Soon-to-be-delivered daVinci tandem with 700c wheels. Standard tire is 700cx28. I've searched for specific guidance, but can't find information that reconciles the 1/2-bike attitude toward tire pressure versus load and the use of 28 mm tires (even at max pressure) on a heavily loaded tandem. Tandem and team (DW) will be right around 400 lbs. When I ride with one of my sons we'll be upwards of 450 lbs. I'll use appropriate tires (Ultra Gatorskins, probably), but I'm looking for some soothing words to convince me that this size tire is ok. I mentioned in a previous post that we suffered a double blowout on a test ride after I hit a particularly nasty piece of gravel - not a confidence builder.

The engineer in me would sacrafice a bit of speed and go with 26" wheels, but I've been overruled by executive management. I'm going to also explore whether the standard DV rim will accommodate 32mm tires.

Same tires you mentioned in the same size. We are about 400 with bottles and rear bag. 115 to 120 (max rated) for the rear and 110/115 front.

I had asked about this a couple months back, it should be easy to find.

PK

We run 28's without any trouble. I like the Continental GP 4 seasons but I've also used the wire bead version in the same size (Gatorskin?). Total team weight 300 lbs. We don't tour with the bike but we ride some pretty rough roads in Quebec. The only flat we had last year was a torn sidewall from a beer bottle. We run 90 to 100 psi.

Our team weight is 340-350. We ride 25c's with 115lbs without any problem with pinch flats. At 400lbs I think you'll be fine with 28c's at 110-115. At 450lbs, I'd push it up to 120lbs.

Of course there is some individual variance here. Road conditions vary, and some people are lighter on wheels than others.

We are 400 lbs and have had good results with 700x28 gatorskins at 120 PSI. My only complaint is they seem to wear more quickly than the tires on the singles.

I'm feeling better already! I must say that irrespective of the tire pressure the CF fork on the daVinci transmitted almost no road vibration to my hands versus my road bike which is a killer on chip seal even with a CF fork. I'll probably be taking the tandem out for solo rides.

There was a big article about this in Bicycle Quarterly in 2007. Bottom line is that it turns out that a larger tire at a lower pressure is more comfortable and has a lower rolling resistance.

Pounds per square inch, the size of the contact patch and deflection of the tire are some of the factors to consider.

Putting skinny tires on a big bike is like putting a car tire on a truck. Do you actually think that trucking companies whose profit margins are set by fuel efficiency would not put on smaller / skinny tires if they are not more efficient?

It is fashionable to put on skinny tires, and some very expensive full carbon tandems don't have the room to do so because of poor design and/or the limitations of carbon at this critical spot on the frame. A lot of people want to ride the same stuff that the guys in the Tour de France ride, but they don't weigh 135 pounds - kind of like wanting to wear the same stuff that the anorexic model has on at the Paris fashion show. (What do you think if women had to put their dress size on the outside of their clothes?)

Grand Bois 32's are lighter than Gatorskins 28's so you end up with a lighter weight tire that has a lower rolling resistance. Flat protection is reasonable, but they do seem to wear out faster than the Gatorskins. The larger tire is WAY more comfortable. The downside of the Grand Bois are that they are expensive.

Pannaracer has a inexpensive 30/32 tire that works well but it is a little heavier.

Barring a last second decision to go with 26" wheels I'll probably ride the 28mm tires until they wear out and then give something wider a try.

At your team weight (which is about the same as ours) I'd be scared to even contemplate 28's. Peter White recommends strongly against 700x28 for a team of our weight. Many of the respondents to your query are lighter than you. You've already blown out a set of 700x28's. Having a tire blow at 40 mph in a downhill turn is not my idea of fun. The 28's aren't faster, so why risk it?

Panaracer Pasela 700x32's with wire bead (non-Tourguard) can be bought for about $15 apiece. They're nearly as fast as the Grand Bois tires mentioned by "reversegear" above. I've ridden on them for about fifteen thousand miles on my single bike and for nearly 4000 miles on the tandem. (I've ridden about 5000 miles on Grand Bois -- they're very nice tires, but at $50 a pop, I only use them for time-critical rides.) We run the Pasela's at 95 pounds. Because we're now doing brevets that go into the night, and we're running sidewall dynamos, I've switched to Schwalbe Marathons. They've got more robust sidewalls, and you pay for it with a slower running tire. Still, we finished our 300K brevet (190 miles, 11000 feet of climbing) on Saturday in 15-1/2 hours, which isn't too bad. We were not fully recovered from the previous weekend's 24-hour, 230 mile fleche ride.

Not bad at all!

