Gearing too high. Swap front or rear?

Discuss light weight issues concerning road bikes & parts.
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853guy
Posts: 160
Joined: Sat Dec 30, 2017 8:48 pm

by 853guy

Hello everyone,

So I’m rocking Dura Ace 7800 front and rear with an Ultegra 6600 cassette. Up front it’s 52/39 and then 12-25 on the rear and I’m constantly getting smoked on climbs. (Let’s at least pretend it’s the gearing rather than my cardio, shall we…?)

I’m not interested in a whole new drivetrain, since this is only an interim bike that will be replaced sometime next year.

Given I’m hopeless at maths, am I better to swap the 39 for a 34, or swap the 12-25 for say, a 6700 11-28 or 12-30?

Also, am I going to run into the short/medium rear derailleur problem associated with 11-28 and 12-30 cassettes?

Any help appreciated.

Cheers!

853guy
Last edited by 853guy on Fri Oct 12, 2018 8:43 pm, edited 1 time in total.

by Weenie


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FIJIGabe
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Location: The Lone Star State

by FIJIGabe

Since we're talking about 7800, rather than 9000 or 9100, you're limited to changing the rear cassette, since 7800 different BCD's for their full size vs. compact cranks, so you won't be able to put a 34t on your current setup.

Try running an 11-28 cassette and see how you like it. If you need more gearing, then get a new crank and put a 50/34 on the front and try it again. You won't have any problems with an 11-28 in the back, with your current RD. I'm not sure about the 12-30, though.
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mattr
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Location: The Grim North.

by mattr

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

Let the internet do the maths for you.

But yes, 28 at the back is the "easy" option. Depending on hanger geometry, chain length etc, you MAY be able to get a 30 on.

RussellS
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Joined: Wed Feb 03, 2010 1:31 am

by RussellS

Besides the cassette change to a 28 or 30 cog, the only other "cheap" way to get lower gearing is to change your inner chainring from 39 to 38. Shimano 7800 cranks use a 130mm bolt circle diameter. It fits down to a 38 tooth inner chainring. Sugino or some other off brands sell this size chainring. It will help a little, not a lot.

Hexsense
Posts: 558
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

You may warrant the purchasing of the entire crank by buying power meter crank.
You new bike won't likely come with a power meter, so you transfer your 110bcd power meter crank to your new bike.

Also, here is the link to exact comparison you are looking for:
http://www.gear-calculator.com/?GR=DERS ... 8&UF2=2135

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vejnemojnen
Posts: 392
Joined: Fri Nov 08, 2013 7:11 pm

by vejnemojnen

you can get a 36t ring with adapter to your chainset of 130bcd

https://www.ebay.com/itm/50T-36T-BCD130 ... SwmOZbmNWp

I'd go for a 13-29 cassette as well though. NB: less difference between smallest and largest gear means, that you can keep your short cage derailer, and even offers finer increments, which can be economic.. : )

other choices for different size-bcd rings
https://www.ebay.com/sch/doval_su/m.htm ... pg=&_from=

wintershade
Posts: 123
Joined: Mon Apr 16, 2018 7:12 pm
Location: San Francisco, CA

by wintershade

FIJIGabe wrote:
Fri Oct 12, 2018 7:27 pm
Since we're talking about 7800, rather than 9000 or 9100, you're limited to changing the rear cassette, since 7800 different BCD's for their full size vs. compact cranks, so you won't be able to put a 34t on your current setup.

Try running an 11-28 cassette and see how you like it. If you need more gearing, then get a new crank and put a 50/34 on the front and try it again. You won't have any problems with an 11-28 in the back, with your current RD. I'm not sure about the 12-30, though.
This approach is also what I'd suggest, as it will be most economical for you. Two teeth less up front will make a bigger difference than two teeth in back. You may miss the 16 cog on the 11-28, so start with a 12-28. Even if you go to a compact, I find I prefer a 16 tooth cog over the 11x50, but I'm comfortable at high cadences.

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853guy
Posts: 160
Joined: Sat Dec 30, 2017 8:48 pm

by 853guy

Thanks for the responses. Awesome.

