Fitting a Larger Chainring
 

I have a Scott CX1 bike which has a 36/46 chainset. I am considering fitting a 52T ring in place of the 46 one and the Shimano FD-CX70 derailleur would seem to accept this as it can handle up to a 16T difference between rings.

However, will this larger chainring provide me with a noticeably faster/better performance than the 46T ring? Or I am pretty much wasting my time here? For interest, the rear cassette is 10 speed - 12-30T

The 16 tooth difference will work... barely. Shift quality will probably suffer.

36 to 52 is a huge jump. What are you trying to accomplish? Are you riding a the road a lot?

Hardly, it's a standard mid/semi-compact crank ratio.

For the usage, if using purely on road, it will work fine, and allow greater speed (if the legs will allow), off road, probably a bad idea.

I recently upgraded from a 56/12 high gear, which was shifting fine for the past 10 years, to a 60/11 setup, and now I'm having problems. The FD was working fine but the bike shop replaced it anyway. Now it takes 4 rotations to jump from my 48 middle ring to the 60 big ring.

I may need to seek out a dérailleur expert, or build my own electronic one with a small stepper motor and actuator.

As for "making a difference," your 46/12 can hit 28 MPH @ 90 RPMs while a 52/12 can reach 32 MPH at the same cadence (calculated with 700 mm tires).

I should have added that I would go with a 50 rather than a 52.

Your can use this site to see the effect of changing to a new gear combination before spending time and money: https://www.surplace.fr/ffgc/

Riding down Mt Everest ?
Once I get over 50 mph, I just coast.

Back when 13t was the Highest gear on freewheels & cassettes 52 made sense , now that they go to 11, a 46:11 or 48:12
should be plenty high.

LOL!

We have a few hills along my riding path with long run outs. I was spinning out at 56/12 but I can't hold 110 RPM for long in any gear. I hit 55 MPH @ 123 RPMs briefly, my new top speed. Not bad for upright handle bars and MTB tires. I would have needed 144 RPMs in my older setup.

It's fun speeding past cars and seeing kids press their faces to the windows when they catch up to me at the next traffic light. Also seeing the moms shaking their heads in disapproval is funny too.

The Shimano CS-HG70 has a 9-tooth sprocket. It requires a special hub though.

Yep, I am riding the road more than off now and the difference between the small and large rings does not seem that significant. In truth, a road bike would make far more sense but I dont have that option just now. The 1 tooth intervals on rear cassette are also a waste of time on the road, but the derailleur will not handle spreading those from 30 to 36 [........I think it was 36]. So, I thought that the larger front ring may make a bit of a difference.

Many thanks for that. I will have a broswe thru that later when I get time. Looks interesting.

Caprio, sensible if the wheel size is a 20"(or smaller) 9-26t, as does the 60t chain ring .. Bike Friday pocket rockets & Moultons.

What are you using for a shifter?

My Rans rocket (20" rear wheel) has a 60/50/39 triple crankset and shifts perfectly. I'm using a friction shifter, however.

I would think that 52/36 should work fine. It's the same gap as commonly used compact cranksets.

The real trick to compact cranksets is your favorite flat road gear combination. If your favorite combination falls at either end of the cassette, you'll find yourself constantly having to make annoying double shifts. If, on the other hand, if your favorite flat road gear falls in the middle of the cassette you'll probably love it.

I have an index shifter.

Suntour XPress, with Shimano Altus FD.

Thanks for that advice. Often, I do ride on the 46/12 combination when on flat roads. Do you mean having to make double shifts to skip some sprockets? I do that often now skipping the single tooth increments. I had hoped, initially, to be able to replace the 12 - 30 cassette with something like an 11 - 36 [.........I think that was what I looked at] but the derailleur would not handle that range.

Your jump from 46T to 52T is about 12%
Your jump from 12T to 11T is about 9%

So you may get similar gains by just adding a 11T sprocket on your cassette.

I run a 54T on most of my bikes. As long as you're not dragging on the bottom of your derailleur, you should be fine with the 52T.

Is your derailleur mounted with a clamp, or some kind of a braze-on direct mount? The braze on derailleurs can have issues with adjustment.

You probably have 110BCD

Keep in mind that Shimano usually recommends that you change the sprockets in pairs, although you should still be able to shift even with a mismatched set.

Depending on your chain length, you may need to add a couple of links to your chain. How is the stretch on the rear derailleur?

I think he means having to shift the front and rear in opposite directions to get a comfortable gear. It is ok to do once in a while with a change in terrain, but it would be a pain if you needed to do that all the time on the flats.

Hey, thanks for the good laugh. :roflmao2:

Should give you an idea how steep the grade is and how long of a runoff.

The Sun tour company has been deceased for 20 years

maybe the old Football tackling dummies will be impressed by the high speed boasting . could care less . :rolleyes:

I dont think that the rear derailleur will handle a smaller sprocket. The range of the tooth difference is at its max just now.

I am pretty sure that the derailleur is mounted by a clamp. I am not sure if it is visible in this image?
http://bikeforums.net/attachment.php...hmentid=478266

And, yep, it is def 110 BCD.

I was assuming that I would need to measure a new chain once the ring was changed. I doubt that the existing one will be long enough now.

Oh, I am with you now! Yep, that would be a pain if it was necessary often on the flat. Need to think about that one more and observe more closely when out on the flat exactly how much I ride on the 46/12 combination.

If your only goal is top speed on a downhill then go for the huge chainring. But understand some caveats:

1. You will lose significant utility of several gears, as the next one or two small cogs will be of little use with the large chainring most of the time.
2. Most of your effort pedaling downhill at high speed will go into overcoming wind resistance. In many cases you can achieve the same increase by merely tucking down all the way and moving your hands inward to a secondary grip.
3. The effort you waste by fighting the wind for another few mph's will not be available for the inevitable uphill.

I have hit 55 mph coasting on a hill that was less than 10%. I can't see a circumstance where a 60/12 gear will be of much utility at all.