Road Fan

I'm building my first tandem, and it clearly has clearances for 700x28, possibly for up to 700x32. Considering me, Mrs. Road Fan, the machine and some gear, we are carrying about 400#, with an extra 40 to 60 if we tour, depending. The problem is: Each tire handles about 200# of non-touring load. To extrapolate the chart, at 200# load the air pressure would have to be about 146 psi, and for 32 mm it would need to be about 116 psi, and for 37 mm it would be about 90 psi. The question is, what tires can actually support such pressures reliably? 42 Hetres are marginal, they would probably need (the extrapolation is not so clear ... ) 80 psi and the max pressure is about 80 psi. For full touring loads the situation is somewhat worse.

So, what tires for tandems? And before we had such a good choice of fatter tires around, what was done?

Carbonfiberboy 

We have had our best results running 28mm Conti 4KIIs, both at 95 lbs. They measure a bit over 31mm on 23mm outside dimension rims. We weigh about 325 all up except for water. We've toured at about 375 lbs. on narrower, real 28mm tires at 115 lbs. on narrower rims. To get some idea of where you are on this, you can contact manufacturers of candidate rims and ask them for a table of max tire pressure vs. tire size. As tire size goes up, rim manufacturers call for lower pressures to reduce the outward force on their rims. I think if you are disk braked you can more safely exceed those pressures by some small amount as you don't have to account for rim wear.

To make a comparison of similar tire drop from different size tires, I do a simple calculation. So we run 31mm tires at 95 lbs. 31*95 lbs.=2945. 2945/28 = 105 lbs. See what I did? For us, ~3000 is a good starting number. For us touring, we'd go 28*115=3220 or 3220/31=104 lbs. for our Conti 4KIIs, Their max sidewall pressure is 115.

If you do these calculations, check out the max recommended pressures. You'll find that not every tire will even be a candidate no matter what size it is. The numbers we are using come from experience, not from any table - we don't pinch flat at those pressures, not even on RR tracks. For some tires it will take some research to discover their inflated width on your candidate rim.

scycheng

Schwalbe's website lists maximum pressure and maximum weight for all their tires. I find that, for the sake of my stoker's comfort, we don't tour at recommend max pressure.

A lot will depend on the expected surface. If you are riding on smooth pavements, you can get away with narrower tires. If we are riding on rougher roads (canal paths, hardpack,...), I want as wide of a tire as can fit and run it at as low pressure as I can.

Paul J

We are a heavier team at 450# all in and ride Conti Gatorskins in 28 at 110psi and 32 at 100psi, this is under max on the 28 and just over max on the 32. We had our first flat for couple of years recently which was a pinch-flat on the rear. We hit a patch of gravel on a decent and flatted quickly, not catastrophically it just went down quickly. We've been very happy with these tire, I know other tires have better "performance" but these have treated us very well over the past number of years.

Road Fan

Interesting - from somewhere you have a criterion, pressure times actual width should equal 3000 [mm*psi] perhaps with a tolerance of 10%. I can see that you are scaling from a case that was successful, 3Where did this come from?

I see your equation, but how does this help with estimating load capability of a given tire with pressure within rim and tire limits?

Road Fan

Berto developed his table back in his Bicycling Magazine days. He set a criterion for loaded tire drop. When the tire is loaded the distance from rim edge to the ground should be reduced 15%. Testing tires with load and presumably a hand pump, plus his engineering and mathematical knowledge, he developed the graph that was first published in Bicycling in the '70s (I think), and by Jan Heine within the past decade. It doesn't go wide (high enough wheel loading) or deep (wide enough tires) to directly cover tandems or wide balloony recent ideas of all-road tires, such as 42 mm Hetres.

But my opinion is that his graph was developed by soundly reasoning from the assumption that 15% drop is a "good" target, so within that assumption I feel confident in scaling the chart to cover my 42 mm bike and the greater mass I expect with my tandem. I think I'll get a reasonable basis for tire width and pressure recommendations IF 15% is in reality a "good" criterion. I would use such a scaled criterion as a starting point for tire setup. I also consider tire max pressure and rim maximum ratings.

