Pedalling too fast?

[Stuart_Kirk]

Please excuse the lack of technical terminology in this post!

I have a 21 gear mountain bike that I use for commuting. The bike has slick tyres on for road use but other than that no other modification.

I am finding that I reach the bike's capacity too quickly - In top gear I am getting to the stage where I am required to pedal too quickly to achieve any further acceleration, there is insufficient resistance in the mechanism. I rarely need to use lower than third on the bottom bracket. I'd like some advice on what modifications I can make to the bike to extend the top end range so that I can accelerate further - I've considered new bottom bracket gears (bigger) or different pedal arms, would like some guidance on what changes I should/shouldn't make.

Hope that makes sense! Any advice gratefully recieved.

 

[RedBike]

To be honest there's not much you can do.

Your bike has a 7 speed freewheel at the rear so there's not many alternatives available there. You might be able to change the front chainrings over to a touring set; however, on budget bikes this isn't always possible and would probably mean a new chainset.

I rarely need to use lower than third on the bottom bracket

There's either no hills where you live or your a very strong cyclist.

Count how many times you push one pedal (say the right) down in a minute. You should get something like 90, 100 if you're pedalling quick.

I will work out your top speed of your highest gear when I get a chance. I would expect it to be something like 30/35mph.

 

[wafflycat]

If you're in the big chainring at the front & the smallest sprocket at the back, then there's not much you can do. MTBs are normally lower geared than road bikes as they are designed for dealing with going up off-road slopes rather than going fast on tarmac.

 

[col]

I have a similar setup and find it does happen(rarely for me)looks like you need a racer now.

 

[Globalti]

If you're that strong you'll find a road bike exhilaratingly fast.

 

[Landslide]

Depending on what set-up you have at the moment, you may be able to fit larger (higher/harder) gears. This might entail:
- Changing the chainrings (large cogs at the front of your drive-train) if they can be unbolted from the chainset (assembly of crank arms (pedal arms) and chainrings).
- Fitting a new chainset (some models are essentially one piece affairs with the rings bonded in some way to the crank arms).
In both instances you'd need to adjust the postion of the front derailleur (front gear mechanism), but all derailleurs have a maximum size of cog that they can deal with, and you may find that your current front derailleur won't work properly with larger chainrings. You may also need to add some links to your chain.

Moving away from the idea of changing your gears, you may want to try altering your cadence (your "RPM"). For a given speed, you may find it more efficient to pedal faster in a lower/easier gear (as opposed to pedalling more slowly in a higher/harder gear). Try to think of this as pedalling in circles, rather than just pushing down on each pedal in turn.

Hope this makes sense...

 

[fossyant]

I have a 46 front chain ring on my MTB and a 13 tooth rear - find it's fine up to about 33 mph pedaling, but that's faster than the cars go on that particular stretch on my commute.

On the rear I have a 13-23 7 speed cassette, so it's more road bike gears at the rear. Never use the middle and lower front sprockets though !

What size is your front ? You may get up to a 48/50 front though !

 

[RedBike]

Right, assuming you have a top gear of 44/13 and you're running 26x1.95 tyres (I've just guessed these bits).

Your tyres have a circ' of 2050mm
Therefore pedalling at 110rpm in top gear you'd cover
110 * (44/13) *2050 = 763,000mm/min

or 0.763km/min = 45km/hr = 28mph

Ok, it's perfectly possible to go quicker than 28mph down a hill; but there's no way you're capable of holding this sort of pace on the flat.

 

[Mr Pig]

Count the teeth on the smallest gear on the 'cassette' on the back wheel. If it is more than eleven you can fit a cassette with eleven teeth, which will be faster.

That and changing the front gears for bigger ones will give you more top end.

 

[RedBike]

Count the teeth on the smallest gear on the 'cassette' on the back wheel. If it is more than eleven you can fit a cassette with eleven teeth, which will be faster.

That and changing the front gears for bigger ones will give you more top end.

With the bike only being 7 speed he's more likely to have a freewheel than a cassette. I'm not sure you can get a freewheel with an 11 (or 12) tooth sprocket.

 

[Mr Pig]

I think you can, and I'm sure you get a 7-speed cassette with 11 teeth.

 

[RedBike]

I think you can, and I'm sure you get a 7-speed cassette with 11 teeth.

There are loads of 7 speed cassettes with an 11 tooth sprocket. However, I've never seen a freewheel with an 11 tooth inner sprocket. (This doesn't mean there arn't any). Shimanos standard 7 gear freewheel is 14/28 !

(I'm just guessing that the bike has a freewheel instead of a cassette. )

 

[Stuart_Kirk]

Wow - wasnt sure I'd get a response, let alone so much advice.

I'm not really sure about rpm, speed etc. I'm not even sure if this is a particularly unique case, maybe I just need a road bike. I'm not claiming to be Chris Hoy or anything!

Am I right in thinking that the changes suggested so far may only have limited effects? I will get the stats that have been requested tomorrow, I can't imagine I'm going at 28mph, just that alot of the time the req'd pedal speed to continue accelerating makes me feel particularly unstable.

Need to try a road bike - thanks for the suggestions so far!

 

[LLB]

I'd be interested to know how fast you think your pedalling is.

It's generally accepted that a cadence of around 90 (rotations per minute) is the best balance of efficiency over power.

Definitely feel the cadence crucial to my energy levels. Too low and I get the burn, too high and legs feel like they are going like the clappers.

 

[Twiggy]

Tangent, just so we can rule out some really simple things before we start suggesting complicated changes to your bike/riding style.

There are a few things that can make you feel unstable while peddling still relatively slowly. Not saying you're doing anything wrong, but, best to check and be on the safe side, no?

The first and most obvious would be the saddle being too low, common mistake, esp with mountain bikes. You want to have your leg nearly fully extended at the bottom of your stroke on the pedals.

Next one that could be throwing your balance off if you're at the right height is incorrect foot positioning on the pedals, the ball of your foot should be directly over the middle of the pedal.

beyond that there isn't much else I can think of that might be throwing you out, so if neither of these are part of the situation, then I must take a moment to say "wow".


Getting a road bike would probably be the most simple(but not cheap) solution.