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swapping chainrings on Shimano triple?(11 posts)

swapping chainrings on Shimano triple?Becky
Mar 28, 2003 3:57 AM
I'm currently riding a Shimano triple (30/42/52) and am toying with the idea of swapping the 42 for a 39 to give myself some slightly lower gearing in the hills without having to drop to the granny gear.
A) Is this possible?
B) Is this a reasonable idea or is it just a waste of money and time?
Thanks for your help!

Ultegra, or what? nmDougSloan
Mar 28, 2003 7:34 AM
Tiagra, actually (nm)Becky
Mar 28, 2003 7:52 AM
Mar 28, 2003 8:10 AM
You can use a standard 39 tooth ring (even an aftermarket 38 tooth), as it's a 130 mm bolt circle, but you'll not have ramps and pins on the inside of it, which might hurt shifting performance to and from the granny ring. It will work, but just not as well.

Also, your 52 ring is designed to best pick up the chain from a 42. Shifting performance from the middle to large ring will be reduced a bit, too.

I've done all this before, using all sorts of non-standard sizes. They always work, but might just require some more careful, slower, shifts.

You can't use the new Dura Ace 39 tooth triple middle ring. It has tabs on it to directly attach the granny ring, which is different than other Shimano triples.

Mar 28, 2003 8:11 AM
More from Sheldon Brown:

Triple Chainrings
For the 2002 model year, Shimano introduced a triple chainring crank set in the Dura-Ace line, with corresponding STI shifters, front and rear dearailers. While other Shimano "road" triples come with 52-42-30 chainrings, the Dura-Ace version comes with 52-39-30 rings. At first this seemed like a welcome upgrade, offering greater versatility to the road/touring cyclist.
Unfortunately, the way Shimano chose to do this is rather wrong-headed and retrograde, resulting in a crankset that is even less versatile than their 130/74 "road" triples in the Ultegra, 105, Tiagra and Sora lines.

Instead of using a crankset with two sets of mounting holes, as with other modern triple sets, they have the 30 tooth chainring attach to the 39 tooth ring, rather than to the crank itself, in the manner of a Willow Triplizer or a TA conversion chainring. "Oh, great," you say, "so then you can retrofit the 39 onto other 130 BCD doubles!" Well, no. Unlike the Willow and TA conversion rings, the Dura-Ace is specifically designed NOT to fit a standard 130 double crank, because the "shelves" that support and center the middle ring get in the way. The "triple" Dura-Ace crank has had these "shelves" machined off flush, so it is the only crank that can accept this chainring, even though it does still use the standard 130 mm bolt circle.

But wait! It gets worse! Instead of using the standard 74 mm bolt circle for the 30 tooth chainring, as they do on all of their other "road" triples, Shimano has seen fit to create a brand-new bolt circle pattern just for this application, 92 mm. Thus, there is NO other chainring besides the stock 30 that can be used with the Dura-Ace triple.
Mar 28, 2003 8:28 AM
Thanks for your help, Doug. That gives me something to think about...

happy riding!
Mar 28, 2003 9:08 AM
You might consider a new cassette, with a larger big cog. It will essentially accomplish the same thing, plus you get a lower gear in your granny.
And people wonder why I hate Shimano! nmdzrider
Mar 28, 2003 10:52 AM
What doug said and..........Rusty Coggs
Mar 28, 2003 5:40 PM
If you look hard you may find a 39 with pins and ramps,but it isn't critical.
What doug said and..........russw19
Mar 28, 2003 7:35 PM
Before you do all this... look into an online gear inch calculator and see what difference it will really make. Often times you end up getting more cross over gears (duplicates) so be sure that won't happen first or it's all a waste. Also, look into what a few other cassettes will do with the same gear calculator. You are most likely going to have to play around some to find the largest range as well as the least cross over gears.

This is one reason why I hate pinned pairs of gears on Shimano and Campy drivetrains. It saves a few grams at the expense of the flexibility of swapping just one or two cogs to help out this very problem.

re: swapping chainrings on Shimano triple?Steve Bailey
Mar 29, 2003 6:16 AM
What you want to do makes perfect sense, especially as that's the most common small ring on a road double and gives you better range then the 42. Note though that the middle ring on a Shimano triple is a pinned and ramped chainiring designed to work with the Shimano STI shifter. You might get poor results using a very common 130 BCD - 39 in a non-pinned/ramped ring. Peter Whites site say's the following:

"The quickest shifting with Shimano's STI shifters can be had only with matched sets of chainrings. For example, the Shimano Ultegra Triple crankset comes with three matched rings; 52 - 42 - 30 teeth. The alignment of the teeth and the positions of the steel pins that lift the chain from smaller to larger rings are carefully positioned to make up-shifts as quick and smooth as possible, even under considerable pedalling forces. With a few exceptions, TA chainrings are designed to be used with many different adjacent rings. The TA rings have pins for lifting the chain, but since you can choose from many chainring combinations, the three rings you end up with won't be matched for tooth position or pin position. That makes the shift a bit slower than if you use Shimano's matched chainring sets. But STI shifting still works. You just can't pedal as hard while shifting as you can with Shimano's rings and still get a smooth shift. Don't worry though, they still shift very well.

He also sell the TA ring:

>Specialites TA 39 tooth pinned for STI 9 speed compatibility $ 38.00

FWIW, I've used all sizes - 52 - 53 & 54 large rings on my STI double shifter for years, none of them are ramped/pinned and I've never noticed a difference. Triple shifters are a different matter and they are finicky beasts.

Steve Bailey