I don't see how to use yours in that way. I think you use it to evaluate a new tire against an earlier installation that was successful, based on your all-up riding weight of 325#/375# touring. You make a decision based on that and considering tire maximum pressure and rim maximum ratings. Mine would be (as I said) 400#/450# touring. I presume I should scale your own criterion of 3000 up to a larger value to accommodate our greater weight, but do you have any guidance for that?

OneIsAllYouNeed

I'm a firm believer in the 15% tire drop target for clinchers. You can go to 20% tire drop for tubular or tubeless. That said, I don't pump my tires every ride. They're often around 30% tire drop when I notice increased squirminess or rolling resistance.
A heavy team we're friends with just switched to Terravail Rampart (650b x 47, rated to 70psi) -- that'd be exactly 15% tire drop for a 450lb team. They're loving those tires. Previously they used similarly sized tires with lower pressure ratings, and ran the tires at the pressure limit -- the tires didn't hold up very well. Other tires with similarly high weight ratings: Schwalbe Marathon Mondial 47mm or 50mm, Schwalbe G-one Allround 70mm, Continental Speedking Racesport 55mm, Schwalbe Big Ben 50mm. Using a wider rim (Velocity Blunt 35 or Spank Oozy) helps to get a bit more width out of tire so you can respect the pressure rating.

Road Fan

Thanks for your input! I find it interesting that it (and that of Carbonfiberboy) do not address premature tire failure or whether the tire can provide rim protection. Is this generally not a concern among modern tandemers?

Road Fan

I like to go more like 10% on tubulars or narrow clinchers, to get a little more rim protection. But I don't measure it, I just raise the pressure a little bit.

One reason I'm concerned is that my frame has canti posts placed for 700c rims. I'm not 100% clear on the lateral and radial clearances, but I'm pretty sure I can't fit a 650 wheel unless I do cosmetic surgery on the frame and fork to relocate the brake pivots. I'm planning to use the Berto chart to see how wide I can fit.

OneIsAllYouNeed

Our old tandem was a Trek T200 with 700c canti posts. It could*safely* fit a 700c x 32mm tire, but we sometimes used 35-37mm. We stuffed 650b x ~50mm tires into it. For the rear brake, I used a Diacompe 990 U-brake on a U-brake adapter plate. It was mounted underneath the seatstays to avoid interference with the 700c canti posts. It was a bit of a kludge, but it got the job done. For the front, we got a disc fork. We've since given that bike away (with 700c wheels and cantilever brakes), but I think I have that U-brake setup in a box somewhere. I've attached an image of the U-brake with the 700c rim still installed.

990 U-brake for 650b rim installed on Trek T200

Carbonfiberboy 

When we bought our tandem in '07, I researched tires and pressures. The tables and graphs I was seeing at that time did not include anything like the weights we would have on the tandem. I started by trying tires and pressures that I saw other teams using and went from there. We've tried many tire models and found most of them unsatisfactory for one reason or another. I developed my pressure equation from my experience with several different tire widths and pressures. My suspicion is that published tire pressure graphs show straight lines and that's probably incorrect.

From our experience, you could obviously start with Conti 28mm 4000IIS tires at 105 lbs. and see how that goes. They're still available. I haven't tried the new 5000 tires, though I hear they're a little narrower. The 5000s now come in 32mm. We ride with 6 other tandems which all use about these size tires and pressures, though none of these teams probably weigh over 300 lbs. We all used to use 25mm tires because that's all there was in decent road tires. We just used a little more pressure.

I like our 23mm outside rims with these tires, though a little wider would be fine too.

scycheng

Rim protection is usually provided by a properly sized (for the road condition and total weight) tire at a reasonable air pressure so the tires won't either bottom out or blowout. If you look at the aforementioned Schwalbe website, they give maxiumum recommended loading for each tire.

Premature tire failure I don't think I have ever experienced. I would attribute the causes to be one of manufacturing flaw, gross over/under inflation and possibly overloading.

If you use skinny tires, you have to overinflate above recommended max pressure to avoid things like snakebites. This pushes the tire closer to the engineered safety limit and put more lateral stress on the rim.

We have had snakebite punctures on 700x37c Top Touring years ago riding a rough canal path even at slightly above recommended max pressure. Switched the tandem to 26" and now use 50mm tires. That probem went away.