I e-mailed daVinci to inquire as to the max tire size the 700c rims would accept. Todd Shusterman replied the same day with the following:

"You will have plenty of room and rim capacity to run 32mm, 38mm, or even
bigger tires.

I run 120 to 125 psi with 28mm wide tires. That is higher than most tires
had listed as "Maximum", but the number tire manufacturer use is half of
what the blow-off pressure was. I have talked to two tire manufactures and
both were very comfortable with 120-130 psi. You would probably use a
little less pressure with 32mm, 105 to 115 should do the job nicely. I do
not have personal experience with 32's. You want to make sure you have
enough pressure to avoid pinch flats."
We'll try the 28mm tires initially. Our riding will be very tame for a few hundred miles in any event. It's reassuring to know I can put larger tires on the frame; definitely a good move on the part of daVinci and one that eases my concern about 700c wheels.

Suggest you run the tires that come with the bike. After you gain a few months and/or about a thoudsand miles experience on the bike, you'll be better able to judge what size tires you want to run.
Our team weight is lower (a tad under 250 lbs) and have run 23mm tires for decades. In our 'advanced age' (mid-70s) we use 25mm Maxxis Re-Fuse (folding/kevlar bead) tires.
Good wear and darn good puncture resistance, even out here in cactus country.
Enjoy the ride TWOgether!
Rudy and Kay/zonatandem

+1 on running out the tires that come w/ your tandem, first.

Like zonatandem we're on the lightweight side, but reversegear made an interesting comment about larger tire, lower pressure and rolling resistance. We roll 27" Weinmann concaves (don't ask...) and tire selection is, well, not as broad as 700c. That said, though, we run 1 1/8 (~28mm) on the front and 1 1/4 (~32mm) on the back, not because I like mismatched tires but because the Conti Gatorskins don't come in a 1 1/8, and mounting a Conti 1 1/4 on the front has the bottom of the crown scraping the top of the tire, essentially like a built-in tire saver. I have noticed that the tandem corners well on technical descents and rolls easily, so I'm a bit curious how the rolling resistance bit plays out. 'Sorry, forgot to mention that we run our tires at 100-105, or so. We're pretty light.

First, let me say "thanks". Second, "light" is in the eye of the beholder! We'll definitely run the stock tires for 2-3 months as we build up confidence: avoided a rock last evening an the tandem definitely doesn't handle like my single.

We have a 700c DaVinci with their V-22 rims. Happy to run the 28mm Contis (4 seasons/gatorskins etc) at 120psi for our 340lb (plus the bike) team in day-to-day riding.

When we head out for a week long tour on our bumpy Aussie backroads we usually run 38mm hybrid tires at 85psi front and 95 psi rear as they absorb the shocks and are bulletproof compared to the lighter, race based tires. The 22mm rims fit them fine and the DaVinci frame and fork has plenty of clearance for the 38mm hoops. This is with discs at each end so no probs with caliper clearance.

Dave

The original poster said that his team weight is 400 pounds, 450 if the son is riding. Does that include clothing, water, and gear weight??? Two pairs of bike shoes weigh about 4 pounds, other summer-weight clothes probably add another 4 pounds (incl. helmets). Five full water bottles, alone, is another ten pounds. You carrying any food or gear on your rides? It's pretty easy to end up with ten more pounds (pump, inner tubes, multitool and repair kit are 2 pounds). So your team weight, fully loaded and standing on a truck scale, could easily be 425 to 475.

So I assume that all of you lightweight teams that are encouraging him to ride on the stock 28's will be contributing to his hospital bills if it doesn't work out?

The fact that DaVinci said to run on tires pumped up beyond spec is a little worrisome. Did you discuss your team weight with them? While it may have been true at one time that tire companies speced a max pressure that was half the blowoff pressure, the competition in max pressure has made it so that tire companies have an incentive to be less conservative. And these are not "tandem-specific" tires -- they're tires sold primarily to singletons, carrying much smaller loads.

A 700x32 tire is only fifteen percent bigger diameter, but thirty percent greater volume, and can be run at significantly lower pressures without risk of pinch flats. We run at 95 psi over all sorts of roads (including gravel), day and night. Lower pressure also means lower likelihood of regular flats, too. You've already had a blowout, so you know how much fun it is. A pair of 700x32 (or even bigger, since you have clearance) Panaracer Paselas costing $30 is pretty cheap insurance. To me, the relevant question is "A week's worth of latte's, or my life: Which is more important."

