I recently started building a bicycle around a frame I discovered is designed for a smaller 650b in front. That is because with a 700c wheel, there are not really any brakes with short enough reach and even a slick tire looks awkwardly close to the fork crown. The frame is a triathlon variety and I understand this is deliberate to a achieve a more areodynamic lower shape in front when riding. I am currently looking for a decent rim for this application with a couple criteria being rim brakes and black. Has anyone heard of the Brn Pro rim or would it be better sticking with something like the Alex DM18. Is anyone else riding with a smaller wheel in front / are there decent road tires available?

This rim also looks good but am confused why there are no eyelets .. can anyone shed light on why there would be no eyelets?

it's designed for tubeless so no rim tape is needed

before you get too deep in wheel replacement : make sure to verify the front wheel is not 650A (vs 650B) most of the triathlon frames with a smaller wheel i've seen are 650A

There's been a number of 650C front wheeled tri/TT bikes made too.

Here's a few axle to center of brake track dimensions of the rim sizes we have mentioned (thanks to Southerlands). 700C- 311mm, 650A- 295, 650B- 292, 650C- 286 and MtB 559- 279mm.

Assuming (yes, I know...) the front brake would have the same reach as the rear you could do the math and discover which rim gives you the reach of the rear caliper. Remember that on the fork there's a couple of mms difference between the center of the steerer/crown at the brake hole WRT the axle as compared to the same axle to but instead taken to the front face of the crown at the hole. Andy

Well 650A is only 6mm larger than 650B which means I just need to have sufficient reach on the brake to go with 650B which should have the best tire availability.

No Tri bike that I remember EVER used a 650 A. It's a common error. Tri bikes always came with 650C wheels.

ah you're right - i always get the 650As Bs and Cs mixed up

Likely that bike is using 650c. 650b was an old French randoneurring size so was not common until a bit more recently when it was adopted for gravel and also used for mountain bikes as 27.5.
650a gets into the ultra messy and super silly 26-1 3/8s size which is multiple sizes and vary weird as you can have the correct size and iso diameter and the tire is wrong because for some reason the tread pattern has a say in it.

The rear brake is good with a 700c wheel, indicative of triathlon style. I am trying to figure out if I will be comfortable riding a smaller wheel in front or if steering will just be too responsive. Guess there may be only one sure way to find out .. is there good supply of 650c tires as well as 650b?

650C road stuff is still available although not anywhere as many offerings as was the case 20+ years ago. At last look Conti, Panaracer and Terry had 571/650C tires. If you can't find a suitable wheel PM me. I think I still have a NOS Mavic Open Pro 32* pair in my stash that we could talk about. Andy.

Wheel size doesn't have much effect on steering, it's more about rolling resistance and ride quality, both of which tend to suffer. Steering will depend on the head angle and fork rake, which determine trail (roughly how much the fork tries to go straight - that can be the same on a lo-pro as with a full size wheel). The advantage of a small front wheel is aerodynamic (allegedly - it lets you get lower at the front). Alternatively it lets you build a better-proportioned frame for a smaller rider.

I would very much prefer to have 650b wheels as I do not see any advantage to using 650c even if the frame era predicts this. Maybe it is worth giving this a go even though I am not totally interested in being lower in front -- I will probably raise the stem a centimeter to compensate for the smaller wheel.