blackvans1234

I just got a new specialized secteur compact (yay!) and was on a group ride, and one of the riders was telling me about a double shift.

I got the concept (I think) but have some questions.


Warning: I will probably butcher the correct terms of shifting
12-28 cassette with a 50/34 compact

So the basic idea of the double shift is to shift the rear in the opposite direction of the front. This will compensate for the jump in cadence / pedaling effort when shifting the chainrings?

Assuming all of this, my main concern is ''cross chaining''. Is this okay within limits? what's the deal?

Mathematical logic would dictate that on an 8 speed cassette, anything past the 4th gear would be cross chaining.

Lets say I need more speed (cadence too high)
Currently, I will go to the fifth gear (slight cross chain?) before shifting the front from the 34 to the 50. At the same time, I would shift from the fifth to the fourth (slightly easier, and larger gear). This will help counteract the large change in effort. Is this okay to do?

The guy on the ride was talking about shifting twice or even three times [sounds like bologna for an 8 spd cassette] in the opposite direction.

Please bestow some knowledge upon this new rider.

Thanks!

merlinextraligh

The point is to ameliorate the big jump in gear ratios when you make the front switch..

For example, if you're in the 50/21 combination, and you shift just the front chainring, at 85 rpm, your speed is going to drop from 16mph to 11 mph.

In most cases, that's a bigger jump than you want; and your speed will drop dramatically, or your cadence will have to jump dramatically.

If you shift 2 cogs down on the back at the same time you go to the 34 in the front, you end up in the 34/16, and you get a smaller change in gear ratio, and s your speed only falls to 14mph.


You can play with this to figure out your desired shifting pattern:

https://www.gear-calculator.com/#

merlinextraligh

And don't worry so much about cross chaining. On most modern drivetrains, you can crosschain a double, all the way across without any problem, other than a bit more noise, and wear.

Even if you're fastiduous about minimizing friction, and wear, you certainly can use 6 of the 8 cogs, with each chainring (i.e the 6 biggest with the small front ring, and the 6 smallest with the large front chainring.)

corrado33

Both my friend and I do this. When I switch to the smaller chainring I also shift in the opposite direction in the back so my legs don't go crazy. Actually I shift the back first and right before it finishes shifting I shift the front. It works well for me and is only marginally longer than a standard shift.

My friend has to actually shift three cogs down in the back to compensate. Different gearing I guess.

generalkdi

Best not to do it at the exact same time, I had a couple chain drops using this, so now I space it by like half a second.

MattFoley

Crosschaining is really only an issue for big-big or small-small combos. Pretty much anything in-between will be fine in the course of a normal ride. WRT the "double shift", I think I understand what your saying, and yes, I occasionally will simultaneously downshift from big ring to small while upshifting one cog....this generally happens when I'm nearing the big-big threshold and want to maintain cadence while jumping into an easier gear without getting into a big-big combo. I don't know that I understand about "shifting twice or even three times [sounds like bologna for an 8 spd cassette] in the opposite direction" as you mention, except to note that Shimano STIs (and I think Campy and SRAM too) allow you do downshift multiple cogs in a single motion (for Shimano, it's up to 3 cogs).

RollCNY

With an 8 speed, I would only sweat cross chaining in your 34-12 combo (small small). My 8 and 9 speed systems always seemed far more forgiving than my 10 speeds on that subject.

The double shift makes sense when you think of it in these terms: a front shift is a 30% change in gear ratio, a rear shift is 10%. So if you shift the front only, you have a 30% drop or jump in cadence for your current speed. If you shift the front, and bang out 2 gears in the back, it becomes a 10% overall shift (-30 front, +20 back). These are all approximations, and with an 8 speed 12-28, your rear shifts may be closer to 15%.

caloso

Conversely, if I'm in the big ring and approaching a hill, I will shift to the small ring but also to a smaller cog in the back. I will be in approximately the same gear ratio and can keep the same cadence, but have now accessed a the lower range of ratios.

merlinextraligh

If you look at the tool I linked and take the 50/21 example, and you want to get to the little ring to access the lower gear ranges, but maintain your cadence for the moment as Caloso suggests, you need to 34/14, which depending on your cassette would typically involve moving 3 cogs in the back.

merlinextraligh

JTGraphics

This is not just for compact crank setup in general its about 2-3 on rear before a front ring change.
Its only to keep your cadence around the same speed once you figure out your setup it will make sense to you.

merlinextraligh

only difference is that the most common double setup is a 13 tooth jump, while the most common compact setups are 16 tooth jumps, which make the effect a bit more pronounced.

Nachoman

It shouldn't be a problem if your derailleurs are properly adjusted.

redlude97

This was the most annoying part of a compact. Switching to a 36T ring enabled me to only need 1-2 shifts in most cases

Leinster

The only times I would front shift without a compensatory rear shift would be dropping the front at the foot of a steep climb, or swinging into the big ring going over the top. Other than that, it's usually a case of going from 39x14 to 53x17 (picking up speed on the flat) or from 53x21 to 39x17 (hill getting gradually steeper) (8spd 13-26).

caloso

Which is why 10-speed Campy is awesome. I can hit both thumb buttons and instantly dump the chain ring and 3 cogs.

Drew Eckhardt 

5. I made the compact mistake (50-34x13-23 9 cogs had the same range and spacing as 50-40-30x13-21 8) where 34x15 was the next gear after 50x21. Going the other way from 34x14 to 50x19 with a 3 cog limit did take an extra shove of the right front paddle. Some terrain called for multiple shifts like that for each mile traveled.

Drew Eckhardt 

With properly adjusted derailleurs I've never dumped a chain to the outside and only bounced the chain off the catcher back onto the small ring a few times cornering on bumpy roads, at least one of those after the lower derailleur pivot gunked up and needed cleaning to immediately tension the chain after a shift.

There's a lot of latitude in how you optimize (height, rotation, their interaction with tension/limit screws) a modern front derailleur so the bumps and curves hit where you want them to and you have the minimum practical chain clearance once it settles on a ring. I find it technically more involved than building wheels although obviously it only takes a tiny fraction of the time.

Homebrew01

Correct, and similar going the other direction. Shift into the big ring and with 1 push shift as many cogs as you want.

DaveSSS 

The difference in shifting a 53/39, compared to a 50/34 is the need to make one more cog shift to compensate for larger chain ring difference. If you make a 2-cog shift after a change between the rings with a 53/39, it will require a 3-cog shift with a 50/34.

IP Freely

I don't think I've ever shifted the front without simultaneously shifting the back as well. 2-3 cogs on the back depending on the terrain. I run a 50-34 compact up front and an 11-25 in the rear.

blarnie

Crosschaining is nonexsistent. Shift at will and enjoy.

sfrider 

No problem; I do it all the time, although I do avoid cross chaining big-big and small-small. But it's more to reduce chain and cog wear than any dramatic rear wheel asplosion risk. And it's also more frequent when rolling onto a hill, so going to the 34 than to the 50. If the front drops the chain more frequently than acceptable, get a K-Edge catcher.

thump55

OP- as far as bike skills go, double shifting is way down on the list of things to learn, especially with an 8.

You really should be focusing on more important things, like wheelies.

Looigi

Mmmm. The issue with shifting F/R simultaneously is that the chain can go quite slack when going to a smaller ring and smaller cog which increases the potential of dropping. When shifting big ring to small, it's best to keep up a small amount of pedal force so that the top run doesn't go slack and and get whipped past the small ring by the spring force of the FD. In my experience, this seems to be more of an issue with compact cranks and triples, where there can be more distance between the DR cage and the small ring.