Benefits of Reducing Chainring/Cassette tooth quantity?

Discuss light weight issues concerning road bikes & parts.
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lalahsghost
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by lalahsghost

Good old West Virginia... We have a love-hate relationship, and I feel like I've finally found my sweet spot in terms of gearing.

Right now, I'm operating on SRAM Rival (grad student here, gimme a break~) with a 110 50/34, and a PG-1070 12-26

I found a really sweet deal for a 33t chainring for $35 and shipping (http://www.homebrewedcomponents.com/store.php/products/aluminum-110bcd-5-bolt-chainring). I had some spare money from trading in textbooks back to Amazon, and I got a 48t SRAM chainring for zero out of pocket. (mostly to be sure there isn't a front derailleur issue with 17t shifting (50/33).

Now, I plan on changing my cassette soon to an 11-25 (the cassette has... 5k+ mi on it)

I know the differences aren't significant, but other than because the pros don't do it, and cycling culture, why aren't we utilizing smaller cassettes and chainrings that are (nearly) equivalent in gear inches, etc?

a 52-12 (114.0 g.i.) is nearly equivalent to a 48-11 (114.8 g.i.)
big.png
Big Rings (Theoretical Setup)

This way, one would lose 0.8 g.i. off the top, but gain 4.8 g.i. on the low end by having a 33-25 (34.7 g.i.) instead of 39-26 (39.5 g.i.) To get gearing as easy as the 33-25, I would need to have a 39-29 (35.4 g.i.) Eeek, that sounds heavy.
small.png
Small Rings (Future Setup)

This in also creates a wider range of gearing, no?

I'm a high cadence rider anyways, so this all seems win-win.

All while theoretically losing weight due to smaller chainrings and cassette sizes!

Are there any downsides to this?

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breezerboy
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by breezerboy

Bigger steps between gears at the top end, apart from that, not a lot wrong.

11T - 12T (2 top gears) = 9.01% gear change. If running 90 rpm that drops you to 82.5 rpm.

12T - 13T (2 top on current cassette) = 8.33% gear change, only dropping you to 83.1 rpm.

Also note that a chain is more efficient on larger sprockets (e.g. Berner pulley conversions).

Small differences, but to some it matters.

by Weenie


RussellS
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by RussellS

As already mentioned, bigger steps between gears when using the 48-33 chainrings with 11-25 cassette. And unless you are climbing hills, the 33 ring won't get used much. And huge jumps between gears when changing rings. Have to do a lot of double shifting when going from ring to ring. Personally, I think 50-34 or 48-33 rings with a 12-23 cassette, or 50-39 rings with a 12-29 cassette makes ideal gearing. I'd run the 50-39 rings for everything but BIG climbs. Unless you are going down a mountain, 4:1 ring:cog gearing is plenty high. My next cranksets will have 110 mm bcd rings to allow a better range of gearing.

lalahsghost
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by lalahsghost

Would there be a noticable difference in reaching a certain level of acceleration, or would there be a higher level of torque?

http://www.physicsforums.com/showpost.p ... stcount=13" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

This, and some of the BMX conversations about gear ratios (Using a 44/16 or a 25/9, etc) make me wonder whether there is a torque benefit with a smaller chainring. With BMX and their VERY SMALL gears, there is crazy amount of stress put on the chains, or so I hear. A derailleur, and higher amount of teeth that the chain is gripped with, would lower the worries for roadies, no?

I know I'm speculating, and I'm nowhere near a physics/engineering master.

What about chainring size as compared to crank size?
I don't know if this will substantiate my claim, but on Sheldon Brown's gear ratio calculator:

52-12 @ 175mm crank = 8.3:1 ratio.
52-12 @ 170mm crank = 8.5:1 ratio.

A 2.4% decrease in ?torque? with the 175mm crank, if you calculate 8.5:1 being "100%" (8.5 / 8.3 x 100)

Is there an increase or decrease in values solely because the radius of the chainring is closer to the center of the crank? (or is this part of the gear ratio calculation?)

