I don’t particularly like riding my TT bike on most of the roads that I ride my road bike on, either

I rarely ride on the kinds of roads you mention. In fact it’s not uncommon for me to drive 20 mins to get to good TT-bike roads

But when I have the right roads, and right conditions, my TT bike is a blast

One thing that hasn’t been talked about. What kinda wheels do you have? Maybe that’s an option to go with a more shallow wheel** (or different shape) and be more stable?

Edited above

70psi migth be a bit high for 28mm tyres (also depending on the rim), no?
On only moderately wide 19.5mm rims I use 25mm GP5000s at around 70-75psi front and I think 75-80psi rear. I might go low end of those ranges on poor roads and high end or slightly above IF I knew the roads would be very smooth. I'm typically well over 90kg including the bike.

I presume you're talking about rim depth rather than width? (shallower rather than narrower)

All should help with stability...

Shallow vs deep (I’d guess all being equal, a 30 in front feels more stable than a 90)
Wide vs thin (allows you to run lower pressure tires)
Modern rim shape (not sure how to explain..: but my 90mm Flo wheels feel pretty stable vs the 40s I used to race on 10 years ago)

It's not too high based on the Silca calculator, rough pavement, 28mm wide tire, and a male of average weight. Of course such calculators only give a starting point and your actual tire width, weight, and personal preference will affect that number. And I wouldn't recommend someone running tubes to push thing toward the low end on pressure as you're doing. In the context of this thread it's enough to likely get him out of the overinflated bouncy range that could affect handling.

YMMV.

I'm riding a Roval Rapide CLX front. It handles better than my Zipp NSW 303 and ENVE 5.6 did. It's a fantastic wheel. I suppose the one knock is it's not tubeless so have to run 70 PSI. I used to train on my ENVE 5.6 at like 55 PSI which was nice. I don't think it's the wheel though. I think it's the psychology of being away from the brakes, the shorter front center of my TT bike, maybe insufficient core strength, and feeling like I'm going to bounce off the pads if I hit a pothole without death gripping the extensions.

Agreed, just wasn't sure this was what you meant by the way you wrote it originally. A shallower rim profile will reduce aerodynamic instability due to position of side area relative to steerer axis. It'll also reduce the magnitude of aerodynamic influence on steering due to gusts.

I agree a bit more give in the tyres should help a lot.
Current toroidal style rims are indeed an improvement over the more V shaped rims. The more bluff trailing edge on the front side of the wheel and rounded leading edge on the rear side of the wheel make the response to variation in Yaw less dramatic. V shaped rims look great on the front side of the wheel in stable zero yaw conditions, but otherwise it's far from ideal.

What tyre and approx weight? I'm guessing you could safely go below 70psi if they're 25mm tyres of most brands on 21mm internal rim width. Still, that's not likely the issue, what you've got seems sensible. You could always try moving yourself aft to unload the front wheel a little. It'll screw up your fit, but if you can't use it anyway, you lose nothing.

Well, from memory my 25mm GP5000 on 19.5mm rims measured about 28mm wide. This is the dimension used by Silca as the tyre dimension input for their calculator. The Silca pressure recommendation at my weight for a tri bike 50/50 distribution is:
For 28mm wide: 73psi

So if your're going by the Silca Calculator, my 70-75psi front 75/80psi rear range is perfectly in line, or conservatively high at that back if anything!
Incidentally I use less at the front versus rear for comfort on the extensions. Perhaps I should drop the rear pressure to match.
I've never had a pinch puncture or any other issue with this setup so I don't see a problem and certainly don't think I'm pushing anything as you suggest. In fact I'll probably start dropping the pressure a little lower when I get back riding outside regularly which I haven't been doing since last summer due to local Covid restrictions.

Hypothetically, if the OP was to use 28mm tyres as you suggest (I think 25mm is enough but 28mm isn't wrong) on his 21mm internal width rims, and if those tyres were GP5000; I think they'll measure about 30-31mm. If an average weight man is 75kg and their Tri bike weighs 9kg, then Silca says:
For 30mm wide: 64.5psi
For 31mm wide: 61psi

So if you're happy to rely on the Silca calculator (which is a better tool than most), then it appears you agree with me and 70psi is a bit on the high side for 28mm sized tyres. 70psi will work but Silca suggest lower and so would I.

I just measured some GP5000 clincher tires 700x25 mounted on Reynolds AR58X Rim brake wheels that have a 19mm internal width. I am getting 25.9 to 26.1mm.

Really?
Pretty sure I got 28mm with the GP5000, and about 29.5mm with previous GP4000sII. I'll measure again in case I'm mistaken.

I've got 25mm GP5000 on a set of Kinlin XR31T rims (19mm), and a set of SwissSide Hadrons (19.5mm).

What did you decide to do for Placid?

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Dimond Bikes Superfan

Took a potentially expensive gamble and bought the Shiv Disc in a Large (was on a medium before) thinking the longer wheel base and better weight distribution might improve handling. And it’s the bomb. I LOVE the bike. Have done several long rides on it and it just flies and the integrated nutrition is a bonus. So riding the Shiv Disc with 321 Disc rear and Roval Rapide CLX front.

Also, running lower PSI helps. I’m running 70 PSI on 26 Turbo Cottons for training. I hear Lake Placid roads are pretty good so thinking I’ll go up to around 85 PSI on the 26 TC with latex tubes for race day.

Cool, sounds like you have a setup you're more confident in. Good luck.

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Dimond Bikes Superfan

Awesome! I started out on a 47cm Cannondale Slice which frankly frightened the shit out of me. I ended up replacing it with a 51cm IA which, although I have to launch myself off the saddle when I come to a stop (5'3" with short legs:longer body problems!), I feel a million times better riding. I'm pleased the gamble paid off for you.