# Am i right in thinking....

Discussion in 'Bicycle Mechanics and Repairs' started by Joe24, 15 Jun 2008.

1. ### Joe24More serious cyclist than Bonj

Location:
Nottingham
That if i put a bigger chainring on the front i will need to either put a new chain on/ make the chain longer or put the new chain ring on the front and chain the gears at the back to be smaller. But that even if i did make the gears lower(if thats right, less teeth) then i would still need to make the chain longer because i dont want the gears to be too small at the top when i go up hills.
I ran out of gears today doing 32mph in a chain-gang and had to just sit behind the back trying to slipstream and keep up. Got harder near the end, couldn't keep my legs going fast enough to keep pressure on.
So am i right or do i just make no sence?

Cheers

2. ### RedBikeNew Member

Location:
You haven't said what sort of bike / gearing you've currently got. Assuming a road bike with a compact chainset (50 tooth outside ring) and a cassette with a 12 inner ring, 700c wheels (2100mm circumfrence).

Then:
50/12 = 4.166 (gear ratio)
Pedalling at just 90rpm, 90 x 4.166 = 375 rpm (wheel speed)
375 x 2100 = 787500 mm/min = 0.7875km / miin = 45 km/h or about 31mph.

This means even with these relatively low gears you should be able to keep up at 30mph without spinning out.

In most cases you can just switch one chainring for another; but there is the chance you will need to start switching rear mechs, cassettes, chains. It all depends on what you've currently got and how worn they are.

3. ### bonj2Guest

what gear ratio were you in when you were 'running out' at 32mph?
On my road bike i've got 30/42/52 chainrings, and and 11-23 cassette. The middle ring is good for 30mph, i.e. I'm doing a comfortable cadence at that speed on 42x11. Any more than 30 and i'll need my big ring, but even if you've only got a 50-tooth big ring and a 12-tooth small sprocket, that should be plenty enough gearing for high-30s mph...

4. OP

### Joe24More serious cyclist than Bonj

Location:
Nottingham
If i am right, the other ring is only a 44 or a 46, the smallest is a 12 i think. Not 100% sure, cant remember what the numbers are. It is a road bike.
I was thinking of taking the outer ring upto a 50 or 52.
The rear cassette was new-ish. But has done about 1700miles i think. Chain-rings done 3743miles.
Chain hadn't come off, i was spinning and even with keeping on flicking to move to the next gear i was out of gears The other people still had about 2-3 gears left.

5. ### fossyantRide It Like You Stole It!

Location:
South Manchester
I can do about 32mph on my 46 x 13 on my MTB commuting, so you are about right (being comfortable - I'm no spinner)... You might need a bigger ring..Ahem !!

What are the rings on the bike now, and what is the BCD (if printed - Bolt Circle Diameter) - other than that - what's the chainset ????

I have a 53 x 13 on both road bikes -and spin out at about 40 - never really need to pedal that hard, as it's tuck at that point..... and off for 45...50...55....60.................soil pants.....

6. ### bonj2Guest

so your current outer ring is 46? and you were spinning out at 32mph on 46x12? that's about the same as 42x11, so yeah that sounds about right actually.
What's your middle ring then, or have you only got a double?
I would be tempted to say just whack a 52 on, but you ideally don't want a jump of more than 12 teeth on chainrings. However, some people have compact chainsets which obviously do have more than 12 tooth jumps. I don't think i'd personally get on very well with anything other than a triple, but having decided that you probably want either a 50, 51 or 52, you probably now also want to have a think about what you want for your other chainrings.
Like I say I'd strongly recommend a triple as it means you've nearly always got scope to spin up a hill rather than honk, but it's obviously only your decision...

7. ### RedBikeNew Member

Location:
These sound like MTB / hybrid ratios. I'm afraid you might not be to fit a significantly larger ring to your existing cranks.

8. ### bonj2Guest

might be time to get a new crankset?

9. ### RedBikeNew Member

Location:
It all depends on what the crankset is (and what front / rear mech you have). You can probably fit a 48;

10. OP

### Joe24More serious cyclist than Bonj

Location:
Nottingham
Thought about a new crankset to upgrade to go slightly lighter. I'll find out the other stuff tomorrow. Its a copact double, not keen on going to a tripple though.

11. OP

### Joe24More serious cyclist than Bonj

Location:
Nottingham
Its standed Giant SCR4 stuff, but has Sora tripple shifters.

12. ### RedBikeNew Member

Location:
CRANKS TruVativ Touro 2.0 36/50
(Off the web /08 model)

Mind you it also lists the shifters as shimano 2200 not sora.

13. OP

### Joe24More serious cyclist than Bonj

Location:
Nottingham
That will be right then. My shifters were upgraded because LBS didnt have any 2200 ones and it didnt come with any, it was ex-factory. I'll check the large ring, but im sure i saw either 44 or 46 on it.

14. ### PatrickPendingGuru

Location:
Leicester
mmmm I wonder if the bike didn't have any of the drivechain and they just put a front chainset that's a bit too small on it?

You could try more teeth on the front chain as suggested - if you do go to a triple you may need a new long cage rear deralieur and a new front - if it were me I'd go 52/42 on the front- though this depends on how many hills you encounter etc

Good luck.

15. OP

### Joe24More serious cyclist than Bonj

Location:
Nottingham
I was wrong, it does have a 50tooth on the front. Smallest on the back is a 13tooth.
Got a long cage rear already.
Abit hilly around here(i think) to put on a 42/52 instead of the 36/50.
So, would putting upgrading the rear block so the smallest is a 11 work, or would taking the larger ring up to 52 and changing the block work better?