Gear Ratios

supergunn

Hi Guys hope you can help me out once again.
I recently converted my 10 year old plus GT to a single speed for commuting to and from work.
I bought the DMR tensioner kit which has the 16 rear sprocket and ditched my Inner and outer chain rings and left the middle ring which is 32T.
I have been riding the bike for about a week now and done a 20mile run on cycle track last weekend.
Every thing has worked out fantastic and I have no problems with chain slipping or coming off or any other issues
I am now finding the bike too easy to pedal and could do with just a wee bit more resistance but not too much.
I want to change the middle ring anyway to a dedicated singlespeed ring instead of the shift aided one I left on the bike.
I am also about to change the knobblies for Schwalbe city jet 1.5s and this will make the bike even easier to pedal than it is just now.
I am running 16T rear 32T front 2:1 ratio.
Would changing to a 36T at the front give me the wee bit more resistance I am looking for .
I am not looking for a morning workout or anything like that as I want to turn up for work without being all sweaty but I would like to be able to pedal more and coast less if that makes sense.
My daily commute is almost completely flat on bike track with roadlike surface and a few cobbly bits 4 miles each way.
I also just purchased a Peugeot road bike with elite carbolite frame and horizontal dropouts from gumtree for £15 it is in fantastic condition and I plan to turn this into a single speed.
I got rid of the gearing and wrapped the chain around the front ring 42T and rear cog 17T just to try it out and this feels pretty good but they say the preffered ratio for road bikes is 3:1 which would mean using a 14T rear cog with 42T front.
Would this be better.
Your advice on this would be very welcome and much appreciated.
Than You
and
Kind Regards
Supergunn.

lost_in_thought

The best way to look at this, and indeed the way most SS-ers do it, is by a cryptic measurement called Gear Inches, or GI.

I believe it's the distance in inches that one pedal revolution takes you.

Anyhow.

I find tables are the best way to look at it, rather than plugging values into a calculator, and I use this one: http://home.i1.net/~dwolfe/gerz/

EDIT: please note it's for 700C road bike wheels.

Now, 32/16 gives you a 54" gear. Higher numbers provide more resistance and top speed, lower numbers are more spinny and provide better acceleration.

I am a habitual grinder, and riding around London (which is pretty much flat) I'm happiest on 46/14 which is an 88" gear. However, techniques vary, and if you're currently riding around on a 54" gear you may prefer something spinnier.

I don't know where you've got this 3:1 idea from, I've never heard it before, so I wouldn't worry too much about that. Stick with the GI.

42/17 is 66", which is still seen as 'quite spinny', especially in a flat area, but some like it spinny. 42/14 is 81", which is probably what I'd go for, but you can only get freewheeling hubs for SS wheels down to 16T.

As yours is a conversion this may not be a problem, but it's worth pointing out. I don't know how your hub is set up.

Hope this helps!

stuaff

What LiT said. The ratio thing I've never heard of either, sounds like nonsense frankly. Takes no account whatsoever of your wheel diameter and tyre sizing, both of which are absolutely vital to calculating gearing.

Harry182

Sheldon Brown has a gear inch calculator here - http://www.sheldonbrown.com/gears/

LiT's chart seems a bit out compared to SB's calculator. (Maybe because the chart doesn't account for tyre sizes?)

I am currently commuting on a slicked mtb and run a 44x17 with 1.5 tyres for just under 75 gear inches. Seems do be a good ratio for not too hilly London.

PS - Ask LiT the gear inches on her fixed.

supergunn

Thank you,

LIT and StuAff,
After reading your comments and looking at the link this has all became very clear to me now.
Regarding the 3:1 and 2:1 ratios I'm sure I read somewhere on Sheldon Brown's web page that road bike users converting to singlespeed had a preference typically for a 3:1 ratio rear cog being a third of the size of the front chainring and MTBs a 2:1 ratio rear being half of the front or at least that's what I took it to mean.
But it did state that there were no hard and fast rules and what was good for one may not be good for the other.
It also advised to experiment.
I am running 2:1 ratio 32T front and 16T rear and my legs are going like billyo.
I also understand that weight of bike, type of tyres, windy weather, wheel size, terrain etc all come into the equation.
I will refer to LITs link , it makes so much more sense.
I was just looking for advice from other members of what gearing they use with similar commuting to my area ie flat and smooth and you have provided the answers.
I am now a wee bit wiser.
Thank's again
and
Kind Regards
Supergunn

wgwarburton

Hi,
OK... there's probably a better explanation on the 'net somewhere but...

Gear ratio is how many turns of the wheel you get per turn of the cranks, so a 32t chainring and a 16t sprocket give you a 2:1 ratio, same as a 36:18 or 42:21.

Gear inches is the effective diameter of your rear (driven) wheel- so with a 27" diameter wheel and tyre the above combinations give you a 54" gear. Historically, this is the diameter of the front wheel of an Ordinary bicycle (penny-farthing).

Most fixies are set up with a gear around 72", give or take quite a bit. Some are a bit less (I used to run 64"), others more (LiT's 88"). An Ordinary is limited by the inside leg length of the rider, typically to about 48-54", since you need a frame & saddle.

