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Old 09-10-17, 03:53 PM   #1
merzcm
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Repeated blowouts on rear of tandem

Hi All,

This is a weird issue I am having - hope one of you has an idea.

Rear Tandem wheel, 40 spoke Araya VX300 on a Santana tandem

Tires are Panaracer Pro Tite 700x32 new - I am using them on another Tandem bike just fine, no issues

I usually run them at about 120 PSI on the tandem (I just checked and they are only rated for 95).

I have had 3 blowouts in 2 days - twice when I wasn't even on the bike.

Checked the rim for cracks, spokes sticking through the nipples, etc and can't find anything. Replaced the rim strip and tried a new tire just in case the one was defective.

Funny thing is I rode the bike about 20 miles last week - no issues.

Aside from lowering the tire pressure, does anyone have any ideas?
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Old 09-10-17, 06:16 PM   #2
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Tube pinched between the bead and rim?
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Old 09-10-17, 06:20 PM   #3
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Three blowouts in two days, rear wheel only. Twice when you were not on the bike. Even with a new tire.

Two things come to mind based on personal experience.

1) A very fine piece of wire stuck in the tire somewhere. Even the new tire. Would be invisible to the eye, and can only be found by carefully running your thumb around every inch of the inside of the tire before installing a new inner tube. I would also try to locate the hole in the inner tube, and approximate where to look on the tire.

2) Embarrasing to me, but after successfully changing my own flat tires and those of others for 5+ years, I had a similar experience because the new inner tube I was installing on my bike kept getting stuck between the rim and the tire bead. I was in denial and blaming the new inner tubes I was blowing. I returned to the bike shop demanding a refund. The shop manager showed me how I was not paying attention to keep the inner tube from getting pinched between the rim and tire bead.

Now, I take my time. Put a little air in the inner tube before setting the tire bead to keep it from getting stuck between the rim and tire. Then I pump it up to 60 PSI and look carefully at the tire for any signs of 'puckering' in the tire sidewall before I pump it up to full pressure.

Hope you find my 'confession' helpful!
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Old 09-10-17, 06:26 PM   #4
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Most tire manufacturers rate max pressure at 1/2 the pressure the tire blew off the rim in testing, so 95 means that the tires blew off the rim at 190 so running at 120 should not be a problem.

Some questions:

Is the tire coming off the rim or is the complete tire still on the rim?

If the latter, are your tubes too small for the tire size tire you are running?

New tires and/or wheel? Or has the wheel been used for a while with no issues or the tire was on a different wheel with no issues?

New rim strip? Is the rim strip off center or too wide and interfering with the tire bead settling in the bead groove of the rim?

A little more info may help drive towards an answer
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Old 09-10-17, 06:47 PM   #5
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Please define what you mean by "Blowout".
  • Tire coming off the rim?
  • Tire being completely destroyed in pieces?
  • Loud BOOM and broken ear drums?
  • Flat tire?
  • Slow leak?
If the tire is being blown off the rim, but appears intact, perhaps the tire is getting old or loose. Replace... but you've done that, with a BRAND NEW tire?
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Old 09-10-17, 06:57 PM   #6
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A better description of EXACTLY what's happening would be helpful.

Is it a tube puncture near the rim or belly, the tire blowing OFF the rim, or the tire blowing OUT, meaning though the sidewall?

However, whenever I hear of repeated issues on what had been OK prior, My first instinct is user error. In this case it might be poor seating, but I have no basis for a guess without details.
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Old 09-10-17, 07:59 PM   #7
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Repeated blowouts on rear of tandem

The wheel isn't new and has a few thousand miles on it.

The tires are new front and rear and I have tried two new tires on the rear. No issues on the front.

After the first failure, I replaced the tube and rode the bike about 2 miles home. The Velox cloth rim strip was suspect so I replaced it with a Velox plastic rim strip this morning.

The tubes are sized 700C 28-35 and the tires are 700x32.

Loud boom like a gun going off 2 of the three times, the other was more like a big woosh like you had pushed on the valve.

Tire was blown partly off the rim twice.

Two of the three blowouts have had large rips in the tube about a foot long. The other was a single hole.

None of the tube failures have been in the same place. The first tube rip was on the tire side in the center, the second was on the sidewall side. The hole was on the rim side.

