Bike tire width/pressure nerd out
 

He's presenting this as a kind of myth-busting, but by and large I think conforms to the current conventional wisdom regarding tire width and pressure. Nevertheless, it's interesting.


One interesting thing that I didn't realize is that a fatter tire will feel harder than a skinnier tire at same pressure because the fatter tire will indent less to achieve the same contact area.

GCN Says 15 Compression
 

If you hunt through the GCN videos you'll come upon one from last summer where a Tour De France team mechanic was quoted as saying their teams starting pressure goal was a 15% height compression. In theory on a near weightless bike, the contact patch would be a small dot. So bike and rider weight should set the tire pressure. I'm still not running my CONTINENTAL GRAND PRIX GP 4000 S2 28 MM /700C tires as low a pressure as the 15% test indicates but the lower I go the fast I go. I sweep a weekly ride of twenty with three out and back legs so I've have been using where I meet the leaders as my indicator. The lower pressure the lower the actual rolling resistance as the tire absorbs road vibrations that would normally consume power as the mass of the bike and rider is moved.

That's an important issue he didn't cover much in the video.

It's the same for sailing, and well understood.
 

When racing sailboats they move everything they can to the center mid point of the boat so it uses less energy going through chop. On a bike the motion is much smaller but motion is wasted energy just the same.
We like crystal smooth water to race on but that comes with no wind. Velodrome track smoothness only comes with going no where, everywhere else there's surface roughness.

maaaaaaan , i don't think any lab test can translate to real world performance , contact patch smontact patch , you will always have a varable that counters the point you are trying to make , it can go on and on , solve two variables , and 3 more pop up .

heavier weaker rider , vs lighter stronger rider , bumpy jagged road vs perfectly smooth wood track , there is no end .

loose watts in the tires make it up int he muscles , loose strength in the muscles , make it up in aero gains, use a tubeless tire on a basic wheel to get a boost , or use a standard clincher on a high end wheel to get a boost , on and on !!!!

I don’t understand the point of this post. Are you trying to say that it is impossible to make a bike/rider faster using... science?

Most changes involve pros and cons. Nothing is “free”. But for a given course, there’s usually an optimal bike setup and an optimal physiology. Part of what makes bike racing fun is that a setup/rider who is really good on one course may be really bad on another.

I’m very confused by your statement that no lab test can translate to real world results. Plenty of people have aero bikes, kits, helmets, wheels and faster, wider, grippier tires that allow them to go much faster than they did when they didn’t have those things. Maybe your average speed on a solo ride will only change a little - but what’s easy to ignore is how much training is required to be able to gain that speed increase with no setup changes.

Science is my thing.
 

In fact I was a test engineer and program manager at the largest test lab west of the Mississippi. While I started out programming IC testers and military hardware environmental testing I moved up the food chain quickly and took over doing all the off the wall test request.

This is no different, but I don't think this calls for an indoor "lab test". I see it as two bikes, one a test vehicle and the other a tow eBike. Using boost cut off speed of 15 MPH. Add a strain gauge and a recording device.

Not sure I understand the video's foundation. He compares different width tires, all inflated to the same PSI. And then he compares different width tires, all inflated to a PSI that garners the same measured amount of drop (eg. 5mm). But what he doesn't do, which is what I believe most tire pressure calculators recommend, is inflate and test all different width tires to yield the same PERCENTAGE drop. Why not?

This. At least give the tires a chance to compete on a level playing field.

Because hysteresis is definitely real-- I changed the wheels on my wife's bike without her knowledge, from 15c to 19c; the 700x25 tires were unchanged, but went from 24mm mounted to 27.5mm mounted. She noticed the difference almost immediately. We were going down a typically SoCal chunk of cracked and heaved road and she said, "Did you change something? Because I'm not getting beaten up as much as I usually do."