If you run SS instead of fixed, you can go lower, 'cos you can freewheel when you get too much speed up. It is traditional to set up a flip-flop hub with your preferred fixed gear on one side and a couple of teeth bigger freewheel on the other. You can then flip the hub without needing to change the chain length and get a "get-you-home" gear that lets you climb hills slowly and freewheel down if you run out of beans...

There are several online gear calculators- the rabbit applet (GIYF) is good for fixed riders, as it has useful features like rpm vs speed and equivalent cog sizes.

Hope this helps...

Cheers,
W.

Salsa

I have an old steel Marin MTB set up for short (sub 5 mile) commutes. I started out with 32/16 on 26" 1.6" slick tyres & personally found that unusable, sprint, spin out, coast, repeat.
I think I then tried 36 or 38/16 but even with a tensioner I couldn't get it too work (too loose with tensioner/chain not long enough for no tensioner).

I then got a really nice 44t Salsa SS chain ring very cheap (£8.50) & thought I'd give that a try, it's a little on the high side for really steep hills but switching to SPDs solved that problem & it gives a very usable gear ratio with the tensioner.

I've now changed ratio once again as I built a Surly SS hub wheel up that I got the parts for cheap, a friend gave me a trials version 18t White Industries freewheel.
Amazingly enough it gave a perfect chainline & without taking/adding links from when I ran 16t, I now have that magic no tensioner ratio! It looks so much cleaner & the gears a good compromise between speed & ease of use, I love it when these things just come together

wgwarburton

Salsa wrote:

...I then got a really nice 44t Salsa SS chain ring ... a friend gave me a trials version 18t ... the gears a good compromise between speed & ease of use...

44:18 on a 26x1.6 is about 61". Low for a fixie but works if you have a freewheel...

Cheers,
W.

N0bodyOfTheGoat

63" freewheel ( 42/18 ) on the Tricross works for me on the commute from Bitterne to Shirley. The comprimise needed to get me back home via Chessel Avenue or Mousehole Lane (both ~ 6% gradients for ~2Km) means that on the way in, it takes about an extra 5 minutes, due to the reduced gearing compared to the Pylon8.

stickman

I started on 46/18 (2.55) outgrew it, now on 44/16 (2.75) which is excellent for me.

wgwarburton

stickman wrote:

I started on 46/18 (2.55) outgrew it, now on 44/16 (2.75) which is excellent for me.

OK, but is that on 29er with a 2.3" tyre (80" gear) or a 26x1" slick (66" gear)... or maybe something in between? :-)

Cheers,
W.

stickman

It's 26x1.4 slick but I think the wheel size is not relevent because everyone goes with the ratio that suits them and they've got to find out by trying at least one then maybe going higher or lower. The best thing for me was writing out a load of ratios for reference to see big or slight differences then deciding what to get.

wgwarburton

stickman wrote:

It's 26x1.4 slick but I think the wheel size is not relevent because everyone goes with the ratio that suits them and they've got to find out by trying at least one then maybe going higher or lower. The best thing for me was writing out a load of ratios for reference to see big or slight differences then deciding what to get.

The ratio that matters, however, is the one between the distance moved by your foot and the distance moved by the contact patch between the tyre and the road. The wheel + tyre size is a significant part of this, so it is relevant.

The "Gear Inches" number gives you an easy way to compare different ratios combined with the wheel size. I say "easy" because the number you get is simpler to interpret and compare than a fraction to two or three decimal places.

Crank length ought to be factored in as well (Sheldon Brown, RIP, developed a system that caters for that: http://www.sheldonbrown.com/gain.html but it's not widely used).

You're right, though, that many will need to pick a starting point and then change according to how they get on. I went from 64" to 69" or so as I got fitter.

Cheers,
W.

asprilla

I find this to be the best gear calculator out there:

http://software.bareknucklebrigade.com/ ... pplet.html, especially since it provides some comparison to roadbike gearing allowing you to see what you are comfortable with first.

wgwarburton

Asprilla wrote:

I find this to be the best gear calculator out there:

http://software.bareknucklebrigade.com/ ... pplet.html, especially since it provides some comparison to roadbike gearing allowing you to see what you are comfortable with first.

+1 This is the "Rabbit applet" I was suggesting googling for upthread (Is that a word? Should it be?). Best option available for SS/FG riders.

All credit to Asprilla for making the effort to post a link...

Cheers,
W.

northstar

Might as well bump this, I am converting my MTB to single speed and I've narrowed down to using a 38/18 setup as I still want to take it off road now and again, does this sound about right to anyone who has done this before?

Thanks

stickman

Depends on the offroad, it would be low for firm grass on park land but for tough wooded areas it would be better.
The Specialized Tricross Singlecross cyclocross bike I think is 42/18 or similar.

northstar

It will mostly be common's and places like richmond park, newlands corner, nothing that difficult.

Thanks stickman

Am I likely to spin out on the roads easily on a 38/18?

wgwarburton

northstar wrote:

It will mostly be common's and places like richmond park, newlands corner, nothing that difficult.

Thanks stickman

Am I likely to spin out on the roads easily on a 38/18?

Yes, but if it's a SS you can always coast....

Cheers,
W.

ex-pat scot

42 x 16 for me on my Langster fixed wheel bike.
Gets me bowling along nicely on the flat at 18-22mph.
I can crank up the local 15/20% hills just about.

northstar

Cheers chaps, might go 40 now.