I used talc on the tubes when I reinstalled them except when I changed the tube yesterday while riding.

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Old 09-10-17, 08:14 PM   #8
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These are classic examples of blow OFF.


99% of these are user errors, mainly failure to properly seat the tire, and they tend to happen fairly soon after mounting, which confirms it as seating error.

There are three main types.

1- the tire was seated too low in one area, allowing enough slack in another area for the tire to blow off - most common on rims with squared off seats on either side, and deep well in the middle
2- a flap of tube trapped under the bead. Usually happens when the tube is installed flat as it came out of the box, rater than inflated round
3- tire not seated at the valve. Common with narrow rims, where the valve and thick area of the tube keep the tire from slipping between the area and the rim to seat properly.

I added the descriptions, so you can decide if any may apply, and correct it next time around.
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Old 09-10-17, 08:18 PM   #9
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Ok, so at least two of them were blowouts, as in blowing the tire off the rim, but not necessarily destroying the tire (although it may stress or stretch the tire).

I don't think I've had an ordinary puncture blow a tire off the rim. I don't think they leak out that fast, unless you have some extraordinary sealing of the tire/wheel, and perhaps a vastly undersized tube.

What I have encountered was an old MTB tire that mounted very sloppy on the rim. So, one had to be very careful while inflating it that the bead was perfectly even all the way around the wheel. I think, as long as centered when inflating, it worked ok.

However, if your tire is mounting very loose, then I'd replace it.

I presume pinching a tube during installation could also cause a blowout, but I am careful to make sure the tube is inside the rim before pushing the bead over, and then often double check by pushing on the side of the tire all the way around to make sure the tube isn't being pinched, before inflating. One bike with a blue rim strip, it is easy to look down for the rim strip below the mounted tire.
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Old 09-10-17, 11:36 PM   #10
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The cause is simply that you ran too high of a pressure. That rear rim could be manufactured slightly differently than the front rim, with the difference not visible to the naked eye. The combination of higher pressure and a slight defect in the rim itself is causing the blowout. As the tire's internal volume increases, one must decrease the pressure to maintain the same tire casing tension. That's why a narrow tire is rated to a higher pressure and a larger tire (32 is huge!) has vastly reduced pressure rating. As an analogy, a 32 tire pumped to 120psi creates the same tire casing cord tension as a 23 tire pumped to 200psi. I made up the numbers but hopefully you see the point. The actual math for a tire's cord tension, etc. can be found on the Silca blog. I'd recommend that you keep both wheels under maximum rated psi. Although the front is holding for now, the high cord tension could pull apart the rim bead over time, which will result in a blowout without warning. Your rear wheel might have been damaged already, with the rim bead being slightly bigger than when the rim was new.
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Old 09-11-17, 02:06 PM   #11
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Tires can blow off any time they exceed the sidewall rated pressure. They're typically *designed* for double that pressure to allow for manufacturing tolerances on the rim and tire. If you run across a combination of rim and tire that blows off, it's time to try a different combination.

In my experience, I've had one tire blow off a tandem rim and one blow off single bike rim due to slightly excess pressure.
On the tandem, it was a fairly new Velocity Cliffhanger rim with fairly new Vee Speedster 27.5 x 1.95 tire-- both tubeless compatible (suggesting tight fit and tight manufacturing tolerances). The tire was inflated 5psi above the sidewall rating and blew off about 1 mile into a ride. We had ridden at least 100 miles on that tire at lower pressures prior to the blowoff.

On a single bike, I had some old wire bead Panaracer Tourguard 700 x 28 or 32 tires. I inflated them outdoors for winter commutes (roughly 20 F), then hung my bike indoors (roughly 75 F). One tire blew off twice on the bike rack at work before I retired it.
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Old 09-11-17, 02:32 PM   #12
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Quote:
Originally Posted by FBinNY View Post
These are classic examples of blow OFF.


99% of these are user errors, mainly failure to properly seat the tire, and they tend to happen fairly soon after mounting, which confirms it as seating error.

There are three main types.

