longe

A few years since my last road race, I still delude myself that I'll actually have the time and energy to race road again. At the same time, my reality is that I belong in the 41 + gravel. So I'm looking at putting a bike together with the new GRX di2 that runs 31-48 in the front.

Just how out of touch am I to hope that running that will still give me the flexibility to be competitive in Cat 3 road races? I'm assuming waiting for the bunch sprint wouldn't be my plan A in this case...

Cypress

Which cassette?

longe

Any one of these:

11-12-13-14-15-16-17-19-21-23-25T
11-12-13-14-15-17-19-21-23-25-28T
11-12-13-14-15-17-19-21-24-27-30T
11-12-13-14-16-18-20-22-25-28-32T
11-13-15-17-19-21-23-25-27-30-34T

Hermes

Some road races are very hilly with steep sections such that lower gearing may be an advantage but probably not.

With a 48t large ring, you give up some top end flexibility. Each tooth is equal to about 2 rpm in leg speed. So if your competitors are spinning 90 rpm at 25 mph using a 53t ring, you will be spinning approximately 100 rpm.

Cypress

I wouldn't run a 48t without a 10t in the back. It's not the extremes I'd worry about. It's long 35-40 mph false-flat descents that would put your legs into a crisis. And as you said, most flat sprints would be out of your wheelhouse unless you pride yourself on your cadence. 100 rpm on the pedals with a 48x11 will only get you ~34 mph. If you could bump your cadence to 120, you'd be touching 41 mph. 120 rpm in a dead sprint in the Cat3 field isn't unheard of, but it's uncommon.

rubiksoval

48x11 is like... ugh. Friction alone wouldn't be worth it. Even less so considering the speed. Any extended downhill or tailwind runs and you might just hate your bike..

Doge

In the big scheme of things rings and cassettes are relatively easy to change and not super expensive. I think it is reasonable to swap for the race.

HTupolev

Maybe. But doing extremely high cadences at peak effort isn't very difficult, or even unnatural.

A big question is being able to rest when someone is hammering the front on a shallow descent. If you're in a 45mph situation where holding the next wheel requires 80 watts, it's going to be more relaxing to do that at 120rpm in a 53-11 than at 130rpm in a 48-11.

If you're building a gravel bike and thinking about dabbling in road racing... I'd say build the gravel bike and dabble in road racing. Don't put together a gravel bike with less gearing than you find useful for your gravel. If you later find the gearing to be unsatisfactory, then go fix it. Changing out sprockets isn't a big deal.

If you're competitors are spinning 90rpm at 25mph in a 53T ring, they're in their 15-tooth cogs. In a 48-tooth ring, you'd have the 13T or 14T cogs to use to get into that same gearing range. Not really a big deal, even if you're not preferring 100rpm in the situation.

burnthesheep

Why not buy a spare chain and set of rings? Rings take like 10min to swap out.

Better yet, keep the rings setup and just buy a duplicate crank.

Then have exactly what you need in both scenarios. You'd have to shift the FD up/down a tad going gravel/road swap, but whatever. I'd leave it gravel to train on then just toss the roadie crank on to go race road.