Author Topic: Lower gearing: change cassette, chainrings or both?  (Read 7112 times)

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Offline mcparsons

Lower gearing: change cassette, chainrings or both?
« on: May 20, 2010, 09:30:39 pm »
I have a 2008 Giant FCR3 that I have been riding for two years including one long tour.  I'm planning an unsupported tour with a trailer and it is clear that I don't have enough gear on the low end.  Even unloaded I never used the third chain ring and spend 90% of the time in 4 or 5 gears.

I'm considering three options:

1.  Replace the cassette, currently 11-28 with an 11-32. That only changes the lower three gears and perhaps by not enough. 

2.  Replace the chain rings (currently FSA Vero 30/42/52).  I think I can move the 42 to the outer ring and add 22/32 FSA Pro ATB to get 22/32/42.  I've never done this but It looks like it would work.

3.  There is also this Nashbar MT2 mountain bike crank set for real cheap that would get me 22/32/44.  I'm a little nervous about this as I don't know if it will work in my current bike (don't know anything about how cranksets match up with bottom brackets).  This cost is about the same as option 2.

Questions:
- I might opt for both the cassette and the chain rings to get a gear range of 19 to 108 inches.  Is this too low?  (I think I know the answer to this one).
- Am I right that I can move the middle chain ring to the outer and add two more inside?  I'll check the BCDs.  The range is fewer teeth than the current so I assume no problems with the FD?  Do I just move the FD down the post a little bit to match the smaller chainrings?
- How can I figure out if the Nashbar MTB crankset will work?  Is this a lot harder to do than the rear cassettes and the rings?  If I had to spend a lot on tools this will be a more expensive option.
- Any other advice?

Sorry for asking for free help but this is essentially my entire biking budget for the year and I'd like to get it right.  :-\

Offline whittierider

Re: Lower gearing: change cassette, chainrings or both?
« Reply #1 on: May 20, 2010, 09:56:06 pm »
If you currently have a 30-42-52, the BCD (bolt circle diameter) of the outer two is almost certainly 130mm, and the BCD of the little one is 74mm.  The smallest you can go with 130 is 38 teeth, meaning you cannot put a 32T in the middle position; but the 74 can go down to 24T.  24/34 will give you your 19" gear.  I don't think you'll be sorry you went so low.  We have 24-42-52 on our tandem since my wife is anything but a good climber, and we have done many-miles-long 10% grades.  It shifts fine.  I do recommend keeping the middle ring bigger than the average of the other two, for better shifting.  IOW, if you have 24-XX-52, the "XX" should be at least 39.  It will also be best to keep middle and outer rings that are mated to each other, ramped, and pinned.  (They will be all three if you keep your current crankset and only change the granny ring.  The new granny won't be mated to the middle ring, but that's life.)

To keep proper mating between the two biggest rings if you change the 42 (which is a B type) to a 39 (which is an A type), you should also change the 52 from a B type to an A type.  Correct mating makes sure that the teeth of the two rings and the ramps and pins are aligned such that when you shift up, the chain gets caught and helped up at just the right place so the rollers will go between the teeth of the bigger ring instead of trying to ride on top of the teeth and more easily drop the chain off the outside of the crank.  Properly mated rings make for very quick, trouble-free shifts.

The small MTB crankset will work with your BB, but not with your front derailleur, especially if it is a braze-on type instead of clamp-on, since it won't go low enough on the seat tube.

You can put a 13-34 cassette on, to get the low end, but you'll have to change the rear derailleur to a MTB type to handle the big cogs.  You will want a normal-high type, instead of normal-low or RapidRise which has recently become common in the MTB world.  Normal high means the derailleur will go out to smaller cogs when you release the cable.
« Last Edit: May 21, 2010, 12:39:39 am by whittierider »

Offline CastAStone

Re: Lower gearing: change cassette, chainrings or both?
« Reply #2 on: May 20, 2010, 11:12:24 pm »
I recently replaced the cassette and smaller 2 chainrings on my bike. I got a SRAM 970 and 2 XTR chainrings, total cost $106 with installation. If you can find cheaper chainrings and are willing to do a SRAM 850 or Shimano HG-40 Cassette it'll save a lot of cash over what i paid.