We could also talk about whether there are benefits in terms of chainline efficiency (whether or not this is a big deal 8) )

And @Russel - The hills around here are all either 3-4% or 8%-16% (more often about 10%), so the rationale for the 33t is to keep a higher cadence (Ex: 70rpm on the 34t, or hopefully 75rpm on the 33t :P)
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speeds.png
Preferable speeds, and how it relates to gear choices.

eric
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by eric

Make sure the 33t ring actually works before you get too excited. I tried to fit one for a climbing race and found that the chain's side plates would contact the ends of the crank arms before the chain rollers bottomed out on the chainring, making it unuseable.

thisisatest
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by thisisatest

the spider tabs can be filed a little. but...
as mentioned, gears with less than 16teeth start getting less efficient. personally, im going to go the other way, with big tt rings and a mix of mtb cassette and junior cassette.

Chris_W
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by Chris_W

Efficiency is greatly reduced when getting below 13 teeth, so the 52-12 may be 0.5%-1% more efficient than a 48-11.

Having said that, I love the 33-48 gearing that I have on my compact. I use a 12-30 cassette (10-speed), which allows me to avoid the dreaded constant double-shifting that is often needed with a compact crank because I can stay in the big ring for much longer, plus I have a real low bottom gear to get up the long Alpine passes around here with a decent cadence. The 12-30 cassette even has great spacing, there are no big gaps (better than either SRAM's or Shimano's 11-28 in this regard).

In the summer, Shimano has said that they'll release an Ultegra 12-30 cassette, but until then there is only a heavy Tiagra version available. I took that and combined it's largest cog with a Dura Ace 12-27 cassette, minus the 16-tooth cog, plus a couple of spacers, and it works great and is reasonably light. I can spin pretty fast and use shorter cranks than normal, so I can still put in some power at 65 kph with my 48-12.

lalahsghost
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by lalahsghost

Chris_W wrote:Having said that, I love the 33-48 gearing that I have on my compact...


Oh? What groupset are you using, and how do you like the shifting from the 33 to 48? Does it feel better/faster/etc than a 34/50 shift in your opinion?

Valbrona
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by Valbrona

lalahsghost wrote:I know the differences aren't significant, but other than because the pros don't do it, and cycling culture, why aren't we utilizing smaller cassettes and chainrings that are (nearly) equivalent in gear inches, etc?


The cycling indistry is very backward in some regards, gearing being one.Said by someone who has worked in the industry for an OEM.

Higher cadence gears are better suited to lots out there, especially the recreational riders. To get lower gears, however, lots just think in terms of increasing sprocket size rather than decreasing chainring size.

FWIW I use an 11-23 - 26/34/42 triple. Double shifting, like you get with doubles, doesn't half suck.

lalahsghost
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by lalahsghost

Well, homebrewed components finally came through and sent me my 33t 110bcd black anodized chainring... nearly six months after ordering it (huge thread about many customer problems on MTBR) I'm now running a 33/48 with a 11-23 cassette.

My weight savings from the cassette was 31.5g from switching from a 12-26 og-1050 to an 11-23 ultegra.
I forgot to weigh the chainrings, but I'm betting that I'm saving about 10-12g.

All in all, I'm pretty happy with the gearing. Mission accomplished. Cadence has stayed at a higher average than before, and speed has not gone down!

bm0p700f
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by bm0p700f

Please don't get confused with crank arm lengths changing the ammount of torque that gets transmitted to the rear wheel. far more useful to think about work done. Ultimately the same ammount of work is put through the crank no matter it length and it's the rate at which work is done that determines your power output. Difference between your power output and the reistive forces will determine your acceleration.

Torque of a couple (using lipless pedals make the cranks a couple) is simply the force applied by one leg muliplied by the diameter of the circle i.e. the lenght og both arms added to together. this is the same as the force aplied by one leg multiplied by the length of the crank arm added to it self.

Work done by your legs will be the torque muliplied by 2 pi radians. However as torque is funtion on crank angle the ammount of work done changes over one full rotation.

Torque and work done are inextricably linked and therefore as the work done by the rider is "fixed" then increasing the the length of the cranks decreases the force applied through the pedals to keep the work done fixed. Other wise you a perpetual motion machine where by if you had crank arms that could extend you would end up with more and more free energy.

DaveS
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by DaveS

Smaller chainrings will create more chain tension, which can't help chain life. If you want a certain percentage change between the rings and cogs, figure that out first and look for the largest of both that will accomodate your needs.

thisisatest
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by thisisatest

shimano ought to come out with metric pitch chains and gears again, but for multispeed bikes this time.

by Weenie


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