I’ve recently switched from a Trek with skinny tires (25 mm) to a Specialized with deeper rims and wider tires (34 mm). The bike feels great in every way EXCEPT on high speed descents. Especially in crosswinds, the bike doesn’t feel stable beneath me and almost like it wants to slide out from under me. Whereas before on the Trek I could descend at high speeds without ever touching the brakes, I feel somewhat tense now and am constantly on the brakes. I’ve gotten a few rides in and it’s feeling a little better but is there anything else I can do to improve my descents?
How deep are the rims? I find wider tires with appropriate lower pressure better for descending as they ride smoother and track better over road imperfections, especially leaned over while cornering. My 50mm deep carbon rims def respond more to crosswinds and move me around a little, but I just got used to it and basically ignore it in most all situations. Only in pretty severe crosswinds and gusts does it become a significant problem and cause me to slow down.
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They are 38mm depth rims. The movement beneath me is definitely uncomfortable compared to the last bike, but if it’s normal/expected I can work more to try to ignore it.
The flip side of what @Saddlesaur said is if you’re running too LITTLE pressure, the tire can feel squishy, which makes cornering feel wobbly. Do you have mtb experience? If so, you’ll know that feeling. Have you checked your pressure against the calculators?
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There’s a definite art to it. I’ve just got back and all the best descenders had 38mm rims and some of them had rim brakes. They were tearing me apart on descents on shallow rims. Another rider was also tearing me apart and was probably the best descender with the exact same hire set up as me.
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I think @Pbase is on the right track here. Double check your tire pressure against your preferred tire pressure calculator. If you are in line with the recommendations, try a slightly higher pressure. This will stiffen the tire casing and increase your confidence when cornering.
Rim width is an important factor in this scenario. A tire on a wider rim can tolerate a lower pressure while a narrower will need a higher pressure. A good calculator should take this into account.
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Aside from the feeling with crosswinds - Maybe the wider tires are isolating the road vibrations more, so the bike feels less planted on the road, and more like it’s floating or vague?
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Going another direction: what are the geometry differences between the two bikes? I’ve had a couple of Specialized bikes Indoor Bouldering the past decade (Venge and Tarmac) and hated descending on both of them. The geometry just didn’t work for me. The Trek Madone I had prior to them was good, and the Canyon Endurace I have now is amazing.
So, maybe it’s not the wheels or tires, but the frame.
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I’ll double check pressure calculators. I’m running tubeless setup so pressure is ~40 psi lower on the new bike. And no not a lot of mtb experience.
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Thanks, am also going from Trek Madone to Specialized Tarmac so maybe a geometry difference.
I’m reading the book “Climb” by Selene Yeager and just getting to the chapter on descending. One of the first things she mentions is reducing profile by getting more aero, but I’ve been descending completely upright, so that’s something I’m going to start practicing on my local hills. It seems counterintuitive because it increases speed, but I guess improves center of gravity and stability.
I always descend in the drops for aero and to lower my center of gravity. This gives me stability as well.
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Yeah, make sure you put in rim width and the tire width as measured, not just what it says on the casing, plus weight of the bike, full bottles, you and your gear, etc. Lots of room for error in there. Good luck. Hope it helps.
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And be sure your weight distribution is correct, and to put some force into the outside pedal when cornering.
Additionally: do NOT try to emulate Tom Pidcock!! 
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FWIW: Road, gravel, and MTB, I start with a calculator’s pressure, then start trying lower and lower until something I don’t like happens, like tire squirm or rim strikes, then I go up a few PSI. I always wind up less than the calculator recommendations. In my experience, most run pressures that are higher than optimal.
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What rims do you have? Specifically, what is the internal diameter? Maybe the tires are a little too wide for the rim?
Back in the day, I tried 28mm tires for the first time on my wheels at the time. (They were 15mm internal designed for 23/25mm tires). At a correct pressure the 28s were too wishy washy and felt like they wanted to roll over. The were very light bulb shaped. I could pump up the 28s hard and they were a lot better but then hard as rocks. I finally had to drop back down to 25s.
Anyway, maybe your rims are much better suited to a 28/30mm tire?
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It sounds to me like tire squirm. With larger contact patches and lower pressures, the material of the tire can bend and deform during high speed turning. This gives the feeling of the tire trying to move sideways a bit. Low tire pressures and bigger riders will cause this to happen. Try, as others have suggested, raising the pressure a little to see if that mitigates the feeling of sliding out from under you. Usually worse on brand new tires also. Do take the advice to use a tire pressure calculator.
You can really rip down descents with 34s. My fastest road times are with tires that wide. One thing it could be is if your tire pressure is off due to an inaccurate pump gauge. Most are, and I’ve seen nice pumps of by 15 psi. With respect to the wind, deeper rims catch more, but it sounds like that’s not the issue. If your trek and specialized have different geometry or setup, the weight distribution could be off as well, which can also lead to these feelings.
Sometimes new bikes and setups just take a while to adjust to, but for example, my road tires are 32gp5000s on 30mm internal rims, and I’ve used 32s on 25 rims and thought they were fairly similar, so I would be surprised if that led to poor handling
Other’s have mentioned tire pressure and stuff so I’ll stay away from that.
But something else is that a larger tire will increase a bikes trail figure (assuming other geo like head angle and fork rake stay the same). IME, that can make the bike feel more stable going straight downhill but can make it feel like it wants to understeer in fast corners. For example, I don’t love doing road descents on my gravel bike with the 43mm or whatever tires I have on there. But if I put my road wheels with 28mm tires on that bike then it feels 90% as good as my road race bike.
The larger tires will also raise the bottom bracket several millimeters. That alone probably won’t cause huge changes in feel but raising your center of gravity combined with increased trail and increased tire squirm can all make fast descents feel much different.
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