When I had Peter White build up a wheel for me (our tandem team weight is about the same as the original poster's) I asked what is the smallest size tire we could run on our Velocity Dyad rim. He told me that we could run a 25 in an emergency, and a 28 for a little longer, if we had to, but that in his view running anything smaller than a 700x32 with our team weight is flirting with disaster.

Nick Bull

This whole discussion comes down to deflection of the tire and increased rolling resistance. You have X lbs. of load and X lbs. of pressure in the tire. At 120 psi (pounds per square inch) you will have a 3.33 square inch contact patch with a 400 pound load. That size contact patch will not change if you have a 23 or a 32 tire - the size of the contact patch stays the same. Consequently, with a narrower tire you have a long narrow contact patch while with a wider tire you have a shorter and wider contact patch. (Some people try and compensate for this by increasing the tire pressure, thereby decreasing the size of the contact patch - but this does increase the potential for blowouts and reduces comfort.) No big deal until you consider deflection. With the longer/narrow contact patch the tire deflects more, flat against the road instead of round thereby increasing the rolling resistance. All the crap you will ever read or hear about "fast" tires comes from the "super special secret sauce compound" that a tire company is using to reduce rolling resistance.

Putting a narrow tire from a single bike on a tandem increases your rolling resistance and decreases you safety and comfort.

Narrow tires weigh less and therefore have an advantage in acceleration, etc. and are the tire of choice for a 135 lbs. dripping wet pro on the lightest weight bike available but the increased rolling resistance with that same tire on a tandem overwhelms any weight benefit according to the studies performed by Bicycle Quarterly.

In my opinion, the best tandem riders can do is look for a tire size that minimizes deflection and rotating weight for their size team.

It is so refreshing to see that you understand the relativity of contact patch size, and the fact that it is the air within the tire that holds you up! So few do!

I think you need to divide by 2 since there are 2 tires on the bike. So contact patch is about 1.65 sq inches per tire assuming equal loading.

But yes, you can only adjust contact patch by changing pressure. A wider tire at the same pressure will get the same area, just a different shape.

You folks have to stop this! You made me think again! This is terrible.

Let us suppose that what we want to avoid are pinch flats. Let us further simplify by assuming that a pinch flat occurs when the rim contacts the road surface (even though it doesn't quite). So I made a quickie AutoCAD drawing of a 622 wheel with 23c and 32c tires on it. One obvious thing is that wider tires aren't just wider. They are also deeper in section by about the ratio of their widths. So let us assume that our tires are approximately square in section, i.e. 23mm X 23mm and 32mm X 32mm.

Drawing this out, we see that when the rim contacts the road on a 23c tire, the contact patch is 9.65" long, whereas on a 32c tire it will be 11.46" long. If we ignore the flattening of the tire and just use the specified width, we get a contact patch of 8.74 sq. in. for the 23c and 14.44 sq. in. for the 32c.

Our tandem carries a 300 lb. team. We have racer friends who run 23c Vredesteins at 140 lbs., and they are quite a bit heavier that are we. But they are so fast that I decided to run the same tires, which in fact work perfectly for us. So we multiply the 23c 8.74" patch X 140 lbs, getting 1223 lbs. Multiplying the 32c 14.44 patch X 95 lbs., we get 1372 lbs.

Thus for a 400 lb. team to blow both tires, they are taking a 3g hit, which is quite believable, and the 32s offer some margin, but not that great. We could pump our narrow tires higher, to get more pinch flat resistance, but the rims won't take it. The rim pressure limit for wide tires is obviously much lower than the rim limit for narrow tires.

Wide tire advocates, like Jan at BQ, ignore the higher pressures possible with smaller tires. Tire resistance comes from two sources: wind resistance and deflection. Wind resistance is obviously lower with narrow tires. Deflection resistance is due to the deformation of the tire carcass. The energy absorbed becomes heat energy. That's why you can blow a tire on your car by running it at too low a pressure. The heat comes from the deflection of the material and has three modifiers: 1)Amount of deflection 2) Width of material being deflected 3) Thickness of material being deflected. Which last two terms may be multiplied together to get area of material being deflected. Hence the thinner the tire carcass, the lower the rolling resistance.

Assuming a 225 lb. weight on one wheel, 140 lb. pressure produces a 1.6 sq. in. contact patch. At 95 lbs. pressure, we get a 2.37 sq. in. contact patch. Dividing 1.6 by 23mm, we get a 1.77" long contact patch. Dividing 2.37 by 32mm, we get a 1.88" long contact patch. So we see that the 23c tire deflects less at the same loading, and additionally has less material being deflected, hence its rolling resistance is less. However, the 32c tire has a greater resistance to pinch flats, as it is a taller tire.

