Hey guys, first post.
I'm about to buy my first gravel bike and was hoping for some input from the more experienced.
Okay, so I've been riding for a year on a 2019 Trek FX 2, this bike has no compliance and runs 35mm tires.
25 years ago I broke my pelvis pretty bad but hasn't been a problem at all until this year.
I enjoy long rides and built myself up to doing a 5 hour ride up to the Highwood Pass in Alberta.
Legs were good but I was in so much ass pain that I couldn't ride for a couple of weeks.
So now my question.
Being about 220lbs what size tubeless slick tire do I need to run 40psi or below, and how much extra comfort can I expect from the carbon in a 2021 Giant Revolt Advanced 0?
Thanks for your input and I hope I posted this in the right place.

If you have a regular cylindrical seat post, consider a short travel suspension seat post.

On your tire pressure question, there are several online calculators you can use. I like the Silca one here - if you give your name and email address so they can spam you a bit, you can use the full "pro" version.

Is it only really bumpy 5 hour rides where you have the ass pain? Bike/cleat fit, saddle type, pedaling motion, etc are big issues. I found that a bike fitting that ended up moving my seat up and my cleats back really reduce knee and butt pain on long rides, and doing hip flexor stretches really extended my pain-free time in saddle after that.

I have never ridden the Giant, but I have a carbon Trek Domane that has "compliance" bushings in the seat post and head tube areas. It definitely smooths out road buzz but on the few times I've ridden it on gravel (with 32mm tires at 70 PSI) it certainly wasn't as forgiving as my steel Jamis with 40mm tires at 45 PSI.

I may just not very sensitive though - I did 123 miles on a rented aluminum bike on the first day of the 2019 Seattle to Portland road ride and really couldn't tell that much difference between the Domane and the Fuji ride (nearly identical geometries). I also weigh 220, maybe much of it is padding I carry around on my rear...

No experience with the Revolt, but having run tubeless exclusively for the last four or five years, both on road and gravel bikes, I'll say that the most comfortable ride I've experienced was with a Trek Domane SL with 30mm Schwalbe G-One Speeds at 70psi. Post-Domane, I moved to some wider rims (Zipp 303s) with 28mm tires (Schwalbe Pro One Addix) at ~60psi - that was an improvement over the older wheelset/tires, both in terms of comfort and handling. Long and short: if I were looking for a comfy bike for long rides, I'd be looking at the new Domane and some wide tubeless wheels - either the Zipps (if you're comfortable with the prospect of only being able to run tubeless tires) or the new Bontrager Aeolus wheels - and tires in the 30-32mm range.

Reading the OP, I didn't see one critical bit of information. Having "built up" to a 5 hour ride, what was your longest ride before that long ride?

It's possible you spent (lots) more time sitting on the saddle than you were used to. If that sitting time included long stretches going downhill, the resulting discomfort isn't surprising. Perhaps you might try "posting" (i.e., using your legs to lift yourself off the saddle) more in the middle of the long downhills, or putting a bit more power into pedaling (with the same result -- keister separates from saddle).

I have to agree.

A 5 hour ride is a substantial amount of time in the saddle with or without compliance. One really important factor is the distance travelled. A 5 hour ride at an average speed of 15 kph is going to feel very different than a 5 hour ride at a 25 kph average. When I ride at a relaxed pace I put a lot more weight on the saddle than when I ride fast. Bike setup also matters. If you ride as upright as possible your center of gravity shifts back to your saddle. The more of your weight on the saddle, the more likely it is that you will feel pain after a long ride

Seems like you need a different saddle, not necessarily a different bike.

But in regards to tires: run the widest tires you can fit in the bike (Giant's website should have the specs), and run them at the lowest psi you can hit without bottoming out on the rims or burping air from them.

Honestly, to get to 40 psi tire pressure at your weight you’d need nearly 2 inch tires. The other problem you’ll have at 40 psi on anything narrower is going to be with rolling resistance. It goes up significantly as you get to those kinds of pressures.

But you are looking for comfort in all the wrong places. Your problem has more to do with technique than equipment. We call the seat on a bicycle “a saddle” for a reason. Like a saddle on a horse, it is there to support you but it is not something that you sit on like a chair. If you sit on it with all your weight, it becomes uncomfortable very quickly. Impacts from the road go straight up the frame to your delicate nether regions.

What you should be doing is using your legs to hold you above the slightly above the saddle. You are in contact with the saddle but you aren’t putting your entire weight on the saddle like you would if it were a stationary chair. It requires a bit more from your core muscles to do this but you’ll find that you glide across things you would have just thumped before.

