tiger1964 

I have a friend who wants to repurpose a bike as an errands bike; he has plenty of shops in a tolerable radius. The bike is a custom-built (by Bilenky) upright tourer and for various Euro trips he has moved on to other bikes. The Bilenky looks right, and is equipped with a Rohloff hub (for this entire bike, $$$ doesn't begin to describe it, more like $$$$$$$); plenty sturdy and has all the right bits for brakes, etc. And he wants to electrify it. So far -- cool. Alas, he's worried about weight capacity: the bike is something like 38LB and you have to add in the electric conversion, plus the cargo load, and he's a pretty big guy. So, before he starts throwing money at the project, is there a way to determine what the weight capacity is of a given tire? For instance, he rides on Continental's GP5000 and also Gatorskins on other bikes, so I hit their website -- all kinds of info but no max weight capacity (like a car tire would). The bike is on 28mm tires now and a rudimentary check shows 35mm no problem, not so sure about 42mm. Any thoughts on how to calculate whether he can do this?

Leisesturm

It really wouldn't be the tires that have a weight capacity limit (GVWR) it would be the wheels. HOW does he plan to equip the bike frame to carry the weight of ... errands? I don't know seems like an easier solution is simply to buy a trailer. I also think some e-bike rated tires like Marathon Plus would be part of the upgrade. Their GVWR is irrelevant also but the Conties are deffo not up to the demands of being thrashed by an e-assist.

tiger1964 

Thanks, food for thought, I am passing this along to the bike's owner. Unsure how to calculate a wheel's max weight capacity. Use to p*ss him off I could suggest Campagnolo high-flange and tubulars...

ThermionicScott 

For a first-order approximation, you could use one of those online "15% drop" calculators to figure out how much pressure you'll need for the load, and then check if that is over the maximum pressure rating of the tires.

tiger1964 

OK, you lost me there. I did a search and found a % calculator, alright, but I am not sure what values you're suggesting I put into it.

ThermionicScott 

This one is as good as any. Just pick the tire width and plug in the total weight of the rider, bike, and gear: https://www.renehersecycles.com/tire...re-calculator/

tiger1964 

Now that makes a lot of sense - thanks! Passing this along...

Leisesturm

Just saying, those calculators ... any of them have input for multiple riders? Or a 'single rider' that weighs 350lbs+. (tandem team)? Heck there are plenty of triplets on the road. At least a dozen Quads. They all use regular tires. I've seen a 10 man bike that used moped tires but it could just as easily have used a standard bike tires. They would have handled the 2,000lb(!!) load just fine. I ride a tandem, with and without, my Stoker daily which is a 150lb weight difference for the tires. Even if I wanted to, there is no practical way for me to adjust the tire pressure for the load change when the Stoker gets off. And I have found that, in practice, it's irrelevant. Inflated to Max PSI, there is no difference in ride quality or anything else when 150lbs. are suddenly taken out of the equation.

Road Fan

The specific thing Thermionic is talking about is the Frank Berto tire pressure chart. Usually by googling "Berto Tire Pressure" you can find a number of different renderings of the chart that Frank developed while at Bicycling Magazine, in the 1970s or 1980s. It's also printed in Frank's classic book "Upgrading your Bicycle," and in at least one of the editions of "The Dancing Chain," also by Berto.
For a given tire width and the expected weight the tire must carry (bike weight plus rider lard weight plus cargo load for your tour, your commute, your romantic afternoon picnic, or whatever), divided by two for a first approximation, the Chart will tell you what is the inflation pressure which will let the loaded tire sag by 15% of the unloaded sidewall height?) If that tire pressure is above the max inflation pressure printed on the side of the tire, that tire is too small for the intended use and you need to go to a wider tire which contains more air volume.

Berto's chart was eventually licensed (is that the right word?) to Jan Heine (Bicycling Quarterly, Rene Herse Cycles). Heine refined it into something that tells you best pressure for low rolling friction, and this might not be the answer you're looking for, though randonneurs and committed long-distance riders find it very useful. Josh of the Silca site also has a online tier pressure calculation tool, intended to dial in race-day pressure on a day of serious racing. ( If you lose, you can blame your tires.)

If you can find a nice clear image of the earlier Berto chart, print it out on A4 or 8 ½ x 11 inch paper, and mark out your parameters (tire width and wheel loading) with a ruler and pencil. It should tell you the pressure you need to eliminate pinch flats and rim strikes, but not to provide any other optimization. That will let you see if your tire is fat enough to withstand your expected usage.

For for example, I weigh 190 now, and my bike plus a little gear is about 35#, for a total of 225. Assuming roughly 50% weight distribution, each of my wheels carries 113#, and my nominally 28 mm tires measure 26 mm using a caliper. The scales are not exact, but for 110 psi and 26 mm I get about 90 psi. For 28 mm I get about 70 psi, and for my 650b with 42 mm tires I get about 34 psi, as target pressures. If something does not agree for you, you do not need absolutely to use those values.

hodala

As my experiences,with rear hub motor(4kg
),1.75" width tire is absolutely not enough

In this case as the picture, 26"x1.95 tire,the rear hub motor + folding chair + empty pannier box are less than 10kg, rode fast through uneven concrete surface 2 days ago ,the tube broken since the tire on the bottom touched the rim

The front tires 20"x1.75 are enough since the front suspension is excellent

Rick

Schwalbe tires give the maximum rated load on there site. I have Schwalbe marathons on our 26" wheeled Tandem and have had a combined weight of over 700 lbs without any problems. Schwalbe's Pick-Up tire would be my current choice for large loads.

noglider 

I think capacity of a tire is a worthy concept. I'd say it should correlate roughly as a function of the tire's weight. A 500 gram tire is not good for much weight.

I'm not sure what the function would look like. Perhaps I would use an exponent such as w^2 or w^3.

Leisesturm

I don't know. I still submit that the weight capacity of a tire is basically unlimited. The wheel itself will fail long before any reasonable tires limits are approached.