52/39 Chainrings.The 39 is too small
 

I have a 105 52/39 chainset,but don't really use the 39 ring much.Do shimano do a bigger one like a 42 - 46 inner ring to use with the 52

If you were to replace the 39 with a 42, it would do little to increase the useable gears. You'd just be using one cog larger to get the same gear ratio. A 42/16 and 39/15 are nearly identical. The other difference is you'd shift one cog less, after shifting to the little ring, in order to reach the next lowest ratio in the progression.

Of course you can buy different chainrings, but they work best in matched pairs and you might have to buy an aftermarket brands to get what you want.

I really like the 42 over the 39.
Have it changed.

While I changed my 42T inner to a 39T inner to appease my knee, I prefer the 42T. Not much difference between the two, but enough for me.

Brad

What cassette are you using? Put something like an 11-21 on there and see if the 39 is too small :)

Yes, they make larger rings. I haven't ridden a 39 in a while due to my love of Biopace. I have a 52/42 double and I like it just fine.

Shimano's older road triples (105 and Ultegra) all came geared 52/42/30 and if you find one of those NOS you may be able to buy it for less than a replacement chainring. Swap both of the larger chainrings to your crank arms (the bolt circles are all 130 mm) and you will have the gearing you want and a matched set of rings with no shifting issues.

More likely explanations are that your large cogs are too big and/or you never developed a decent spin.

Assuming you have ten cogs and like a 12 starting cog you don't get good spacing if the last cog is bigger than a 23 and eschewing the fully cross-chained combination you'll be down shifting when you're spinning along in 52x21 and hit the first false flat that drops your speed below 19 MPH or on the first hill you grind up at 15 MPH.

With 9 cogs or an 11 starting cog (allowing for a 21 big cog so you're using 52x19) the down shift happens at 17 MPH grinding and 21 MPH spinning.

With the same 12 starting cog 39x13 is good for cruising at 24 MPH. Solo that can take 300W on flat ground which is a substantial (and unsustainable) effort for many riders.

the Paris-Roubaix racers use 53-46 combinations .. 1) because they are strong,
the race is fast,
and the course is not mountainous, and the likelihood of the chain falling off
is lessened. [less chain slack]

Yes chainrings are cut with a different tooth number
than the ones sold with the bike.
you just need to buy the appropriate ones..
your LBS has sources to order from ..
even if they don't have one bought on Spec hanging on a peg.

Shimano may make them , but there are other after-market sources also..

which are comparable..

I have two Ultegra cranks on two different bikes. I changed one to a 42 inner chainring and quite like it for flatter terrain (Illinois). The other I've left with the 39 and do a lot more "easy gear" spinning with it. I think there's a noticable difference between the 39 and 42. I'd suggest making that change.

The difference between a 42T and a 39T chainring is 7.5% which is almost exactly same as the difference between a 14T and a 15T cog so it's reasonable that the change is noticable.