My family and I are heading to RAGBRAI this summer, and I'd like to put some skinnier, higher pressure slicks on my wheels for RAGBRAI. (We're currently stuck with 26x2.0 tires because our house requires us to ride 1.5 miles each way over nasty, loose gravel to get to paved roads to ride.)

I've seen a few 100psi 32mm and 35mm tires I'd like to try, but I'm not sure how narrow I can go and still have the tire adequately support the weight of the bike. What is a safe width for tires that need to support 250+lbs each? (If it matters, the wheels are 26" w/36h hubs.)

The minimum width is dictated by the inside bead seat width of your rims. Everything else about tire width is pretty subjective based on rider expectations for comfort, reliability and perceived performance qualities. The tire sizes you mentioned -- 32mm and 35mm -- are more than adequate for even the heaviest tandem teams and touring on less than ideal roads.

And you can find a handy chart that will tell you what the range is at Sheldon Brown's website.

Annoyingly, most rims don't have the inside bead seat width printed anywhere on them or on the specs you might be able to find on the web. Meaning you get to take the tires off to measure. Unless you happen to know, or someone else knows for your specific rim.

I can't tell the brand from looking at the photo.

Awesome pic, btw.

It's most likely something like a Two Knives DX-3000, or some other similar model from Two Knives... as that's what Micargi specs for most of it's fat-tire bikes.

A quick check with Vernier calipers with the tire partially or fully removed from the rim will probably yield a width suitable for a 1.5" and even 1.25" slick or semi-slick urban tire. 1.5" with about 10psi more than suggested on the tire sidewall should keep you rolling along just fine and with a lot less effort than your stock 2.0" tires.

However, do yourself a favor before RAGBRAI and have a bike shop inspect the wheels for proper and even spoke tension. That's one of the potential soft-spots with mass-produced wheelsets like the ones used by Micargi. You certainly don't want to start breaking spokes on the ride.

WE think the pic belongs above the mantle! You all have guts
R&J

Assuming your rims check okay and spec to size, Kenda Tandem tires are 26 x 1.5 rated for 100 psi. I have run a set. They're heavy but great for gravel trails, and they're okay for the road if you're not a speed demon. I bought mine from Precision Tandems.

I'm doing an Adventure Cycling trip on the GAP/C&O Canal in September and I'm considering using them on my hardtail MTB for the tour.

https://www.precisiontandems.com/catframepart.htm

https://www.precisiontandems.com/cat_pics/trkenda26.jpg

So, I've made progress. I'm rebuilding the rear wheel with a Deore 36h hub so that I can upgrade to a 9 speed drivetrain. When I had the wheel apart, I put the calipers to it and found the inner width was 19.3mm, plenty small enough for a 28mm or larger tire (according to Sheldon Brown).

My initial assumption is to go as narrow, light and as high pressure (100psi) as I reasonably can.

• Is there any reason to consider lower pressure tires? The soft, squishy steel frame of the triplet absorbs most bumps, so I don't know how much absorbtion the tires need to do. (But, I will listen to anyone with experience)
• Any disadvantages of 28mm or 32mm tires, opposed to a 35mm (1.5") tire?
• Are "Tandem" rated tires necessary, or should I be fine with any reasonable 26x1.5ish tire? I'll probably only use this set of tires for 6-700 miles, because I can only use them when I'm driving the bike and don't need to drive the gravel on road.

BTW, the tire I'm currently leaning towards is the Serfas Seca 26. I'm new enough to this that I'd love to hear if there is anything bad about this tire that should sway me against it.

Here is what DaVinci puts on their tandems https://www.davincitandems.com/options.html
The smaller, higher pressure tire will roll better, but it will be more prone to pinch flats, cuts, and have a harsher ride. I'm assuming that tight handling is not a concern for your train. I would think a 28 or 32mm tire would be appropriate for you. I would suggest running at least the max pressure the tire is rated for. A Conti Gatorskin 28c is rated for 120 psi.
I don't know of any tires being tandem rated.

The real question is what pressure to run your tires at. Most tires can handle somewhat higher pressure than their rating. The answer, in your case, is probably no. The usual answers are that overinflation stresses the rim, makes the ride harsher, and reduces traction. You probably have enough weight there that you're not likely to run into issues. And with a tire in the narrower half of the range for the rim, you shouldn't be stressing it, even at somewhat higher pressure than rated.
Selection, mostly. For instance Schwalbe makes typically one (559 BSD bicycle) tire in each size until you get up to 1.35".
Ain't no such thing as a tandem rated tire. Most of us are willing to pay a little extra for improved puncture resistance, if only because there are two people inconvenienced if you flat. But those tires aren't specifically tandem rated. I don't think you'll get much information on the Serfas on this forum. Last time TandemGeek did a survey of tire brands and satisfaction levels Serfas didn't show up unless it was in the "other" category. And there were 450 responses.

I agree, 32mm tire is probably wide enough.

Unless you are riding on a steel drum, high-pressure, narrow tires are slower rolling than lower-pressure wider tires. On real roads, extensive testing (see published reports in Bicycle Quarterly) shows that lower-pressure wider tires are faster. This is because high pressure tires cannot absorb road irregularities, thus forcing the entire load of bike plus rider to be lifted up and over the road irregularity. The resulting suspension losses exceed any decreased rolling resistance on ultra-smooth surfaces. Lower pressure means that the tire can deform over road irregularities. Obviously, pressure has to be high enough to prevent pinch flats.

Note, you have to use decent quality tires, whether they are narrower or wider. You can get narrow tires that are slow or wide tires that are slow, particularly if they are marketed as being highly flat-resistant.

Nick