I very much doubt it. Probably a very marginal benefit.

I'm actually surprised they recommend 25s on that. The external width is 30.6mm and the internal is 21mm, so I wouldn't expect 28mm tyres to balloon much.

At any rate extremely marginal to no penalty running 25s in the back.

From an aerodynamic perspective, the tire you run in the back has no dependency on the one you run up front.

Also remember that the wheel performance is highly impacted by all that happens to the air before it reaches the wheel. A disc is typically tucked behind a curved seat tube, and the airflow has been disrupted by other tubes, the drivetrain, legs, etc. As a result, rear discs are less sensitive to tire width than other wheels. This is especially true of a rear disc, since any airflow separation at the front will always have a chance to rejoin along the length of the disc. The performance difference between a 25 and a 28 is probably negligible.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Sorry, just noticed we are talking Vanquish disk, not Jet disc. What I had posted was about Jet, so don't mind me.

It is a bit odd to recommend 25 on some of the wheels and 28 on others given they have the same widths, but I would trust HED did their homework. Given it's in the rear, though, 28 shouldn't hurt.

Also important to note that the aero chart is for GP4k, so the sizing recommendations may be based on that tire. Given the GP5k is smaller, I would go with 28 front and rear

Thanks all for the info!

Sounds like minimal difference, so I’ll likely go 28s for the slight comfort advantage. A happy taint is a fast taint, right?

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Aaron Bales
Lansing Triathlon Team

I mounted the 28 and it sure passes the “eyeball aero” test.

My Wheelbuilder rear cover had stickers that fit over the valve opening. Do people commonly tape discs like the Vanquish up, or leave it open?

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Aaron Bales
Lansing Triathlon Team

Yes, taping over the valve opening is pretty standard. You can buy some Zipp "stickers" that are pretty nice (below from Daytona), or just go with heavy grade 3M packing tape (cheap and doesn't leave residue).

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Imagine how much better Paula could have done if she put her feet inside the shoes 😂

Thanks for the 3M tip. I would use electrical tape on my Wheelbuilder stickers. I still have some, so I might just use that combo, but with 3M tape.

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Aaron Bales
Lansing Triathlon Team

Personally, I always keep a roll in my transition bag. It also ends up being useful for attaching bike numbers, and helmet numbers (stick the number on the tape and then the tape to the helmet...easy to remove post race).

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

pretty sure I tested covered and not covered for the valve cover and there was no gain by taping or covering it

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

Interesting - thanks!

I'll probably start a bidding war for corporations to put their logo on my valve cover. That new GM logo would look nice... While my agents are sorting that out I'll ride coverless without fear.

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Aaron Bales
Lansing Triathlon Team

IIRC Flo has windtunnel data of a test with and without valve hole cover on their site somewhere. I think it was around 1W difference.

I would personally cover it just to avoid getting water, dirt, etc. inside the wheel. A piece of black gaffa tape work fine and looks clean.

I wonder how much time you're losing running the 858 backwards. She could also learn to orient the front skewers better.