I'm new (actually coming back into bmx) and I'm pondering my grearing. I couldn't find any rule that says I can't run a 48T front ring. I can push that size ring (please no rants on how hard it is) out of the gate but it just doesn't seem fair to those guys running a 42T. Is there any rule (NBL) that limits your gearing?

There are no rules about gearing but you need to do some math as there are better ways to achieve your desired gearing. The chain ring size is one of three factors in determining your gearing, you need to factor in tire size and the rear cog size. Assuming you are running a 16 tooth cog and a 20 * 1.75 tire you are looking at 58.5 gear inches, if you have a 1.50 inch tire then it is 57.7 gear inches. There are gear charts all over the internet so you don't actually have to understand this.

Using a big chain ring is not terribly bright. By using a big chain ring you are increasing the weight of your chain ring and your chain as the chain needs to be longer. You would get exactly the same gear inches by using a 42 * 14.

In any event, anyone can push that gear, it is just not likely to be very efficient on most tracks.

A lot of racers change chainrings depending on the track (long straights or short straights). There is no "set" chainring size as such for racing. The idea is not to over-spin or under-spin at the end of the straight. Maximum efficiency is key.

It's also a matter of preference and riding style because different riders have different arm-leg proportions and strength capabilities. Some are more inclined to be "spinners" and others are "grinders." My daughter and son did BMX racing for 6 years and their gearings changed as they grew up and got stronger. Most of the serious racers I knew had stashes of various chainring sizes in their toolboxes... and the dads were always trading these like baseball cards when our club decides to change the track layout. Oh, those were the days!

Find what works best for you throughout the ENTIRE track, not just the gate. I'm assuming you've got a lot of leg strength so 48T could very well be your thing, if you're winning races with it.

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I've never raced. Is ground clearance likely to be an issue on the track with a ring as large as 48?

No, ground clearance isn't an issue. But I know J&R's has a really simple gear calculator. If you go on the site, it's in the sizing chart. But, I would run something smaller with the same ratio.

why dont racers run a smaller ratio, like a 25-8 or something?

Because on that rear 8t cog, only 3-4t are actually connected to the chain. This is an issue that comes up with FG Track bikes too. If there are so few teeth connected to the chain, there is the possibility of chain slippage or even stripping teeth if you are really powering it.

Correct me if I am wrong, I assume that it is the same for BMX as FG.

I've actually wondered the same thing DirtDawg.

+1
Yep and one of the results is early and extreme wear on the teeth. Even though similar gear inch ratio is achieved, it actually puts a lot of strain on the cog and chain. The 8T barrel also has really small pawls on it's ratchet mech, so there's another area that could suffer.

It's be fine for freeriding or park stuff, but it probably wouldn't survive the stresses generated during gate snapping in BMX racing.

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36/13 is alright for racing isn't it? That's about 8t in contact with the chain right?