Note that we don't race so I have no experience with running 700x25c tires on low-spoke count wheels. From what I read and thought about, you can get away with it most of the time but once in an inconvenient while, KABOOM.

My stoker does not appreciate the risk for KABOOM so I try not to go there.

Carbonfiberboy 

I don't understand the concern. I've seen "premature tire failure" and rim damage when a buddy of mine tried to bunny hop a 4" asphalt lip and hit it instead. There's a photo on here somewhere of a tandem which hit a similar obstacle while descending at speed. Wheels were destroyed.

Other than stuff like that, I've never had an issue with either of those concerns. There's always an issue of sidewall cuts with both tandems and singles. I don't think one is more liable to them than the other. That's why I always carry a light spare on both tandem and single. No, the tire's not going to provide rim protection. Tubies can do that to some extent but I don't know of a tandem that runs tubies. If you flat at speed you have a good chance of staying upright if you hit the correct brake, hard. You also have a good chance of needing a new rim, but can probably continue on the old one if you have a spare tire.You also have a good chance of the tire getting caught by the fork and taking you down. Bicycles are inherently unsafe.

OTOH the great thing about a tandem is that the captain can put their entire attention on the road every second. Even with that, I've hit quite a number of potholes as speed and just skipped right over them. That's when you don't want to pinch flat. I've never blown a tire -I don't even see how that could happen. The wheel would be destroyed first.

Road Fan

Back to the beginning:

Just a simpleminded view (I am not a tire expert) is that tires that are run a few dozen psi above rating have casings with excessive stress, which by my simple engineer's logic should lead to a premature failure of some sort. I'm assuming problems of wearout, mainly in the carcass and structure, not so much in big impacts like 4" curb jumping. I have a rear admiral who will physically explain to me the error of my ways, in such a case.

As an engineer I have some clue what other engineers will probably do, and I think there isn't a limit printed without some reason, which the lawyers have agreed to. I don't know enough to predict the nature of such a failure. Certainly tread wear is an obvious point. In cycling we normally mitigate this (if not by dieting and working out harder) by using tires with larger cross-section. The loading of my tandem, which is set up for 700c tires that are not very wide, is pretty high from my naive point of view. Using the Berto chart to estimate a tire pressure to achieve enough ride height to protect the rims under most road conditions, the pressures needed are well in excess of the sidewall pressures.

That looks like a problem.

conspiratemus1

"Simplificate and add lightness." -- unknown (to me) aeronautical engineer quoted in Flight (Time-Life series of yesteryear.)

OneIsAllYouNeed

Running tires beyond (or well beyond) the sidewall pressure rating comes with a variety of failure modes. You never know until it happens, though. In every case, the only remedy is to replace the tire. In my personal experience, here are a few of those failures:
Vee Speedster 27.5 x 1.95 on Velocity Cliffhanger rim. Rated psi: 50. Tested psi: 55. Tire blew off the rim about 1mi into 2nd ride on the tire. It was colder outside than where the tires were pumped, and the brakes hadn't been used, so the tire pressure didn't increase during the ride. The tire got tangled in brake pads and was destroyed.
Continental Gatorskin 700c x 28mm on Velocity Dyad rim. Rated psi: 116. Used psi: 110-125. These tires failed slowly over thousands of miles. The sidewall delaminated from the wire bead.
Panaracer T-Serv 700c x 28mm on Bontrager Clyde rim. Rated psi: 116. Used psi: 110-125. These tires failed after thousands of miles. The wire bead stretched such that the tire would blow off the rim at reasonable pressures.
Panaracer Pasela (various versions) 700c x 32 and 35 on Velocity Dyad rims. Typically used at 0 to 15psi over sidewall rating. We've put many thousands of miles on these tires over the years. Eventually, the sidewall rips away from the bead.

Except for the Vee Speedster, the tires all had been used >2000mi and >2 years at time of failure.

Paul J

So, what Tandem are you building-up? And we always like to see pictures. :-)

conspiratemus1

So how do you know that the tires wouldn't have failed in the same way, at the same mileage, even if they had not been inflated (sometimes) over rated pressure? For each of the tires you mentioned, you would need a control tire inflated only to the rated pressure during the entire time and ridden in the same way. And you would have needed enough tires in both groups (i.e., more than one each) to draw meaningful statistical inference about any observed differences in failure mode or longevity between the groups. All you've managed to say here is that tandems are hard on tires but they still hold up pretty well until they fail.