1- the tire was seated too low in one area, allowing enough slack in another area for the tire to blow off - most common on rims with squared off seats on either side, and deep well in the middle
2- a flap of tube trapped under the bead. Usually happens when the tube is installed flat as it came out of the box, rater than inflated round
3- tire not seated at the valve. Common with narrow rims, where the valve and thick area of the tube keep the tire from slipping between the area and the rim to seat properly.

I added the descriptions, so you can decide if any may apply, and correct it next time around.
+1 To elaborate:

1) Most tires have a mold line right at or slightly above the edge of the rim. After putting ~20psi in the tire, I always pick the wheel up by the QR ends or axle ends, spin the tire and watch for that mold line dipping under the rim edge or being higher in one location. Either can be a blow waiting to happen. I watch this on both sides. I also watch the center of the thread and see if it dips. This takes 6 wheel revolutions.

2) Tube caught under the bead. Easy way to minimize this is first, putting a little air in the tube. I simple inflate by mouth before starting. The highest pressure I can achieve is perfect. I then close the valve. After mounting the tire, I "walk" around the full rim squeezing the tire and looking for the tube. I should never see it. (This also helps a lot with 1) above.)

3) As I mount the tire, I press the valve into the rim until I observe the tire fit down past the bulge in the inner tube at the valve. (I close the valve after the mouth blow in 2) partly so I can do this.)

All this said, there are rim-tire combinations that are too loose to be reliable, just as there are rim-tire combos too tight to mount without broken tools and patience. Both tires and rims are made to tolerances. Different manufacturers and manufacturing runs differ. If you are seeing this (are your front and rear rims the same? Tires?) you might ask at your shop about tires that they find are a tighter fit.

Ben
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Old 09-12-17, 05:43 PM   #13
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I'd try different tubes. I've had bad runs of tubes where every tube failed in some similar manner. Never like the OP, usually the valve, but it happens. The tube splitting on the tire (upper) side of the tube should never happen, no matter what.

I've also had a bad run of tires blow off the rim, but only when heated. I tried different tires with good result, so threw the old tires away.
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Old 09-12-17, 06:29 PM   #14
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As I have stated earlier the issue is simply too high of a pressure. The high pressure causes the two parallel brake tracks to be non-parallel. Think about the amount of force that is exerted outward on the brake tracks when you pump up a tire. When the brake tracks become non-parallel, the circumference of the rim becomes large and the tire bead just comes off the rim. Different models of rims can tolerate different pressures. A rim with a stiff sidewall can tolerate higher pressure. A rim with a soft or flexible sidewall can tolerate less. Instead of experimenting by trying different rims, it's much cheaper to just lower the tire pressure.
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Old 09-12-17, 11:56 PM   #15
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Quote:
Originally Posted by pdlpsher View Post
As I have stated earlier the issue is simply too high of a pressure. The high pressure causes the two parallel brake tracks to be non-parallel. Think about the amount of force that is exerted outward on the brake tracks when you pump up a tire. When the brake tracks become non-parallel, the circumference of the rim becomes large and the tire bead just comes off the rim. Different models of rims can tolerate different pressures. A rim with a stiff sidewall can tolerate higher pressure. A rim with a soft or flexible sidewall can tolerate less. Instead of experimenting by trying different rims, it's much cheaper to just lower the tire pressure.
Yes, this is a simple and correct fix.

Why run a 700x23 tire at 120 psi when the Panaracer's recommended MAX pressure is 95 psi for Pasela ProTit? Schwalble also recommend 95 psi for their 700x32 tires.

The OP has assumed that higher pressure will make a tire faster or provide enough support for a tandem team. Most teams will inflate 700x28 tires to 95-105 psi, and go up to 100-110 psi for narrower 700x25 tires. I don't see any benefit to over-inflating to 120 psi on a 700x32 tire. By reducing the pressure to 100 psi or less, you'll avoid blowouts and get a more comfortable ride with no rolling resistance penalty.
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Old 09-13-17, 12:17 AM   #16
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Quote:
Originally Posted by pdlpsher View Post
As I have stated earlier the issue is simply too high of a pressure. The high pressure causes the two parallel brake tracks to be non-parallel. Think about the amount of force that is exerted outward on the brake tracks when you pump up a tire. When the brake tracks become non-parallel, the circumference of the rim becomes large and the tire bead just comes off the rim. ......
I strongly doubt this is true. The amount of outward deflection that any rim could tolerate without fracturing is very limited. So, rim deflection can't be a meaningful factor. However, rim SHAPE might be. Older rims had straight sides, more modern (last 30-40 years or so) have hook edges, which work with a shape built into the tire to lock together and work in addition to the wire bead to keep the tire from blowing off.