If your gonna swap the cassette, you might as well go to a 11-34 or 13-34 instead of a 11-32. They cost the same, and you'll get that little bit extra in low gear.

I'd leave the 52, go 38 middle and 24 small in front. With the modified cassette gearing and the smaller middle chainring, you'll find yourself there more often. 42/11 is too small for a high gear IMHO.

Shimano's been making some really bizarre BCDs in the last few years, so make sure you measure or look up your part number on the Shimano website (or whoever made your crank) to ensure you get the right parts.

Offline rvklassen

Re: Lower gearing: change cassette, chainrings or both?
« Reply #3 on: May 21, 2010, 07:16:01 am »
If you make radical changes in either the largest cog in the back (34 from 28 probably isn't radical enough) or the smallest in the front (30 to 24 probably is), be sure your RD can take up the chain slack.  The other thing to do is get rid of that 52.  You say you rarely use that chain ring.  Something like 24-36-48 will give you somewhat less range, but with an 11-34 in the back you can find an RD that works (e.g. Shimano XT). 

Offline whittierider

Re: Lower gearing: change cassette, chainrings or both?
« Reply #4 on: May 21, 2010, 11:14:07 am »
Quote
(34 from 28 probably isn't radical enough) or the smallest in the front (30 to 24 probably is)

Your current 30/28 gives 30/28*27" = 28.93" gear.
A 30/34 gives 30/34*27" = 23.82" gear.
A 24/28 gives 24/28*27" = 23.14" gear.

The difference between a 23.82" and a 23.14" is less than 3%.  You won't feel a difference of less than 3% when climbing.  They will both be basically the same benefit over what you have now.

Quote
The other thing to do is get rid of that 52.  You say you rarely use that chain ring.  Something like 24-36-48 will give you somewhat less range, but with an 11-34 in...

A 52/13 gear (108") is actually quite a bit lower than 48/11 (which gives 118"!), and the 13-34 cassette will give you slightly tighter spacing between gears, which is a good thing.
« Last Edit: May 21, 2010, 11:15:54 am by whittierider »

Offline mcparsons

Re: Lower gearing: change cassette, chainrings or both?
« Reply #5 on: May 23, 2010, 02:37:17 pm »
Thanks for all the input.  Everything I've got is spec'd for MTB (square bracket interface, long RD and a clamp on FD) so I think I can go with an MTB crankset without much trouble.  I can't find a 13-34 cassette online.  This is an 8 speed and I assume its a lot more work to change to a 9 speed cassette (shifters at least?).  I was looking for a conversion suitable to my limited skills but looks like I'll be seeking the services of my local bike mechanic. 

As far as gearing, based on the last two years of riding this is my target:

Climbing: up to 45 inches max
Flats:  up to 65 inches
Downhill, good road, tail wind:  up to a 100 inches

That may not sound like much but I'm more interested in enjoying the ride and keeping a pace I can handle for 8 hours a day.  The lesson for me is: Next time get a properly geared touring bike.


Offline whittierider

Re: Lower gearing: change cassette, chainrings or both?
« Reply #6 on: May 23, 2010, 03:51:38 pm »
Quote
I can't find a 13-34 cassette online.

Hmmm... I didn't have any trouble finding it in 7-speed, so see if your LBS can order it from QBP or some place like that.  There are places that will build up custom combinations.

Quote
This is an 8-speed and I assume it's a lot more work to change to a 9-speed cassette (shifters at least?).

It should be no harder and no more expensive to change the shifters than to change the crankset.

