I'm thinking of stripping my road bike to build a cross bike. It presently has a 34/50 compact chainset. Will this leave me under/over geared for racing?

It's what I use on both my race and training cross bikes, though I will be fitting a 46 outer later this year.
I only used 50 on a couple of tarmac sections.

I've never been off the 34 ring in a race
What you are planning will be fine although I agree with the above that 46 would be better

As above, Ive got had a 34/50 but swopped the 50 for 46- much more useable

I've an Uncle John I use as a second-bike for racing. I fit a 36/46 chainset during the racing season, and a standard 34/50 compact chainset for just riding around on in the off-season.

Single 42 on the front, wide spaced 9 on the back

Dont bother with 2 on the front, ditch the mech, ditch the cables and inner ring, save weight

Learn to run 😉

Single definitely appeals for racing, but I want to use it on the road too. Maybe I should think about swapping the 50 for a 48. Maybe I should think about blowing a grand on a Jake the Snake?

I can't imagine anyone using the big ring for any cross race apart from the three peaks. Do a cross race and you'll understand immediately.

racing on hard grass courses = big(46) ring samurai, and Im crap so the the good lads are defo on it

Thinking a little differently, what about running a 53/39 and just leaving it in the inner ring for cross? Would this still leave me undergeared?

yes

Bugger it. I'm just gonna run two chainsets.

I run 38/44 and a 12/27 cassette and when racing I've only been undergeared a handful of times. Running bigger than a 44 might be nice on the road but unless you're one of the big boys you'll never use it in a race, so two chainsets might not be a bad idea.
34 inner would be too small for my liking.

46/38 with 13-26 cassette for racing. Never use the top gear and I finish top 10 in the london league races.

Lots of people do keep their 50 outer but they don't get used much! Change the 50 for a 46. If you want to ride it on the road, a 46 11 is plenty big enough 99% of the time

I've ordered a Jake the Snake this morning, which comes set-up as 46/36. I'm just going to buy a 50t (maybe 53) ring and swap back and forth for racing/training.

Thanks for the advice, everybody.

I'm thinking of putting a 34 in place of the 36 on my jake the Snake just to have something in reserve for those especially steep sections of trail or for the end of a long day in the saddle. I'm not that strong and can get up most stuff with the 36 but once or twice I've been left wishing I had a little left in reserve.

36/46 chainset, 12/27 rear

Sponge.
I brought Nicks Scott cx from S/fork yesterday as a general training bike and it came with 34/48 rings. I intend on using it mostly on the back roads of Exmoor but with the option of some cross country when needed. I found a couple of years ago that with the hills we have surrounding us and the cadence acquired from cross country that a std road setup was a severe push on the legs. Ever tried riding up Whitefield hill or Porlock on a road bike? Give me a shout when your up for a ride.

sponge

Hope you enjoy the Jake – nice machine.
Just an observation, but won't it be a bit of a faff swapping the big front chainring, as you will need to reposition the front derailleur each time.
Unless you are TDF fit, I would imagine a 46 big ring would be OK for most of the time, plus it's good to spin!

Azarat, haven't seen you or Snaps for ages. Are you intending to race your new bike too? I've found 50/34 compact has been ideal for North Devon road riding. Would definitely struggle up some hills on a 39 inner.

As for gearing, I'm intending on running the Jake as a road bike for most of the year, so no faff as I'll change the ring at the start of the season, then back to road set-up in December. 46 will be fine off-road, but too easy on.

On another note, is a carbon post a really bad idea on a cross bike?

My cross bike is used almost excluively off-road. I use 44/34 rings with a 12-28 cassette. Flattish off-road stuff is fine in the big ring, small ring for climbs. Seems to work well.

I don't think a carbon post is a bad idea on a cross bike. Why would it be? We use them on both road bikes and mountain bikes. The extra bit of comfort is worth having.

carbon posts are fine, especially as there's usually less leverage on one than on an MTB post.
there's not much that isn't carbon on a top cross bike now

Got a 46 outer on mine, you spin out on the tarmac a bit but for 85% of the time it's perfect for me.

"On another note, is a carbon post a really bad idea on a cross bike?"

Can't see why it would be a problem. I've got carbon post, stem and bars on mine. No real performance gain but they look nice!

I've bodged an FSA MTB triple on my Surly Cross Check. Got a Deore cassette on the back. Lovely gearing for the steep bits…

I have a compact on my road bike 50/34 I think.I also have a 25 block on the back. The big ring gets used a lot and I'm not one of the top lads( I'm not slow either though).
In summer when the ground is hard and open the the big ring gets used a lot, I am a grinder though, I like to push big gears and in a 50 min cross race it's not going to break you.
I would like a smaller inner ring and a bigger outer for the 3 peaks though.It's always too hard up hill and I spin out on the way into Horton!
In winter mud though I do consider a smaller outer ring and bigger inner ring than this a good idea.