I regularly ride bikes that are as stiff or stiffer than you Trek with narrower, higher pressure tires (for better rolling resistance) and using harder saddles than you are using. Most of my road bikes have Brooks saddles that are unpadded and are hard, stiff leather. My Brooks aren’t broken down and soft either...they show not dents at the sit ones with tens of thousands of miles on them. But I do ride in a much more dynamic way without “sitting” on the saddle.

I do have bikes where I run 40 psi but those are mountain bikes ridden in even more demanding conditions. I still ride them while hovering over the saddle.

The Revolt should fit up to a 45mm wide tire, which according the Silca calculator would put you just under 40lbs.
Are you wearing bike shorts? A lot of pain is actually due to friction and chaffing, not the outright pressure. Lack of compliance typically leaves your joints and back feeling beat up, not necessarily the skin.

Get up out ot the saddle for a short while more often.

Interesting, I've never heard that before

Probably a 60km in about 3 hours. But the Highwood Pass in Alberta is a bit of a beast 75km to the top and back.

Most people haven’t and most people complain about saddles. When you coast, where are your feet? If you say that one is down at the bottom of the crank and the other one is at the top, you are likely to be “sitting” on the saddle. People who hover tend to coast with feet together at 9 and 3 on the clock. They can put a little upward pressure to take the weight off the saddle.

Mountain biking, by the way, goes a long way towards teaching you how not to sit hard on the saddle as well as many other useful skills.

I rarely agree with cyccocommute about much anything. On this one he is completely correct, it is a matter of technique. Back when I was a lad, which is a very long time ago, everyone was taught to ride ‘light in the saddle’. There was sort of enforcement on that, clinchers then were very slow, if you wanted to ride much it was tubulars. Tubulars were even more fragile then than they are now. Tubular rims were very fragile indeed, they flatspotted if you looked at them wrong. If you could manage to get light in the saddle the bike needed a whole lot less maintenance. Even now, bikes ridden lightly last much longer. And of course there will be comfort.

Other things being equal more weight will be supported by the legs if your saddle is back and down. In any case experiment a lot with position. Move things around. But wherever the saddle ends up good technique is mostly up to the rider. First step is to know it is possible and to keep it in mind.

If you did nothing at all the Giant should still be more comfortable than your old Trek. Should be six or seven pounds lighter. Think of your bike as a hammer and your butt as the anvil. A much lighter hammer is going to do much less damage. The Giant also comes stock with decent tires, way more compliant than what the Trek would have. Most compliant tire going currently would be (in your size) the Rene Herse Snoqualmie Pass, 700x44. Like riding on clouds compared to OEM on your Trek. The Giant is a whole lot more bike than an FX2. Not a crack on the old bike, but you are stepping up several levels here. Enjoy.

Can you develop that thought further? I'm 210 lbs riding 42mm tires at about 38 psi on trails, and I'd like to go faster. I can't brake 20mph top speed on level ground.

What kind of trails? Are you talking about smooth paved trails or rocky mountain bike trails? If you are talking about rocky mountain bike trails, is your bike rigid or suspended?

If you are on smooth trails like bike paths or even smooth gravel paths, lower pressure causes more deformation of the tires which results in more energy loss through the constant flexing of the tire. That creates friction which is energy lost where you don’t need it. There is a balance, however. If the pressure is too high, the tires will bounce on impacts so that your energy goes into lifting the bike rather then propelling forward. I would say that the balance is much higher than 38 psi on your width tires at your weight on a solid surface. Tires on roads and paved trail should be firm enough to not squish down too much but not hard enough to bounce off road imperfections. If you can feel your tires bounce with each pedal stroke, they are too soft. With the proviso of smooth (or smoothish) surface trails, I’d run pressure at 20 to 50 psi higher. The bike will roll faster.

If you are talking about rocky trails, I’d increase the width of the tire to at least 55mm (2.1”). The pressure you are using now would be good for that wide a tire. Much lower for your weight increase the chances of the rim impacting the ground on big hits. That can lead to rim damage which is an expensive repair. The lower pressure will probably make you slower but rocky trails are usually thrilling enough that the slower speed makes little difference.

I’d also add suspension if you don’t have it now. Controlled suspension...not just springs or bumpers...keeps the bounce from impacts from controlling the bike. You can keep a straighter line because when the tire hits something and wants to bounce off-line, the shock absorbs the bounce and allows the bike to keep running in a straight line. Suspending your body above the bike also helps. Your legs and arms absorb shocks as well.

