Gear Ratio Change

For a MTB a 48 big ring and an 11 tooth small cog on the rear cassette is it. 😉

I use Shimano TX71 Cranksets with 28/38/48 tooth chainrings and 11-24 tooth Sunrace Cassettes.

See: http://www.cyclingforums.com/t363004.html

You bought the wrong bike.

You probably already have an 11-tooth cog on the back so that's the fastest gear available for back there.

Changing the front chainrings can turn into a fubar of a project. You probably have a 46-tooth big ring now. Depending on your crankset you may be able to buy a 48-tooth chainring that will work but I doubt that will make enough difference to satisfy you. Going beyond a 48-tooth chainring can get you into all sorts of issues - chainring scraping on the bike frame, different interface with the bottom bracket, and front derailleur problems.

Your either very fast or you ride with very slow leg speed. do some training in the lower gears to get your leg speed up. around 60 - 70 rpm. A good tip is listen to some funky techno music on the ipod and keep in time with it (most techno is about 140 beats per min. devided by 2 legs gives you 70 rpm)

MarkInNC said:

I have been riding my new Trek 6500 for about a month now. This is a mountain bike that I ride on the road. I have replaced the tires with smaller road type tires. I find that most of my riding uses the largest gear up front and the four or five smallest gears in the back. I almost never have occation to use the middle front gear. I never use the small front gear.

Has anyone changed he three gears up front to eliminate the smallest and add a larger big gear. Is this possible, practical, easy?

Or should I look at changing the cassette in the back?

thanks,

Mark

What cadence are you pedaling?

I guess I should have talked about cadence in my original post. My computer does not show cadence so I have to look at the clock and count to get the number. So, I don't have as good a picture as maybe I should. However, I am just over eighty in the highest gear going down a grade and at 18-20mph. (20 is the highest speed Ive seen going downhill with the wind) My normal flat road no wind cadence seems to be around 60 or a few more. Going up hill using a lower gear my cadence varies between about 50 to 70.

MarkInNC said:

I guess I should have talked about cadence in my original post. My computer does not show cadence so I have to look at the clock and count to get the number. So, I don't have as good a picture as maybe I should. However, I am just over eighty in the highest gear going down a grade and at 18-20mph. (20 is the highest speed Ive seen going downhill with the wind) My normal flat road no wind cadence seems to be around 60 or a few more. Going up hill using a lower gear my cadence varies between about 50 to 70.

According to the Sheldon Brown gear calculator, sheldonbrown.comgearsOpen ↗: 26"x1.9" tires, 175mm crank arm, MPH @ 80 RPM, 44 chainring, 11 cassette = 24.5 MPH.

Either your computer speed is wrong or you're not counting cadence correctly.

Good catch, but there is at least one other possibility which is the case in this situation. I bought the bike with differant tires which have a diameter and therefore circumference differant than the stock tires. This differance affects the speed made good per revolution, of course. In my case, the effect is a bit lower overall gearing.

I have played with the computer to set a custom wheel size based on measuring the distance for one revolution. I have a handheld gps which is very accurate and I have compared the two. My computer is close, but not exact.

Mark

MarkInNC said:

Good catch, but there is at least one other possibility which is the case in this situation. I bought the bike with differant tires which have a diameter and therefore circumference differant than the stock tires. This differance affects the speed made good per revolution, of course. In my case, the effect is a bit lower overall gearing.

Actually, I assumed a smaller-than-stock tire size of 1.9" based on an earlier description of yours. 18 MPH compared to 24.5 MPH is a large difference, 36% to be exact. The only way to get a good circumference reading is to perform a rollout test with the tires at normal PSI and you sitting on the bike (sheldonbrown.comcyclecomputer calibration.htmlOpen ↗).

Back to your original question, you could change your large chainring to a 48 tooth max, but you will probably also need a new front derailer. Rather than eliminating the small and middle chainrings, you could change them from a 22 and 32 tooth combo to larger gears such as a 34 and 42 combo. As stated above, your 11 tooth cassette gear is already as small as possible, but you could get a cassette with a tighter high-end span such as 11-23.

To be honest and not trying to sound like too much of an ass, I just can't believe a beginner cyclist is running out of gears at 18-20 MPH with a cadence of 80 RPM going downhill. Even though you're on a mountain bike with lower gearing, something just doesn't add up here.

Scotty_Dog said:

Actually, I assumed a smaller-than-stock tire size of 1.9" based on an earlier description of yours. 18 MPH compared to 24.5 MPH is a large difference, 36% to be exact. The only way to get a good circumference reading is to perform a rollout test with the tires at normal PSI and you sitting on the bike (sheldonbrown.comcyclecomputer calibration.htmlOpen ↗).

Back to your original question, you could change your large chainring to a 48 tooth max, but you will probably also need a new front derailer. Rather than eliminating the small and middle chainrings, you could change them from a 22 and 32 tooth combo to larger gears such as a 34 and 42 combo. As stated above, your 11 tooth cassette gear is already as small as possible, but you could get a cassette with a tighter high-end span such as 11-23.

To be honest and not trying to sound like too much of an ass, I just can't believe a beginner cyclist is running out of gears at 18-20 MPH with a cadence of 80 RPM going downhill. Even though you're on a mountain bike with lower gearing, something just doesn't add up here.

Yes im pretty sure thats the case too, i have the same gearing on my mtb and i have no problem crusing at 40kmh, Bigger profile slick tyres could help you too, i dunno why anyone runs those tiny skinnys on a 26" wheel, there are 2" slicks available and at 60psi they handle good on the road.

