Spreggy

I am looking at replacing the chain and cassette as a maintenance item, and want to look at the options. What I would like to accomplish is to move the transition spot of shifting from middle ring to big ring. It currently is at around 20 mph/95ish RPMS, which means it seems I'm always shifting front rings because I am cruising at both sides of that speed often. I am hoping I can move the sweet spot up or down by selecting a different cassette.

I like the gearing just fine now for climbing, but a larger bottom cog may help me avoid shifting to the granny.

I don't know anything about choosing cassettes, so any wisdom is greatly appreciated. Do any of the recommendations below look like they might meet this goal?


The bike:
2007 Trek 1000
Sora 8 spd shifters
Tiagra frt & rear derailers
Triple 52/42/30
SRAM 850 12-26 cassette.



A few options:
Interesting one from Sheldon, is someone carrying on the shop now? Shimano
8-speed Cassette, 11-34 "Nexave"
https://harriscyclery.net/itemdetails.cfm?ID=729

Shimano HG70-I 8 Speed Cassette - Cog Range: 11-30T


Shimano HG-50 8-Speed Cassette 12/25 or 13/26

hernick

Well, I don't think your rear derailleur can handle a 34T large cog. It's probably rated for a 27T max, but it would almost certainly work with a 30T big cog. Anything larger, like a 32T or a 34T, is probably not gonna work.

A wider cassette is going to have bigger jumps in size between each cog, and you're going to feel it when shifting if you go with that 11-30.

I suggest you play with Sheldon's gear calculator. My favourite measurement for comparison is speed@rpm, like MPH@90RPM.

Also, I was wondering, when you upshift to the big front chainring, do you do a "synchro shift" and downshift the rear cog at the same time? Vice-versa, when downshifting the front chainring, do you upshift the rear cog at the same time?

Al1943

I think the wide range cassettes would be a step in the wrong direction. Of the cassettes suggested the only one I'd consider as an alternative to what you have now is the 13-26. A closer ratio cassette will reduce the number of times you'll need to shift from the big chainring to the middle chainring. I think you should be able to go as low as 18 mph on the big ring and still maintain a good cadence.
I ride a 53/39 double with either a 13-26 10-speed or 13-26 9-speed. I'll ride the 39 inner ring up through 20 mph. If it appears that I'll be able to maintain 19 mph or more I'll go to the big ring. If I drop to 18 mph and I think the drop is temporary I'll stay on the big ring. I like to ride with a cadence of 90 to 100 rpm at all times but don't mind dropping into the 80's for a short while if I can see that I'll be able to get back to higher cadence without shifting the front.

Al

Gonzo Bob

I think the 12-26 should have plenty of overlap for you. According to my calculations, a 42/13 @ 95 rpm is 24 mph (with 700x23C tires) and a 52/21 @ 95rpm is 18.4 mph. Are you using all the gears that you can? You should be able to use 6 of the 8 cogs from each chainring (the smallest 6 from the big ring, the middle 6 from the middle ring).

A tighter cassette like a 12-23 or a 13-26 actually reduces this overlap.

A wider range cassette like an 11-30 will increase this overlap. The Tiagra rear derailer is spec'd for a large cog of 27 but it may run on a 30. I've run my Ultegra on an 11-30 8sp cassette. However, the cog jumps get pretty big and you may find yourself double shifting (shifting both front and rear) to get the next biggest or smallest gear.

Spreggy

Maybe my problem is in adjustment perhaps? I definitely don't get 6 cogs per ring. The big ring is clean on the top four, and the middle is good on the first five.

Yes! What does that indicate?

I have it adjusted per the Tiagra pdf manual off Shimano's site. Is there something I should be doing differently?