do overshift the cable a bit when actuated and then release it to the indexed position, so you can just hold the lever until the chain jumps.

Konstantin Shemyak (konstantin@shemyak.com) / Cycling / How does hyperglide work