I had to use my calculator to convert 40 kilometers to 24mph. My max so far is about 20 miles per hour going down a grade and with the wind. We don't have any big hills on the coast here but have some grades or gentel hills.

I have 1.5 inch continentel tires on the bike now. I am not sure what they are called but they are pretty much slick in the middle and have some tread grips on the edges.

It sounds as though several folks who know more than I do believe I have calculated something incorrectly. The one thing with the greatest chance for error is my cadence counting so I will repeat that at as close to 20 mph as I can maintain and see what I get. The gps pretty much agrees with the bike computer at 5, 10, and 15 mph so I would think that at 20, (unchecked), the computer should be very close.

thanks all,

Mark

MarkInNC said:

I had to use my calculator to convert 40 kilometers to 24mph. My max so far is about 20 miles per hour going down a grade and with the wind. We don't have any big hills on the coast here but have some grades or gentel hills.

I have 1.5 inch continentel tires on the bike now. I am not sure what they are called but they are pretty much slick in the middle and have some tread grips on the edges.

It sounds as though several folks who know more than I do believe I have calculated something incorrectly. The one thing with the greatest chance for error is my cadence counting so I will repeat that at as close to 20 mph as I can maintain and see what I get. The gps pretty much agrees with the bike computer at 5, 10, and 15 mph so I would think that at 20, (unchecked), the computer should be very close.

thanks all,

Mark

Woops turns out your right mark, this problem was bugging me so last night i measured a 26x1.75 tyre and did the calcs to work out your rpm's,

44t x 11t @ 70rpm = 21.72mph

If i work it back from your 25 mph it turns out you are doing 84rpm
Anyway this got me thinking that your problem is not realy top speed, i would say that its more that you only have a few gears in your speed range that you can use, which is probably because you have a mountain bike rear cog set which has large spaceings, eg 11 - 32

If you changed that to a road bike cog set it would give you alot more gears in your range eg 11-21. Kind of like haveing a close ratio gearbox.

Hope that helps

Insaneclimber said:

Bigger profile slick tyres could help you too, i dunno why anyone runs those tiny skinnys on a 26" wheel, there are 2" slicks available and at 60psi they handle good on the road.

Following the same argument, why didn't Lance use 2" tyres?
The answer is "wind resistance". I run 1.3" on my 26"-wheeled tourer.

Thanks for spending the time and effort to figure out where I am cadence v speed wise. Yes, you are right, with a mountain bike I don't expect that I will be able to get my speed much beyond where I am now. I was just concerned that as a new ridder with a new bike and very little knowledge that I was very slow compared to where I should be. That said, you are also correct in your opinion that the only real issue is that I have a bike with a whole bunch of gears and I am only using the top four or five. Thinking this through, that is not really an issue it is just what is.

thanks,

Mark

Insaneclimber said:

Woops turns out your right mark, this problem was bugging me so last night i measured a 26x1.75 tyre and did the calcs to work out your rpm's,

44t x 11t @ 70rpm = 21.72mph

If i work it back from your 25 mph it turns out you are doing 84rpm
Anyway this got me thinking that your problem is not realy top speed, i would say that its more that you only have a few gears in your speed range that you can use, which is probably because you have a mountain bike rear cog set which has large spaceings, eg 11 - 32

If you changed that to a road bike cog set it would give you alot more gears in your range eg 11-21. Kind of like haveing a close ratio gearbox.

Hope that helps

Insaneclimber said:

Woops turns out your right mark, this problem was bugging me so last night i measured a 26x1.75 tyre and did the calcs to work out your rpm's,

44t x 11t @ 70rpm = 21.72mph

If i work it back from your 25 mph it turns out you are doing 84rpm

What do any of those numbers have to do with this post?

He has now stated:
26"x1.5" tires
80 RPM downhill

My assumptions for a mountain bike:
175mm crank length
44x11 gear combo

= 23.7 MPH

But he claims to only be going 18-20 MPH. Therefore, something is amiss. Because he has verified his MPH with a GPS unit, I believe he is actually pedaling at only 60-70 RPM, and could see gains in speed by pedaling at a higher cadence. That might take a little time and practice, but should be readily achievable.

Scotty_Dog said:

What do any of those numbers have to do with this post?

He has now stated:
26"x1.5" tires
80 RPM downhill

My assumptions for a mountain bike:
175mm crank length
44x11 gear combo

= 23.7 MPH

But he claims to only be going 18-20 MPH. Therefore, something is amiss. Because he has verified his MPH with a GPS unit, I believe he is actually pedaling at only 60-70 RPM, and could see gains in speed by pedaling at a higher cadence. That might take a little time and practice, but should be readily achievable.

Quote "I have a bike with a whole bunch of gears and I am only using the top four or five"

Not everyone is a racer. Maby you could offer some tips on how he could improve his cadence.

Insaneclimber said:

Maby you could offer some tips on how he could improve his cadence.

google.comgoogle.comOpen ↗

Insaneclimber said:

Quote "I have a bike with a whole bunch of gears and I am only using the top four or five"

Not everyone is a racer. Maby you could offer some tips on how he could improve his cadence.

He should buy a computer with a cadence function. It helped me speed my cadence up when I started. After a few months I could pretty much tell without it.

I commuted about 4 miles each way an a MTB which would not accept a front ring larger than 48. Instead of putting a bigger ring on I used a cheap crank with a 44 paired with an 11-21. I was probably running 1.75's but it been a few years.

I had hills and still could have used a bigger ring.
Eventually I switched to a 700 which was better.