Carbonfiberboy 

IMHO you are absolutely correct. I've never run a tire over sidewall pressure. However, if you look into it a little more deeply, you'll find that tire sidewall rating and rim manufacturers' tire/width tables are about the same. Which makes sense. Of course a tire manufacturer won't put on a max pressure rating which might result in a rim failure, a very big deal.

Ignore the Berto chart. You don't need that much pressure. Buy some tires, pump them up, ride them. Get experience. OTOH, it doesn't hurt to use my formula to guess at tire pressures. You'll find tires which seem like they'd work fine, but have much too low a pressure rating. Don't use those. Look at the manufacturer's tables, do some calcs, buy 2 tires.

JanMM

I haven't plugged numbers into the calculator recently, when I last did that I ended up using 85 front/75 rear for our recumbent tandem with Schwalbe Marathon Racers 1.5" tires (406/559). This calculator: Bicycle tire pressure calculator Weighed both of us on the bike twice, once at each wheel, to get F/R weight bias, to calculate weight per wheel. Max pressure of tires is 85.
Our total weight (us plus bike) is around 350 now, but I was probably 10 pounds heavier then. We could probably go down at least 5 pounds each tire. Never a pinch flat. No punctures on this set of tires that are into second season and second thousand miles. Previous tires were Schwalbe Marathon Supremes 1.6" which we used for three seasons with one radial tire wire puncture. A bit less pressure per tire, as I recall.

bwebel

I think you are envisioning more collaboration between tire manufacturers and rim manufacturers than is actually there when it comes to max pressure ratings. From what I've read, the sidewall number is determined by the pressure at which a tire blows off the rim, specifically, it is half of this number across some range of samples. Unless a tire manufacturer is recommending a specific rim, it's up to the user to make sure that the tire pressure is appropriate for the rim they have chosen.

Carbonfiberboy 

I'm thinking lawyers, not collaboration. As long as you stay under the max tire pressure, doesn't matter what rim you use. They're all designed to about the same standard from the tables I've received from manufacturers.

Road Fan

It's a steel Meridian frame, among the last Meridian made before folding. A friend of mine bought it new and a bunch of parts he had in mind. He's a veteran tandemer, but he now has a big problem with hand numbness and can't ride an upright bike any more, so he sold the stuff to me. It has room and canti placement for 700c wheels, absolute max size 32 mm, maybe 35. Cantilevers will be Tektro 720 and the brake levers are TRL. The fork blades, seatstays and chainstays are rather stout, but not thick-walled. The fork is Unicrown type for threadless headset, the seatposts are 29.8 mm. So far the front seatpost will be a Kalloy carrying a Selle Anatomica NSX and the rear one could be a Cirrus. The rear mech and bar-ends will be Microshift, the cassette 12-32 or 34. The rear chainset is a really nice SR, now equipped 52/32/30. It could change to a double, but I have some details to sort out, like how to handle the timing chain.

Road Fan

From Carbonfiberboy:

1. "Ignore the Berto chart. You don't need that much pressure." I guess what you mean here is, don't use the excessive pressures the Berto chart leads to when you try to make existing tires work within frame constraints." To "Ignore the Berto Chart" makes no sense. What makes sense is to expand it to it covers my case, even if it's only on an estimated basis.

2. "Buy some tires, pump them up, ride them. Get experience." Please don't imply I have no experience. I'm new to tandems, but I've been working with tires since I was 8 years old and took the bike to the local gas station to get air.

3. "OTOH, it doesn't hurt to use my formula to guess at tire pressures." Your formula does not make sense. I don't know where it came from or how it was determined. It depends on a constant around 3000, with NO rationale for how that number is chosen and it it suits additional cases. If it does not suit my case, then I can cause myself a LOT of bodily harm on the road. No, I will not just use your formula if it does not have some transparent basis or come from an authoritative source.

Carbonfiberboy 

Well, you obviously know a lot more about it than do it and I certainly wouldn't want to be responsible for anything you do. Yes, please ignore all my advice. I take it all back.