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31/48 up front?

Old 10-02-19, 07:41 AM
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longe
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31/48 up front?

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...
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Old 10-02-19, 08:02 AM
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Which cassette?
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Old 10-02-19, 08:07 AM
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longe
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Originally Posted by Cypress View Post
Which cassette?
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
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Old 10-02-19, 08:13 AM
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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.
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Old 10-02-19, 08:37 AM
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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.
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Old 10-03-19, 04:54 AM
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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..
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Old 10-03-19, 07:58 PM
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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.
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Old 10-03-19, 11:53 PM
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Originally Posted by longe View Post
I'm assuming waiting for the bunch sprint wouldn't be my plan A in this case...
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.

Originally Posted by Hermes View Post
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.
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.
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Old 10-07-19, 01:50 PM
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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.
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