General Gearing Question

[Jeffrey Zelevansky]

I know this topic is addressed in countless ways online, so I'm looking for more of a real-life take on the concept of gear inches.

I'm running the wide/low double 38x24 (with guard) and an 11-34 on the rear but feel like I am spinning out at the top end of the range on my Hillborne. I'm a heavy guy, riding a big heavy bike and, momentum being what it is, I can get up to a decent clip on flats and downhills. My Wahoo trainer has an 11-28 installed and even when grinding up a 10-12% grade in Watopia, I never use my 24 up front. I occasionally use it on the road, but mostly stay in the big ring...34 seems plenty low for me. 

I think I might enjoy having a bigger gear up front but am not sure how much of a difference it would make. I have a reasonable sense of what rear cluster gearing means, but less so up front. 

Is there any way to quantify or assess or describe what might be gained by going to a 42 or 44 over the 38? Can I just swap in the larger gear or will I lose the guard? 

Thanks to anyone who has read this far!

[J G]

The difference between a 38t front and 44t front on the top end is probably the difference between spinning out at 26mph and spinning out at 30mph.

Check out the Sheldon Brown Gear Calculator page for looking at impact of gear changes in gear inches, mph at cadence etc.

While we can probably assume what crank you are running, would be good to verify for anyone who wants to speak on how changing the big ring would impact your overall setup.

-Justus

Mpls, MN

[Sebastian Rodriguez]

Hi Jeff,

I have the same gearing on my Homer (650B), I use Sheldon Browns Gear Inch calculator (https://www.sheldonbrown.com/gear-calc.html).  

The highest being 89.8 gear inches (38x11) , bumping it up to 42x11 you would get  99.3 gear inches.  

What does that mean....i think you would get that sweet spot in higher speed, i also have the silver 2 crankset, you can just get the 42 tooth ring, you would still keep the chain guard too.

I don't recommend you go higher than 42, as an 18 tooth jump for your front derailleur (42 to 24) is prob stretching it, doable, but stretching it.

I say go for it!

[Jeffrey Zelevansky]

Thanks for the replies and the explanations for the layman! Crank arms are 173mm. 

I will spend some time with the Sheldon Brown calculator. I have been trying to wrap my head around the info on the page on the Riv site but somehow how it's still confusing to me. 

Jeff

[Sebastian Rodriguez]

With the numbers i gave you, with every pedal revolution you'll go about 10 inches further.  Crank arm length doesn't make much of a difference, more leverage in theory, all things equal, it may take you a little longer to spin up but once your at your natural cadence expect 10 inches more per pedal revolution (80 to 100 rpm) will equal 800 to 100 inches more so 60 to 80 feet more per min. so on and so on......

[Jeffrey Zelevansky]

Ok! As Al Pacino said "the inches we need are everywhere around us."

Would a 42 from this page be what I'm after?

Thanks again, the "10" further per revolution" explanation resonates with me.

Jeff

[J G]

Good luck with the gears.  

For reference when I stated the ~4mph difference you would note, I made the following assumptions, correct or not:

700x44c

175mm cranks

mph@90rpm

38 and 44 front ring

11 rear sprocket

Regarding cranks, sounds like you do have the Silver 24/38 with guard, so yes would need a new guard if increased ring size to 42t or 44t and Riv sells theirs up to 42t.  Else, could get a 110bcd guard in 44t max likely elsewhere, but likely would be black.  I am not a fan of spreading my big and small rings too wide, so if in your position I would consider new small, big, bash if you need the top end.

-Justus

Mpls, MN

[Sebastian Rodriguez]

Yep the silver cranks use a 110 BCD, so you would mount them on the inner holes of the ring you linked.

[J G]

Yes, any 110bcd compatible ring will work for your cranks in the middle (and outer) positions.

[Garth]

This graphic gear calculator is so much easier to use and compare what all combos look like on a graph.  Something like even a 30-32t small and a 42-44t big rings would be fine. Just point your mouse and drag the rings and cogs around, select your cassette or make it custom, it's completely customizable. Kids stuff ! However going to that large of a 74mm BCD ring may not clear the stays, only you can know that with your frame. Otherwise just use the two 110BCD tabs and make it a 34/44. It'll shift easy peasy and you may actually use the 34 !

https://www.gear-calculator.com/?GR=DERS&KB=30,42&RZ=11,13,15,17,20,23,26,30,34&UF=2230&TF=90&SL=2.6&UN=KMH&DV=gearInches

https://www.gear-calculator.com/?GR=DERS&KB=34,44&RZ=11,13,15,17,20,23,26,30,34&UF=2230&TF=90&SL=2.6&UN=KMH&DV=gearInches

[Ben Miller]

Jeff, you can make this dead simple if you know 2 things:

1. How fast you can comfortable go now without spinning
2. How fast you want to go without spinning out

It can be in whatever units you want: miles per hour, meters per seconds, megaparsec per millennia. Once you know that, take the ratio of item two vs item 1 (2/1). This is how much you need to increase your front chainring by.

For example: 

1. How fast you can currently comfortable go 15 mph
2. How fast you want to go 20 mph
3. 20/15 = 1.33; that how much bigger your chainring needs to be
4. In this case, you'd need to increase your chainring from 38T to 50T (That's big, but this is just an example! Obviously need to round up or down to nearest whole size, they don't make fractional teeth :)

Since everything else is constant (crank length, wheelsize, cassette, etc) you don't actually need to account for them. 

If you don't know 1 & 2 for you, maybe you can estimate it from previous ride's data. Or go on a typical ride and just note those speeds approximately. 

Ben

[Ben Miller]

Quick addendum to my above post:

1. It works the other way around to: going from a 38T to a 42T will have an approx. increase in speed of 11% (42/38 = 1.11). 38T to 44T is ~16% (44/38 = 1.16). Again, everything else being equal.

2. Hopefully this is obvious, but do not shoot for you max speed on some big downhill. Max target speed should be close to the average speed of some prolonged gentle declining gradient that you normally encounter. (For me that's like a -2 or -3% gradient, which if I'm honest barely exists on my normal rides :)

[Ben Miller]

Addendum to my addendum :) I swear this is my final note, then I'm going for a ride.

A neat thing that this makes obvious is how much you highest (smallest) cassette cog matters. You get an 10% increase in speed when going from a 11T to 10T cog, or about the same you get from a 38T to 42T chainring!

Okay, time to stop replying and go riding!

[Jeffrey Zelevansky]

These are great resources...the graphic gear calculator is especially interesting.I think I'm going to swap the 38 for a 42. It sounds like an easy swap, though there are apparently differences of opinion as to whether the guard will still fit. I think I'll email Riv for the tie breaker.

[Jeffrey Zelevansky]

To close the loop, Will at Rivendell said I would need a larger guard if I go to a 42 from a 38. If it ever warms up, I want to put some more miles on outside to confirm that this change is worth the money. 

Thanks again everyone...I have enjoyed educating myself via the gear inch representations.

[Danny]

Hey Jeff,

Since you're jumping up 4 teeth in the front on your big ring, you'll want to add a little length to your chain too. Plenty of calculators out there for that, but I've had good luck using the big-big method Park describes here: https://www.parktool.com/en-us/blog/repair-help/chain-length-sizing

Danny
Madison, WI