Well ive decided to spruce up my old bmx racer and go get some laps in on the local track, maybe some casual racing. It's a mid 90s dyno nsx. 44x16 gear and a 17.5 TT. I'm just under 6' and I realize this is small for me. I ride an undersized road bike for criterium races and wonder if this 17.5 TT frame will be fine for me.
Technically I'll be saving some weight with a smaller frame, what are the cons to me running a kids sized rig instead of a 21" TT like I should.
TIA

Constantly hitting your knees, feeling cramped and awkward, and because its old and possibly deteriorated, it could buckle under you and your weight (which I'm sure is more than it was when you originally rode that bike). If you want to race, it would be best to go for something in your size.

OT, what road bike do you ride?

My knees don't hit the bars, perhaps I'll test drive a 21" and see if the difference is worth getting a new bike. I have a 56 cm for touring and leisure rides. Ride a 54 for crits and races. The bigger bike is a steel Italian vintage and my nice (relative for my budget lol) bike is a tirreno raza 3.0, carbon tail/fork with 105

Are the bars really low that they don't hit? or are you sitting down while pedaling with the saddle jacked up?

Seat slammed. Bars pretty much perpendicular to the ground. Riding while standing.

Crazy. Does it feel cramped at all though?

Well after months on a 29er this summer it feels like a clown bike. Going to test drive some well fitting ones tomorrow

Sounds like a plan man