New to Cyclocross: Should I change 46 ring to a 50?

[dustyC]

I am a MTBer. Have been gradually changing my non suspension bike to a
commuting/touring bike and now the only way to take it further is to
get a new bike.
I have decided I'd like a Cyclocross as it would suit me for touring
and the CAADX Tiagra currently appeals
BUT...
It comes with 36/46 chain rings.
I want this bike to be a fast commute/tourer.
Could/should I change the big ring to a 50? or maybe even bigger?

I'll be changing the tyres for semi slick/hardpack towpaths anyway
(something similar to my MTB Maxxis wormdrives)

 

[tafi]

The 36/46 or 36/48 are favourite combinations for cross racing, which is very stop start and often on grass/mud. They are probably a bit low for commuting, but on a compact you probably don't need to go higher than 50 for commuting.

So yes you can change if you need to but it is probably best to figure out if you need to first (ie: buy the bike as is, safe in the knowledge that you can modify as necessary).

 

[daveryanwyoming]

+1 on Tafi's post, bikes equipped for cross racing generally have 46 tooth big rings which is about right for most courses as it lets you ride most if not all of the course in the big ring and then run up the really steep stuff.

Unless you really wind it out fast on some long straight descents during your commutes you should be fine with the 46 tooth big ring matched up with an 11 tooth cog in the rear. That's still nearly 114 gear inches or more when you consider the large radius tires you'll likely run on your cross bike (~113 gear inches with stock road wheels). That's a big gear and roughly halfway between a 52/12 and 52/13 which is plenty of gearing for fast riding. At 100 rpm the 46/11 combo rolls along at roughly 34 mph.

FWIW, my cyclocross bike is set up for racing but I commute on it a lot during the winter months, I've got a few long fast descents on my commute that take me upwards of 30 mph but I've never felt I was spinning out the 46/11 gearing.

-Dave

 

[dustyC]

Thats brilliant advice. Thanks to you both.

On my MTB with slicks I'm in top gear 42/12 all the time on the flats and so was thinking I'd need to get much lower gearing (faster). I wasn't thinking the wheel/tyre would make much of an extra difference. Thank you for explaining that to me.

The CAADX Tiagra comes with 36/46 and 12-26 so perhaps good for touring and I can always update the bigger chain ring, or can I?!
As Tafi says "buy the bike as is, safe in the knowledge that you can modify as necessary"
But I can't find out if its possible to get chain rings to fit!

Spec says its a FSA Omega MegaExo 36/46. Can I even get a bigger chain ring for this?
I'm guessing I'll need a longer chain too.

Thanks for you help so far, its been great and a relief that this bike might finally be my choice!

 

[daveryanwyoming]

Originally Posted by dustyC .
...Can I even get a bigger chain ring for this? I'm guessing I'll need a longer chain too...

Yes, your crankset has a standard 110mm BCD bolt circle which is what defines a 'compact' road crankset. You can run any compatible 110 BCD chainrings. There's typically no problem going up to at least a 50 tooth big ring but if you go much bigger and stay with your existing 36 tooth small ring you might need a longer cage rear derailleur to take up the extra chain slack (your cyclocross bke might already be equipped with a medium or long cage rear derailleur in which case this isn't an issue for you).

If you swap to a larger big ring you'll likely need to raise and readjust the front derailleur slightly so that it clears the bigger ring and you may or may not need a longer chain depending on how much chain slack you currently have in your big ring to big cog gear combination.

BTW, your current MTB top gearing of 42x12 with 26" wheels works out to 91 gear inches.

The stock 46x12 combo on the Tiagra works out to ~104 gear inches.

Changing the big ring to a 50 and keeping the same cassettes and you get a 50x12 ~113 gear inches

Keeping the same 46 tooth big ring but swapping to say an 11x27 cassette gives you 46x11 ~ 113 gear inches

And swapping both the big ring to a 50 and the cassette potentially gives you a 50x11 ~ 123 gear inches which is absolutely huge for a commuting bike unless you happen to be riding a lot of long straight alpine descents on your commute where you'll be holding 40 mph or more for sustained sections.

Bottom line, you can definitely change your front rings as long as you get a compatible (9 speed, 10 speed, etc.) chainring with a 110mm BCD bolt circle. But it's generally easier to make gearing adjustments by changing your cassette and the 46x11 combo gives you the same top end gearing as a 50x12 combo and either is plenty for most uses outside of racing very fast events.

-Dave

 

[dustyC]

Amazing.
Thank you for a very informative answer.
I can see what the overall solution is; just buy the fecking bike and stop procrastinating!
It would appear that its going to be simple enough to change it to meet any of my future requirements.

Thank you for all of your help.
Is there a good source where I can see how to work out the gear inches?

As for my commute, Portsdown is nice, but its not quite "a lot of long straight alpine descents on your commute where you'll be holding 40 mph or more for sustained sections."
Though that would be a commute worth doing!

(I wouldn't fancy the ride home as much though ) )

 

[daveryanwyoming]

There are a lot of good online gear calculators like this one: http://cycleseven.org/bicycle-gear-inch-calculator

Or this which is even more complete and gives you a lot of ways to compare gearing: http://www.sheldonbrown.com/gears/

But it's pretty simple. gear inches are used by track racers a lot to compare gearing combinations for different events and basically represent the equivalent wheel diameter you'd get if you had a big direct drive wheel (think kids tricycle with pedals directly on the wheel or old fashioned high wheeler Penny Farthings) that was equivalent to your normal wheel size plus the gear ratios. You calculate it as simply:

Chainring_teeth / cog_teeth * wheel diameter in inches


-Dave

 

[dustyC]

Dave,

Thanks for all your help. I spent the weekend in London trying bikes and settled for the CAADX. It seemed the ideal quick commute and comfortable tourer.
Its not as fast as the road bikes I took out and I did feel I ran out of gears quickly but thats something I can upgrade as I go. Will probably get a bigger chain ring as the cassettes available aren't too good. 12-27 is a better range for me than 11-25

But...
I liked the road bikes so much I wanted one of them as well! I think that purchase will have to wait till spring time and the cycle-to-work scheme.
Now its time to plan in some long tours of Europe )