Shimergo

For discussions about bikes and equipment.
mas051

Shimergo

Postby mas051 » 22 Dec 2004, 11:31am

Hi

The bikes and bits section of the site has plenty of information regarding addaptation and combinations of rear derailleur parts but what about the front mechs, is there any information available about combining shimano shifters and other peoples front mechs or visaversa

matthew

CJ

Re:Shimergo

Postby CJ » 22 Dec 2004, 1:06pm

So what exactly do you want to know?

Chris

Re:Shimergo

Postby Chris » 22 Dec 2004, 8:49pm

Adding to front mech's, whats the advantage of 3 chainrings with small gaps between the number of teeth between them, and two chainrings with a large difference in teeth between them, considering the ratio's over-lap. Other than of course a greater selection of gears!

CJ

Re:Shimergo

Postby CJ » 23 Dec 2004, 2:32pm

Chris asks: "what's the advantage of 3 chainrings with small gaps between the number of teeth, and two chainrings with a large difference in teeth, considering the ratio's over-lap?"

I don't call 10T a "small" gap. Much more than that requires a simultaneous rear shift in order to avoid an uncomfortably large jump in cadence and torque. Average riders are insufficiently coordinated to make simultaneous front and rear shifts, so they do one or the other depending on whether they're responding to a big or a small change in gradient. Just about the biggest cadence jump most people will tolerate is something like 40%, which is about what you find on all off-the-shelf triples.

But the human engine works better with smaller steps than that between gears. Most people are happy with 15% jumps, road-racers seem to want them as small as can be, but will generally accept 9%. Even with a 10T gap, co-ordinated rear shifts of one sprocket will generally be required in order to deliver a gear not more than 20% different. With larger gaps or a racer's desire for fine tuning, a double or even a triple rear shift may be required, but that takes a lot of skill to do well and is easy to mess up – in which case you can lose so much speed you may wish you'd not bothered!

So: super wide-range doubles are a possibility, but demand more shifting skill than most people can muster. (I had one once: a home-made 34,54 with 1/8 chain and freewheel on a 1930s tandem. By building shifting ramps between them – 10 years before Shimano had the idea – I made something that worked pretty well whilst acquiring some impressive double-shifting skills!)

Another limitation on the gap between the top two rings of a triple, is that a bigger gap puts the front mech too far away from the middle ring for good shifting. Think about it: the inner pair of a triple are a double with a badly-fitted front mech – way too far up the seat tube!

In the past, before Shimanagelo sculpted the cages of front mechs in such a clever way, the only decent shifting triples took a form that we called "half-step and granny ring". The top two rings were only some 4T different and were used to slice the 20+% jumps between sprockets on a wide-range 5-speed freewheel (14-32T was typical on touring bikes), into bite-sized pieces. The granny ring could be as much as 20T smaller than the middle, to get the range. Of course you didn't have any finer tuning when you were down on the granny, but one doesn't seem to need it so much when grinding up steep hills. The only snag was that "dropping into the void" feeling when one knocked the chain off the middle ring. Some of us tourists acquired the knack of thumbing the left lever forward whilst hooking two fingers around the right and simultaneously clawing the rear mech two gears higher. All this with down-tube controls, no indexing and whilst steering a heavy load uphill with the other hand!

If the kind of cycling you do doesn't demand such a range of gears, of course it's possible to get by with a double. And now that a rear cluster may itself deliver a 300% range, in manageable 15% steps thanks to double the number of sprockets, it's not just a matter of getting by! For the maximum gear range however (up to 600% is possible and you can certainly use that on a touring bike – especially if it's a tandem or recumbent): a triple is the way to go.

Chris

Re:Shimergo

Postby Chris » 24 Dec 2004, 7:23pm

Is the reason why racer's have a double chainring more commonly because the riders are more likely to be able to do co-ordinated front and rear shifts, so the triple chainring is just an added weight, making the bike slower?

mas051

Re:Shimergo

Postby mas051 » 28 Dec 2004, 7:51pm

CJ

the reason for the original question was that I have just purchased a set of STIs but have a Suntour front mech. I have discovered therefore that as for the rear mechs different manufactures have different pull ratios for the cable to movement. As with the rear mechs are similar compatibility tables or 'bodges' available to make components from different manufactures work together?

matthew

Si Davies

Re:Shimergo

Postby Si Davies » 30 Dec 2004, 9:43pm

>>Is the reason why racer's have a double chainring more commonly because the riders are more likely to be able to do co-ordinated front and rear shifts, so the triple chainring is just an added weight, making the bike slower?

No, racers are more likely to be strong enough not to need a 'granny' ring up front. Many of them also think that the increased Q factor is a bad thing (although it doesn't seem to bother MTB racers).