I ride a Diamondback Trace Comp, https://www.rei.com/rei-garage/produ...comp-bike-2017 . Changed the flat handlebars to drops, this made a huge difference for me in fit, comfort and handling of the bike.

The bicycle comes with a Suntour NVX-HLO suspension fork, 75mm travel. The fork is a version of this one Details - SR SUNTOUR Cycling

I'm feeling like changing the fork for a rigid carbon fork. I ride with the suspension locked, thus I have no need for a suspension fork of almost 6 pounds, plus the carbon might make for a smoother ride.

I have my eyes on this fork: https://www.ebay.com/itm/1-1-8-MTB-B...r=541738484442 , the tapered version as the axle to crown length is the same as my current fork.

I still have to confirm though that the current fork is tapered. I believe it is, the head tube is tapered, I just haven't felt like taking off the stem to look at the tube.

My question is:

Should I be looking at a fork with an axle to crown length as close as possible to the current since I ride with the suspension locked or do I have to calculate the length based on some give on the suspension? Information I've found on this is all over the place, can't quite get a straight answer searching older threads.

*I'm already saving up for another bicycle, please no "save money for another bicycle' answers.
**The weight difference is secondary, not really playing a part on my decision, although a lighter bicycle would be nice since I have to carry it up and down four flights of stairs every day.

You should be looking for a rigid fork that is 80% the length of your suspension fork this considers compression when you are riding the bike. Alternatively the DB spec calls for a BB height of 310mm measure yours unweighted and subtract the difference from the fork length. Or just spend the $100 for the Ebay fork and give it a try it will probably work just fine if yours is a tapered steerer fork

I'd go for an ATC that figures in a little sag also.

As for the BB spec. The BB height difference will be less than the ATC difference, since when you change the ATC, the whole bike rotates around the rear axle.

The headtube on the Diamondback in the REI listing does not look tapered.

Surly's Karate Monkey fork is 483 mm axle-to-crown, which gets you closer to the 493 mm of that Suntour fork.

Like I said, I just haven't felt like confirming it yet by unscrewing the bolts on the stem and looking at the fork, I'll do that eventually obviously before making a decision.

I've looked at the Karate Monkey and Ogre forks from Surly, they are in my list of possible forks.

I would like to see some pics of your bike with the drops.

Just have a friend measure, with you on the saddle, leaning on the bars,
sag is due to your weight compressing the suspension..

Ignore the front brake cable, at the time of the pics the cable and housing I had were too short but that's been fixed.

Current fork isn't tapered so I went ahead and bought the straight version, 475 axle to crown, 39mm rake vs. 38 in the old one.

If you are living in a place with bad road conditions and lots of potholes, I recommend for not replace your fork with rigid one to you. I have suspension fork with lock on my hybrid but I can use lock only at my recreational rides on seaside parks with good bicycle paths because in my city, nearly all of the roads like MTB paths.

I have a Scott Hybrid I have been thinking about turning into a gravel bike, just not sure I'm willing to go through all the trial and error of getting everything right versus buying an off the rack ready to go gravel bike.

Can't complain about road conditions where I usually ride, the worst streets were recently fixed and I already know the location of the few potholes along the road.

I went with both options. Just bought an used 2016 Louis Garneau Steeple X, for a great price, plus I've turned the Diamondback to a sort of gravel bike, although I'm not taking it off road any time soon, it will probably end up as my winter bike as I can fit 45mm tires on it.

It indeed take some trial and error, I started off with drop bar ends but that didn't work out. I then cut the flat handlebars to fit with the drop bar ends as a 44cm bar with the old MTB shifters and road brake lever until I ended up with actual drop bars and brifters.

Looks like that bike came with 40c tires. Try to get the largest possible tire if you want more comfort in addition to a carbon fork. At larger tire size (and low enough pressure) the fork material becomes irrelevant.

How has the drop handlebars nade a big difference? Do you just ride entirely on smooth surfaces?

Fit wise the flat bars were too wide and made longer rides uncomfortable and my position was too upright for my liking.

The drops extended my position a bit and are now my width making the ride more comfortable.

Finally the steering was somewhat jerky with the flat handlebar, and once I changed to drops the steering feels much better, more stable and not jerky.