Since the advent of hook edge rims, tire makers were able to increase tire pressure ratings without increasing the strength of the steel wire bead. These tires depend on the hook edge on the rim to stay on at maximum pressure, and MAY (not will) blow off an older straight sided rim.


Add to this that the OP didn't always have the problem which started after he replaced the tire. So, the logical suspects based on the timing are the tire and/or his mounting of it.

As for tire width and pressure, we need to understand that this is the rear wheel of a tandem, and has higher than solo axle weight, and so calls for a larger tire at higher pressure, or some balanced mix of both, but more than the normal solo tire design parameters. So, the OP doesn't have much choice regarding pressures.

For my part, I still suspect that it's mounting related, but it might also be a poor choice of tire for his application. If the OP rules out poor mounting, he should shop for a tire with a heavier/stiffer steel wire bead.
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Old 09-13-17, 02:46 AM   #17
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run a ball of cotton wool all over the wheel and tyre, you'll find any fine wires/edges very quickly.
Old bike shop trick I picked up.
Failing that, the tyres pinching the tube when its going in
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Old 09-13-17, 06:03 AM   #18
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Repeated blowouts on rear of tandem

All,

Thanks for all the helpful information.

I am going to try some different tires and see if that solves the problem.

Since the Araya VX-300 is a single wall rim, the Panaracer Pro-Tites are sort of loose fitting.

I have some Continental Gatorskins 700x32 which fit much tighter and I will try to see if they work better on these rims.

I have used a number of double wall rims on tandems - Sun, Velocity and Mavic and never had a problem with Panaracer tires blowing off the rim.

Found an interesting article by Jobst Brant on the Sheldon Brown website that explains a lot about what is going on: Blowouts and Sudden Flats by Jobst Brandt

Bicyclists often report tube failures that they believe occurred inside a tire casing. They believe these are blowouts caused by faulty tubes that split or were cut by the rim tape. However, they also heard a bang, after which the tire was flat. On removing the tire casing from the rim with tire irons, the burst tube is found to have a long slash.

If there was an audible bang, then the tire was off the rim, exposing the inner tube. However, the undamaged tire usually remains on the rim because tires usually fall back into place after exposing a tube. A tube cannot blow out inside the tire with a bang, because a bang is caused by a sudden change in volume, an expansion. Such an expansion is not possible within a tire casing that is essentially airtight.
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Old 09-13-17, 08:41 AM   #19
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I had a similar experience that was caused by a bent rim. The tire blew off several times in my basement before I discovered that the rim had got bent.
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Old 09-13-17, 12:47 PM   #20
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To the OP,

Even if you can find a rim strong enough to withstand the high pressure, I strongly recommend that stay below the tire's recommended pressure. Over time the tire's casing could fail catastrophically, and without warning. The tire rated maximum pressure is there for a reason. This type of failure is different from the bead coming off of the rim which is what you are experiencing now. I'm talking about the tire casing rupturing. Be sensible and safe. If you insist on running high pressure, I'd recommend something like the Conti 4000 S 25mm or 28mm width. They are rated to 120psi. The 28mm version actually measures 32mm on a wide rim. But keep in mind that the 28mm Conti at 120psi will exert a similar force on your rims as your current 32mm tire at 120psi.
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Old 09-13-17, 01:42 PM   #21
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To the OP,

Even if you can find a rim strong enough to withstand the high pressure, I strongly recommend that stay below the tire's recommended pressure. Over time the tire's casing could fail catastrophically, and without warning. ......
We seem to disagree on everything. So don't consider this an argument, just another perspective for the OP, who'll have to make his own decision.

---------------------

What kills tire bodies is flex. Lower pressure means more flexing at the point of contact and shorter tire life. Not only tread life, but also body ply life.

Keep in mind that this is a tandem so you will already see more flex due to the higher axle weight.