Offline digimarket

Re: Lower gearing: change cassette, chainrings or both?
« Reply #7 on: July 27, 2010, 12:27:33 am »
Square taper cranksets shouldn't be expensive - as the industry considers them obsolete.  (used ones on eBay)  Check and see what FSA has to offer with a 48 36 24.  That combined with a cassette that goes to 32 will be enough gears.  You may end up needing to change both derailleurs depending on what you already have.  If you want to make the jump to 9speed - I think it is worthwhile as it allows a little closer jumps in your cruising gears while still having two or three usable granny gears.  I'm assuming you have brifters - if you change to 9 speed barends - you will also gain friction shifting mode for emergencies.  You can sell the brifters for a good price on eBay and pick up some brake levers for less.  For touring I wouldn't go to 10 speed.

Unless I am carrying a very heavy load or a trailer I find either a 26 or 24 with a 13-27 to be plenty low enough and a joy to shift.  I leave the long cage derailleur on the back, and change the cassette for heavy touring.

At this point you will have changed out nearly all the drivetrain (including a new longer chain).  Consider the possibility of buying a two year old bike equipped more to what you want and selling the one you have now.
Bertoni Corsa Montadale, Bob Jackson, Viscount all rounder, Styre, Bianchi Boardwalk, Jamis Aurora, Schwinn Cimmaron, Schwinn High Sierra, Humber 3-speed.

Offline DaveB

Re: Lower gearing: change cassette, chainrings or both?
« Reply #8 on: July 27, 2010, 06:49:03 pm »
IOW, if you have 24-XX-52, the "XX" should be at least 39.  It will also be best to keep middle and outer rings that are mated to each other, ramped, and pinned.  (They will be all three if you keep your current crankset and only change the granny ring.  The new granny won't be mated to the middle ring, but that's life.)
All of the advice in this posting is very good but I would add one thing.  All granny rings are dead flat with no pins, ramps or the other shifting enhancements common to the larger two rings.  I have granny rings that came stock on 7,8 and 9-speed Shimano triple cranks and on a 10-speed Campy triple crank and they are all plain, flat chainrings.  Therefore, what ever size and make granny ring you buy will be "mated" to your other chainrings.   



Offline whittierider

Re: Lower gearing: change cassette, chainrings or both?
« Reply #9 on: July 27, 2010, 09:01:22 pm »
Quote
Quote
IOW, if you have 24-XX-52, the "XX" should be at least  39.  It will also be best to keep middle and outer rings that are mated to each other, ramped, and pinned.  (They will be all three if you keep your current crankset and only change the granny ring.  The new granny won't be mated to the middle ring, but that's life.)
All of the advice in this posting is very good but I would add one thing.  All granny rings are dead flat with no pins, ramps or the other shifting enhancements common to the larger two rings.  I have granny rings that came stock on 7, 8, and 9-speed Shimano triple cranks and on a 10-speed Campy triple crank and they are all plain, flat chainrings.  Therefore, what ever size and make granny ring you buy will be "mated" to your other chainrings.

The granny ring won't have the ramps and pins to help it up from a smaller ring (since it's already the smallest), but ideally the teeth should still be positioned such that wherever the middle ring's ramps and pins help the chain up from the granny, the chain rollers will be ready to go between the middle ring's teeth, not onto the tops of them.  One ring may need to be oriented slightly more clockwise or counterclockwise relative the other to optimize the shifting.  I wish I could find a good picture to show the effect.  It's more of an issue when going to the big ring, as lack of proper engagement on the big ring increases the chances of the chain going off on the outside, something that used to happen more frequently back in the days before ramps & pins and mated chainrings.  (I remember them well.)  Shimano addressed this with A- and B-type rings, where for example the 53-tooth comes in both, and the A-type is made for a 42T middle ring and the B-type is made for the 39T.  Making random numbers of teeth work just as well requires some homework.