If the above were not the case, racers would not be using the tires they use.

Moving to 28's. Mabey stiffin up front wheel feel due to shorter contact patch. Front now very flexy at speed's above 45, will also give bars a lighter feel, hopefully not twitchy, and more rim protection, although these wheels are very tough (Rolf Vigors) running Michelin Krylon 25's @ 110psi., very long lasting, and the bikes ridden 10 times a month,zero flats. Mabey the 28's will be faster! BTW, the Michelins are very comfortable, but the new ones, at least the Pro Race series are made in Thailand

The other issue is rotational inertia, and is probably the main reason that racers moved to narrower (and lighter) tires. In a race, being able to hop onto a breakaway, and being a tick faster in a sprint, makes the difference between winning and losing. For recreational cyclists, being able to accelerate a tick faster is probably of much less concern than riding on comfortable tires.

I'm not entirely convinced by your model for pinch flats. Typically, these do not occur by having a whole section of tire pressed flat against the rim, but by hitting something sharp like the edge of a pothole. So the question is, if you took a 1/4" thick steel bar and put it across the tire (not along it or pointed into it) and then pushed it against the tire, how much force would be required to press it to the rim on a 700x23 pumped up to have the same rolling resistance as a 700x32 (e.g. 140 psi versus 95 psi)?

The one other comment that I'd add about the wide-tire, narrow-tire discussion is that the reason that a wide tire is faster at a given pressure is that the sidewall deflection required to make the contact patch is less. Narrow tires have to have more sidewall deflection because their contact patch is elongated. So to make a narrow tire have the same rolling resistance, you have to pump it up enough that the sidewall deflection is reduced. Of course, the smaller contact patch means that cornering ability is compromised.

Nick

Thanks for the reminder about rotational energy. So true.

I am aware of the failings of the pinch flat model, but it's all I can model without a test lab. I have run different tire sizes and pressures on my rollers and determined that my tire and pressure choice is more noticeably faster there than on the road, as the small rollers emphasize the deflection of low pressure tires. Otherwise I'm guessing, but I think the conclusion would be the same for small objects like a stone as for flat ones, like the road.

As you note, I calculated that the contact patch for a narrow high pressure tire is shorter than for a wide low pressure tire, not longer. Thus deflection is less and less energy is lost. However the difference in deflection is relatively small compared to the difference in deflected cross section, where the real difference lies. Thus if the deflections are approximately equal, the comfort will be also. The only way to get more comfort is to have more deflection.

The most interesting thing from all my calculations is how close a tandem is to getting a pinch flat, no matter what tires we run. We are limited by the strength of our rims, which is limited by our desire for light weight, especially in our rims. I never pinch flat my single.

AFAIK, cornering ability is better on a high pressure tire. You may recall from your Physics 101 that friction is between two surfaces is independent of contact area. So why do race cars have wide tires?
https://www.physicsforums.com/archive...p/t-56486.html

Similar for racing motorcycles. Their tire compounds are designed to run at about 185°F. Again, bicycle racers seem to have no problem cornering on skinny tires, especially since tire distortion is less in a high pressure tire.

It would be very interesting to take a pile of tires and a pump to an outdoor velodrome on a rainy day and see what different tires did at different pressures, just trying to stay upright while riding slowly on the banking. Bring a Michelin Man suit, of course. I know that my Tricomps just barely keep me upright when the painted surface of our local velodrome is wet.

I'll have to admit, the intuition for why friction is independent of contact area has always eluded me. If you think of a tire contact area that is one atom wide, it's hard to believe that that generates as much friction as a contact area a foot wide. Maybe that's because we know intuitively that the one-atom wide contact area will not be robust enough to survive the contact. So it's probably a good thing that I didn't become a physicist.

Friction is not a concept at the atomic level. There you have strong and weak electromagnetic forces. It is in the aggregate of many, many atoms that friction becomes a meaningful concept - the combined effect of lots and lots of atoms. Contact area being greater than a square cm, and atomic separation in solids being on the order of 10^(-8) cm, you're talking more than 10^16 atoms getting into the action.

But jumping back to the macroscopic, if you think about what happens when you press two surfaces together, how hard it is to make them move side to side is proportional to the force pressing them together. This is due to either surface roughness or the affinity of the atoms on one surface to the ones on the other (or some combination, depending on the materials). Double the area and you have twice as many surface variations, twice as many atoms. But the force being applied to each is half as much, so it cancels. And the rule about contact area not mattering only applies within limits. Get large enough or small enough and the surfaces don't "look" the same anymore.