Difference between 27 & 25 teeth on the rear cassette

warrfla

I finally got up Sheskin in one go tonight - 3rd attempt. I usually have to stop at the switchback to catch my breathe and get my heart rate in check. The last 100m was horrible but I wasn't stopping as it's impossible to start again when the gradient is over 20% (I may be exagerating with that %).

My question is I've been attempting this with a 39x25 combination, I thought my rear cassette was a 12-27 (I'm sure that's what I asked my LBS to fit) turns out it's a 12-25, I have a 12-27 on a winter hack wheel, would those extra 2 teeth make much of a difference if I swapped cassettes?
I'm doing the Sean Kelly in august and althought the climb is only 1.5km it takes a fair effort and there's still alot of the Sean Kelly left after this climb!!

blorg

Yes, it would make a fair difference, 8% to be exact. Tough hill to get up on a standard I imagine, the others are longer but not near as steep.

Acoustic

2 teeth

Raam

It will make a big difference. When I started out I had a 53/39 and a 12/25. Found the hills really difficult. Switched to a 27 and it was like someone had run a giant steamroller all through the country. OK, not quite, but it made hills easier.

Quigs Snr

Hi Warrfla, I am intimately familiar with Seskin as it features on our club training rides on a regular basis (Some of us do hill repeats up there). Seskin is easier on a 27 no question, its not a HUGE difference, but like Raam says its significant enough to make a difference to you.

On a hill like that (18% is the last bit by the way, seems worse because of the poor surface) my tip is to try to get over it as slowly as possible. Sounds daft... but I have a kind of default steep climb mode for when I have no legs on a climb like that. I call it the death march and its all about going as slowly as possible just using body weight to push the pedals. It got me over an 80 minute slog over a 24% climb in a 39x25 in a far off land once, made it but felt like I had spent the morning doing squats.

irishmotorist

The Sheldon Brown gear calculator is quite handy and will give you an idea of the difference it will make - work out the gear inches. That said, it's probably impossible to imagine or describe the exact nature of the difference so getting it would be your only way to be sure.

Here's the gear calculator...
http://www.sheldonbrown.com/gears/

Edit:
Looking at the calculator now, km/h at 40 and 60 rpm in the Gear Units box, which might be what you'd expect to be doing on that hill, might be a slightly more 'real world' way of understanding how much of a difference it will make.

Pherekydes

My lowest ratio is a 42x28.

/sulks

I have 52-42 chainrings and a 13-28 cassette. What can I do? Any sort of extended (>10secs) climbing is MHR territory.

blorg

Pherekydes wrote: »

My lowest ratio is a 42x28.

/sulks

I have 52-42 chainrings and a 13-28 cassette. What can I do? Any sort of extended (>10secs) climbing is MHR territory.

You could (1) get a compact chainset (50-34) or (2) get a mountain bike rear cassette and mountain bike derailleur... If you have a 52-42 I am guessing your bike is reasonably old and so you won't be running 10 speed anyway, so (2) is probably the easiest and cheapest option. You can get mountain bike cassettes in ratios like 11-34 so you would even have a faster high end to boot.