I think the rear cassette swap makes most sense (financially at least), but I’ll check with a local bike shop and make sure I’m not going to run into problems with the RD. Pretty confident 11-28 (11-12-13-14-15-17-19-21-24-28) will work fine, but less confident 12-30 (12-13-14-15-17-19-21-24-27-30) will.

I’d rather not go for a non-Shimano inner chain ring for the crankset, as the smallest I can find for the 130 BCD is a 38 which makes hardly any difference, and I’m keen not to screw up the shifting I have which is still pretty good despite its age.

If the 11-28/12-30 still isn’t sufficient, I’ll grin and bare it until I take delivery of the new bike. Mont Ventoux isn’t going anywhere, anyway.

Thanks again, all. Appreciate it.

853guy

P.S. Mattr, I did actually try that exact site. I still couldn’t figure it out. That’s what happens when you get a liberal arts degree, I guess. *Sigh*.

alcatraz
Posts: 1295
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Get a compact crankset next time. Then you don't need to carry as heavy cassettes around with large jumps in gear ratio. :)

I'd still max the cassette and if that's a 12-30 then that's what I'd buy.

Going 30t from 25t is a 20% increase in cadence.

Going 39t to 34t chainring is a 15% increase.

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853guy
Posts: 160
Joined: Sat Dec 30, 2017 8:48 pm

by 853guy

CORRECTION:

So, like I say, I have a liberal arts degree.

That means not only am I deficient at working out ratios despite having a fancy online calculator at my disposal, but also, cannot count.

I'm running 12-24 on the rear. Not 12-25.

I still passed a middle-aged woman on a commuter bike up the hill this morning though. So, something to feel good about I supppose. :roll:

Best,

853guy

Beanicyclist
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Joined: Tue Oct 02, 2018 7:45 pm

by Beanicyclist

As your current bike is 20 speed, then if your next will be 22 (or more?!?) then I'd reccomend just waiting or getting the chainring. You won't really be able to use the cassette again and a chainring should be cheaper (albeit a smaller jump in cadence) but if you've not got long to wait for the next bike then I'd just wait it out.

BC :D
"Eat your green vegetables"- Peter Parker, 2004.

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853guy
Posts: 160
Joined: Sat Dec 30, 2017 8:48 pm

by 853guy

Beanicyclist wrote:
Sun Oct 14, 2018 5:37 pm
As your current bike is 20 speed, then if your next will be 22 (or more?!?) then I'd reccomend just waiting or getting the chainring. You won't really be able to use the cassette again and a chainring should be cheaper (albeit a smaller jump in cadence) but if you've not got long to wait for the next bike then I'd just wait it out.

BC :D
Hi BC,

The new bike will be a custom build with custom geo from a small manufacturer, so there'll be a wait in any case from the time of order. Next summer is most likely.

This means my current bike will become my winter/wet weather training bike. So the modest outlay for a cassette, and perhaps, a new crank is easily justifiable in both the short term and long term.

Cheers!

853guy

Svetty
Posts: 518
Joined: Thu Mar 26, 2009 11:06 pm
Location: Yorkshire - God's Own Country

by Svetty

The 7800 series official maximum cassette sprocket is a 27T. A 28 will work OK but a 30 might be pushing it......

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853guy
Posts: 160
Joined: Sat Dec 30, 2017 8:48 pm

by 853guy

Svetty wrote:
Sun Oct 14, 2018 10:32 pm
The 7800 series official maximum cassette sprocket is a 27T. A 28 will work OK but a 30 might be pushing it......
Hi Svetty,

It won't be Dura Ace 7800, since I can't find any 12-27 cassettes in any case, and the ones I can find are limited to 12-21.

The honour instead will go to good ole' Ultegra 6700, which are plentiful, cheap and to be honest, indistinguishable in shifting quality and real-world weight to the 7800 ones - to me, at least.

Best,

853guy

alcatraz
Posts: 1295
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

You might not think the 11t cog to be important now but get it anyway. If you someday get a compact crank you are going to wish you had.

11-28t or 11-30t.

/a

by Weenie


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