I'm not a fan of running tires above rated pressures, and would always opt for more section over more pressure, but the OP may be limited by frame/fork tire clearance, so will have to do what he can.
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Old 09-13-17, 02:31 PM   #22
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I found the technical article on a tire's casing tension as it relates to tire width and air pressure. The attached chart shows the pressure at each tire's width for the same casing tension. A tire manufacturer would use the formulas found in this article to determine the maximum safe inflating pressure.

FLO Cycling - Why Do You Use Less Tire Pressure for a Bigger Tire or Wider Wheel?

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Old 09-13-17, 02:55 PM   #23
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We seem to disagree on everything. So don't consider this an argument, just another perspective for the OP, who'll have to make his own decision.

---------------------

What kills tire bodies is flex. Lower pressure means more flexing at the point of contact and shorter tire life. Not only tread life, but also body ply life.

Keep in mind that this is a tandem so you will already see more flex due to the higher axle weight.

I'm not a fan of running tires above rated pressures, and would always opt for more section over more pressure, but the OP may be limited by frame/fork tire clearance, so will have to do what he can.
While a tire casing could fail if it's flexed too many times, it's far more likely that your tire will worn out well before the casing will fail from too much flex. Do you have any good examples of bicycle tire failures from underinflation?

Perhaps if the OP indicates his primary motive at running high pressure on a 32c tire someone could offer recommendations that are safer and achieve the same objective.

I have experimented between 25mm (28mm measured) and 28mm (32mm measured) and found the 25mm to be the fastest rolling, the lightest, and reasonable supple at 100psi front and 110psi rear. We are a lightweight team.
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Old 09-13-17, 03:01 PM   #24
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Originally Posted by pdlpsher View Post
I found the technical article on a tire's casing tension as it relates to tire width and air pressure. The attached chart shows the pressure at each tire's width for the same casing tension.
Yes, and I've often referenced "hoop stress" in the past here on BF. You also don't need a chart because the formula is deceptively simple. ------ Pressure X diameter = Hoop stress -------.

So, there is most definitely increased stress on both the tire and rim with larger width or higher pressure (or both).

But you fail to note that this isn't new, and makers have designed tires and rims accordingly all along. The only time it would become a problem is if one were to use tires significantly wider than a maker might have expected, and/or at much higher pressures.

There's plenty of safety margin built into the system because engineers have to allow for brake track wear, inaccurate pressure gauges, heat related pressure increases, and the fact that consumers may use wider tires with high pressure ratings.

BTW - quality tires tend to have more strength reserve than rims, mainly because the tire maker knows the width and pressure rating, and the rim maker doesn't. The typical modalities of failure from over-inflation include tire failure and, mostly with older rims, rim failure.

One last point on this. Currently most rim makers have some kind of brake track wear indicators to show when you've reached the end of service life --- before they crack. However, while the wear markings reference rim strength considerations, they can't factor the variation in hoop stress. I expect that with more wider tires being used at higher pressure, that rim makers will revisit this issue (unless regulators get there first) and publish some kind of max tire width/pressure guidelines. Otherwise their indicator system won't be reliable.
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Old 09-13-17, 03:06 PM   #25
FBinNY 
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Join Date: Apr 2009
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Originally Posted by pdlpsher View Post
While a tire casing could fail if it's flexed too many times, it's far more likely that your tire will worn out well before the casing will fail from too much flex. Do you have any good examples of bicycle tire failures from underinflation?

Perhaps if the OP indicates his primary motive at running high pressure on a 32c tire someone could offer recommendations that are safer and achieve the same objective.

I have experimented between 25mm (28mm measured) and 28mm (32mm measured) and found the 25mm to be the fastest rolling, the lightest, and reasonable supple at 100psi front and 110psi rear. We are a lightweight team.
Just checking that you understand that the OP is dealing with the rear tire of a TANDEM. So he's dealing with significantly (roughly double) loads. You're running 25mm @110psi on a solo, try to figure what proportionately equal pressure/width would be for a tandem.

Bicycles are no different from cars and trucks. Higher axle loads call for wider tires and/or higher pressures.

BTW - you and the OP might find this chart useful, but you'll have to extend the lines for typical tandem loads.
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Last edited by FBinNY; 09-13-17 at 03:15 PM.
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