Gear Ratios on a New MTB

G1983

Hello all,

First time poster here. I’ve trawled through the search function, FAQs, external links etc and can’t find what I’m looking for. If I’ve missed something, please redirect me!

Anyway, I have an MTB with 26” wheels, a 42 tooth large chainring, and an 11 tooth smallest cog. I figure that my gear ratio is 3.81, or a gear inch of 99.27” (if I multiply by the wheel diameter, which is something I read about on the BikeRadar website).

I find that when I am bombing down a forest road, or fire-road, or something else not particularly technical, that I quite often “spin out” even in the highest gear i.e. I am pedalling full tilt but am not really putting any more into the wheel.

I am now in the market for a new bike and want to eliminate this “problem”. Is it as simple a case of me comparing the gear ratios and “gear inches” of new bikes to that of my old bike and making sure I get something with a higher top gear? Or is there more to it than that? e.g. does crank length make a big difference too?

I’m finding that a lot of the bikes I like the look of that have 2*10 gearing (Boardman Pro; Rockhopper Pro; Carve Comp) have lower maximum gear ratios and “gear inches” than I already have, which I am obviously wary of just now.

So I’m now looking more along the lines of 29ers with 3*10 gearing, such as Orange Clockwork S or the Cannondale SL1.

Any advice, or a shove in the right direction, would be very much appreciated.

Also, is it usual to look into gear ratios so much when buying a new MTB or am I over thinking things? Or am I worrying too much about the odd occasion where I get to blitz it full-tilt downhill....?

mrmonkfinger

I quite often “spin out” even in the highest gear i.e. I am pedalling full tilt but am not really putting any more into the wheel

Buy a 48T top ring then.

supersonic

120 rpm with 42/11 is about 35mph. Not usually much call for that speed on an MTB! But if you want higehr gears, as above, fit a chainset with a 48t ring.

snoopsmydogg

supersonic wrote:

120 rpm with 42/11 is about 35mph. Not usually much call for that speed on an MTB! But if you want higehr gears, as above, fit a chainset with a 48t ring.

or even work on cadence? pedalling at 120rpm is some good going in itself.

I have spun out a couple of times but I have never been anywhere near 120rpm so could go faster just by getting more training in.

G1983

Thanks for the suggestions folks. 48t chain rings are not that common though, are they? Will have a look. Good idea.

Supersonic, how do you work out that 120rpm is the "spin out" speed for a given gear ratio? Cheers.

And finally, no, not that common to be up at 35mph but on forest roads etc, which we've got a lot of round my way, I do it... and I feel I lag behind my mates!

cycleclinic

Learing to spin is a useful skill. Since I started road riding I now spin out hitting 140rpm. 120rpm is sustainable for short while now generally on hills.

Pratice spining. It takes a while though. There are 48T rings about but it depends on the BCD of your crankset.

supersonic

Supersonic, how do you work out that 120rpm is the "spin out" speed for a given gear ratio? Cheers.

The cadence of most riders is between 60-120rpm. Given the top end, it gives that figure for speed.

What is your cadence when you spin out?

The Rookie

Unless you train yourself, many people will think they are spinning out at 60-70rpm, when in fact 80-90rpm as regular peddling speed is more efficient and you should be able to go to 120rpm, a friend who single speeds can do 160rpm in short bursts (32/16 circa 26-28mph).

I very much doubt you are spinning out, you just think you are (that's not meant to be negative by the way!)

G1983

supersonic wrote:

Supersonic, how do you work out that 120rpm is the "spin out" speed for a given gear ratio? Cheers.

The cadence of most riders is between 60-120rpm. Given the top end, it gives that figure for speed.

What is your cadence when you spin out?

Sorry, I'm afraid I have no idea what my cadence is... Ever! Ha ha. I've just got a speedo, not a cadence...measuring... device... Or whatever they might actually be called

The Beginner, you may well be right that I am not actually spinning out. It just feels like I am, and obviously i'd like something where i don't feel like it's happening. I guess the crux of my query is: is gear ratio and wheel size the only combination I need to look at to get a higher top gear, or is there more to it...?

Thanks everyone. I appreciate the input!

snoopsmydogg

Easier question then. What speed do you feel like your spinning out?

If its 35ish then a bigger ring will help, if not learn to pedal faster. A bigger ring won't always help.

Cadence = crank revolutions per minute if that helps OP

dv69

G1983 wrote:

supersonic wrote:

Supersonic, how do you work out that 120rpm is the "spin out" speed for a given gear ratio? Cheers.