Offline DaveB

Re: Lower gearing: change cassette, chainrings or both?
« Reply #10 on: July 28, 2010, 06:35:39 am »
No question that the two larger chainrings on a triple should be matched to each other for consistant and good shifting.  My point was that granny rings (and only granny rings) are pretty much generic so any one you find will work as well as any other as long as the bolt circle is correct.

Offline briwasson

Re: Lower gearing: change cassette, chainrings or both?
« Reply #11 on: July 28, 2010, 08:48:35 am »
Another caveat is that if you are buying a new middle ring, make sure it's designed for a triple. Otherwise, it won't have ramps/pins to help with the upshift from the smallest ring. Really only a big deal if you are using STI levers.

A quick look at your bike's specs on Giant's site shows that it comes stock with road components and flat bars ("fitness bike" or whatever the call them). You may have some issues with compatibility between a road FD (Altus) and a MTB crank. The Altus is designed for bigger chainrings, so dropping down to a 44 might be too extreme for it to handle; it might be fine, but the general word on the street is that Shimano road FDs don't work well with MTB chainrings.

I may have a square-taper MTB crank in my parts box. PM me if you are interested in considering a used one.

Offline DaveB

Re: Lower gearing: change cassette, chainrings or both?
« Reply #12 on: July 31, 2010, 05:25:05 am »
..... but the general word on the street is that Shimano road FDs don't work well with MTB chainrings.
Actually it's not the FD's that don't work well across road/MTB chainrings.  What doesn't work well is an MTB fd with STI road shifters or a road fd with MTB shifters. 

Road and MTB front derailleurs have different cable pull geometry so they pretty much have to be used with their own type of shifter unless you are using friction shifters such as barends or downtube.

Offline CastAStone

Re: Lower gearing: change cassette, chainrings or both?
« Reply #13 on: July 31, 2010, 07:54:43 am »
Three things just a little off here:

The actual curvature of the derailer itself is different between a road FD and a MTB FD. A road FD is much flatter than a mountain one. So shifting performance is inevitably affected, although its much more noticeable if you put a MTB FD on a road bike (good luck getting into a 53 with a MTB FD).

However, an Altus IS an MTB FD. Not sure where bri got his info, but its incorrect. Its actually designed SPECIFICALLY for a 44 as its ideal large chainring, 48 as its max.

But that doesn't even matter, because the 2008 TCR3 shipped with an R443 FD, which is the rough equivalent of a Tiagra FD; its designed for Shimano's flat bar road bikes system, with 53 teeth as its ideal. Shimano's tech docs recommend a gap of at least 10 teeth between high and middle gear, and a gap of no more than 22 teeth between high and low gear. I suggest you follow those standards.

So my conclusion is that if you regear, you may or may not find you need a new FD, but if you're having trouble shifting, something like the SRAM x.7 is only $30 and is incredibly powerful. Seriously, the higher-end SRAMs have fantastic springs.

Offline mcparsons

Re: Lower gearing: change cassette, chainrings or both?
« Reply #14 on: August 04, 2010, 08:15:23 pm »
Wow, thanks for all the advice.  It had the salutary effect of convincing me that I did not have the slightest idea what I was doing  ;D and sent me off to my local mechanic. 

I put together a chart of my gearing and based on my first 750 miles on this bike, determined what were my climbing, regular riding and all out ranges.  I also decided that I'm not interested in going over 35 mph and that only on wide open, new pavement. With that I told my mechanic what I wanted in gear inch ranges on each ring.  For about $120 he set me up with 22/32/42 rings in the front and an 11 to 34 sprocket.  Altus RD handled it just fine. I've now got another 800 miles on that set up and I love it.  I went from an effective  4 or 5 gears to fully using all 8 sprockets and all three rings.  My cycling friend says I could climb a tree with it and that's not far off.

I don't regret the $ spent as I would never have known what I wanted if I had not put the time in to learn what gears where right for my load and routes.  I'll put another 1000 miles on this bike and then upgrade to a proper touring bike knowing I'm making the right investment.

Thanks again for all the tips.