Trails are about 50% asphalt, and 50% gravel, and that gravel is a wide mix, from a half inch of gravel or more to mostly packed dirt (sometimes hard, or a little soft) with a sprinkling of gravel. Rigid bike, no suspension. 40 psi seems too high sometimes, as it's a PIA to find the well-worn path in the gravel and keep the front wheel in it, (hard dirt), so I use 38psi. I wonder though whether 40psi would be better for the asphalt. 50 or 60 psi seems like it would be way too bouncy on some parts of the path from my experience.

I ride a Giant Defy Advanced 2 primarily. I'm the type of person that when I ride I want to go, and go, and go. I've found that I need to force myself to stop and get out of the saddle and stretch my legs a little. The longest ride I'll do now without stopping and getting out of the saddle is 40 miles and even at that, the last mile or two, I'm feeling it. This year I rode my first century, I forced myself to stop every 25-30 miles and give my butt a break. I was on the bike for nearly 7 hours that day, but I had no problems as far as comfort in the saddle were concerned. For what it's worth, I have a Brooks B17 that I have about 3k miles on. Here's another valuable lesson that I learned. Midway thru the summer I raised my saddle just a little bit to try and get a little more extension for my legs thinking that it would reduce leg fatigue. After a while I started to get a really sore left sit bone after about 20 miles. I tried switching saddles with some others that I have on hand but I didn't see a big improvement. I decided to lower my seat again after realizing that the pain started occurring not long after I raised my saddle. Voila! The pain went away. I'm talking about less than a 1/2" difference in saddle height was what was causing me pain.
The other thing you don't mention is whether you ride with padded shorts or not. That's also something to consider if you don't ride with them currently.
One more thing, I'm also 220 lbs. I ride Continental GP5K at 85psi.

You are just naturally going to be slower on gravel than on pavement. Even with lower pressure, there is a limit to how well a rigid bike will handle the gravel. The rigid fork doesn’t deal well with the variation of the surface. Any ruts or soft spots will resist changes direction because you need to countersteer into the rut or soft spot. But you can’t countersteer because the fork traps you against the rut or soft spot. You essentially have to plow through it but you don’t have the tools to do that. Lowering tire pressure doesn’t fix the problem.

You’d get more speed on the asphalt bits with 70 to 80 psi but than you have to drop pressure on the gravel bits. Constantly changing tire pressure is a pain. Pick your poison and put up with slow asphalt or bouncy off-road or constantly deflating and inflating tires.

So if I went from 40, to say 50psi, how much faster do you think I could average on the asphalt sections? 60psi? It would make the gravel sections more unpleasant, perhaps even less safe, but if the increase speed on my Strava segments were significant enough, I might try it.

I can’t really give you a hard numbers for speed due to the complexity of the factors. However, you can see trends in the chart here. In all cases in that chart, the amount of energy needed goes down as pressure go up. It’s not linear but it is significant. Look at the first entry...the Schwalbe...for example. There is more than a 35% decrease in going from 30 psi to 75psi. Less energy lost on the tires translates into a higher speed. Unfortunately, wind resistance goes up as well.

Those values are for a smooth drum. Real world data seems to show that the resistance goes up on rougher surfaces but you have to get to much higher pressures...like 120psi...to see much of effect on smooth pavement. On rougher surfaces, however, the resistance goes up significantly at lower pressures.

Well, my idea was to make up time on the smooth asphalt, hopefully pick up 1-2 mph. My terravails however max out at 70psi. I can imagine 70psi would be a handful on the gravel sections. On some of the sections, i'm 5 feet away from a 75ft fall into a river lol. Not sure there's much to be gained for me here. What about a Lauf fork?

At that price, why not just buy a hardtail? I’m not a fan of gravel bikes. I did my time on rigid mountain bikes and I wouldn’t go back for love nor money. A couple of rides on different old railbeds in my area have just reinforced my choice in my eyes. On one of them, there was a very rocky bit for about 200 meters. I had ridden down it earlier and was returning via the same route. When I went through it, I did it about around 15mph. As I was coming back up to it there were a couple of gravel bikiers that were slowly picking their way through the rocks as close to track stand pace. I was faster going through it up hill.

On another railbed, I was at the top of a pass when a couple of people on gravel bikes went by. I was taking pictures (see the Bike pic mission thread) and headed down the hill about 15 minutes after they passed. I caught them at about 4 miles. They had to be traveling at about 16 mph to get that far in that amount of time. I was traveling at almost twice their speed on a smoothish road with lots of washboards. My suspension (front and rear) was working hard and I’m pretty sure from past experience that they arms and legs were really tired (I’ve been down this road many times on a rigid mountain bike). I even got to pass a car later on.

I’ll sacrifice a bit of speed on pavement...I’ve done lots of rides with a mountain bike on pavement...for more utility off-road.