The cadence of most riders is between 60-120rpm. Given the top end, it gives that figure for speed.

What is your cadence when you spin out?

Sorry, I'm afraid I have no idea what my cadence is... Ever! Ha ha. I've just got a speedo, not a cadence...measuring... device... Or whatever they might actually be called

If you've got a speedo you can calculate the cadence:

e.g for 35 mph
multiply by 1760 => 61600 yards per hour
multiply by 36 => 2217600 inches per hour
divide by 60 => 36960 inches per minute

Now divide by the wheel circumference (3.14*26 = 81.64 inches):
36960/81.64 = 452.72 wheel revolutions per minute

If you divide this by the gear ratio you will get the cadence. For gear ratio of 3.818, cadence = 452.72/3.818 = 118.6 rpm

G1983 wrote:

I guess the crux of my query is: is gear ratio and wheel size the only combination I need to look at to get a higher top gear, or is there more to it...?

You missed out cadence but otherwise that's correct. Cadence is multiplied by gear ratio which is multiplied by wheel size. You can get higher speed by increasing any of those three. If you try to go for a bigger outer ring bear in mind that gear changes may not be as smooth or quick if the there is a big jump from middle to big ring.

snoopsmydogg

dv69 wrote:

If you've got a speedo you can calculate the cadence:

e.g for 35 mph

*insert calculation here*

Or you could just use something like sheldon browns gear calculator

http://sheldonbrown.com/gears/

jimothy78

Surely a better answer to this "problem" is to find better trails. If you're whizzing down fireroads at those sort of speeds, then I think most MTBers would consider you're effectively "wasting" all the climbing you've done/will do to get to the top.

Hunt out some nice tight twisty, singletrack descents and you're unlikely to even want to use your top chainring!

G1983

snoopsmydogg wrote:

Easier question then. What speed do you feel like your spinning out?

If its 35ish then a bigger ring will help, if not learn to pedal faster. A bigger ring won't always help.

Cadence = crank revolutions per minute if that helps OP

Cheers snoops. Yes, it is around the 35+mph mark.

And thanks for clarifying re: cadence. I knew what it was, but had no idea how to measure it.

G1983

dv69 wrote:

If you've got a speedo you can calculate the cadence:

e.g for 35 mph
multiply by 1760 => 61600 yards per hour
multiply by 36 => 2217600 inches per hour
divide by 60 => 36960 inches per minute

Now divide by the wheel circumference (3.14*26 = 81.64 inches):
36960/81.64 = 452.72 wheel revolutions per minute

If you divide this by the gear ratio you will get the cadence. For gear ratio of 3.818, cadence = 452.72/3.818 = 118.6 rpm

G1983 wrote:

I guess the crux of my query is: is gear ratio and wheel size the only combination I need to look at to get a higher top gear, or is there more to it...?

You missed out cadence but otherwise that's correct. Cadence is multiplied by gear ratio which is multiplied by wheel size. You can get higher speed by increasing any of those three. If you try to go for a bigger outer ring bear in mind that gear changes may not be as smooth or quick if the there is a big jump from middle to big ring.

Fantastic. Thanks dv69.

The Rookie

I find it hard to believe you're really hitting 35mph off road, I only regularily see just on 40 on road (low drag slick tyres downhill).......

G1983

jimothy78 wrote:

Surely a better answer to this "problem" is to find better trails. If you're whizzing down fireroads at those sort of speeds, then I think most MTBers would consider you're effectively "wasting" all the climbing you've done/will do to get to the top.

Hunt out some nice tight twisty, singletrack descents and you're unlikely to even want to use your top chainring!

Yes, you're absolutely correct :oops:

However, where I stay we have a lot of forest roads leading to/from the car parks up into the trails. Not really a complaint for me as I find them easier to climb on, and I must admit I also get a real buzz from horsing down them at full tilt at the end of the ride!

Cheers jimothy.

G1983

The Beginner wrote:

I find it hard to believe you're really hitting 35mph off road, I only regularily see just on 40 on road (low drag slick tyres downhill).......

Perhaps you are right TheBeginner, and if that's the case it's down to me setting up my speedo incorrectly.

I'm not trying to show off or anything, if that's how I'm being perceived. I really was just wondering, and dv69 has answered, whether gear ratio and wheel size are at the heart of the matter, or whether there are other things I should be looking at.