I first saw this 15% compression criterion in Bicycling Magazine, in one of Frank Berto's tech articles around 1979. It was ressurrected perhaps 10 years ago by Jan Heine in collaboration with Berto, for articles in Bicycle Quarterly (Heine's publication) and in a randonneuring journal.

I'm glad to see evidence that professional racing circles see the idea to have merit! I've used it off and on over the years - Jobst Brandt thought the 15% setting was too soft for 21 mm tubulars when on fast, imperfect paved mountain roads. I sympathize with his concern, that a rim strike when leaned 45 degrees can be very dangerous in the Colorado Rockies, where a trip off the road could be a trip into the air. But Berto made it clear that it was not beneficial or necessary to fill tires up to the the sidewall limit.

It kind of breaks down for road tandems - when built my tandem will have 28 mm 700c tires based on frame limitations, and total weight around 410#. At 205# per tire, the pressures should be 163 psi each tire! I don't know what 28 mm tire is designed for such a pressure!

So you would record a change in test bike propulsion force, assuming test bike is not being pedaled? That change WOULD be significant, but as a former systems engineer on off-the-wall projects, I would plan to isolate the possible tire effect from the other factors in the test vehicle propulsion/drag equation. I'd design in some practical characterization of the test environment - tow bike speed, grade, surface if practical, wind, temperature, perhaps a vertical accelerometer under the saddle, a few confirmation cameras to record the road surface (were you riding on a painted line?) and that the test rider was or was not pedaling.

Sorry to make it expensive, but I think the value add is to use the test data with a model of vehicle propulsion energy balance, and that means we should estimate the additional known factors in consumption of energy. Perhaps we can isolate the energy dissipated by the tire, and estimate the actual change due to the tire.

@Road Fan, is it too late to configure your tandem for 650x38B? The ideal pressure for those would be about 90 psi each, only 15 psi beyond their max rating. :thumb:

Balloon tyres can't cope with the same pressures as skinnies. Mine have a maximum rating of 45psi and as i smash them up kerbs i'm not daring to test that rating with any notable margin.

I'm sure they could be made to take more, but the tyres of my old Mercedes Atego truck took less pressure than the average 700Cx23 bicycle tyre...

On the plus side, true "balloon" tires don't need a lot of pressure to adequately support the load. I think I'd only need 25 psi or so in 50mm tires. :thumb:

ill tell you only what i know from logic , a lab test is not the real world , its almost impossible to account for every unknown variable , when it comes to bikes every variable you solve will have a counter , the only fact is that it will ALWAYS have a counter ,

put , say a 200 watt rider on the bike with all the advancements he goes slower than a rider doing 225 watts on a slower bike with old tech , there is no way to really say what is FACT when it comes to cycling , you can come close , but its like going down the wrong street for a long time in the right direction , you still end up in the wrong place but your close to your goal :)

you can do all the tire tests in the world but you will never know what is really the fastest tire , it seems like most people just try to convince you rather than prove it ,

this guy does try really hard to make a good case and we could give him credit for giving us a base set of rules to follow , but there will always be a counter because there is so many known and unknown variables in cycling , and the many reality of cycling , and this guy is even countering the data we where told by a lab test

no one area will ever be truly conquered , there will always be a way to counter it ,

some areas are easy to nail down like frame stiffness , but try to make a case for best saddle height , best spoke tension , or try to even crack tire vs rim combinations , cleat position, aero gains vs power , BIKE FIT !!!!!!! ect.....

cycling is like a multiverse inside a multiverse , cycling is the physical embodiment of evolution , based in real time natural selection , where the users are nature ,

you say you can get the best results from indoor track but its a lab test where everything is controlled, no wind , no bumps , same track , but there are always going to be variables you cannot prove facts for :(

trying to prove what tire rolls the best just makes me chuckle , we cant even agree on what brand , what era , what material is best , good luck with your " science " though

:)

I never understood why a rolling resistance test using a rolling drum, as is shown on many lab tests, should accurately represent a bike tire on pavement. This situation reminds me of the political poll conducted somewhere in a southwest border state. Pollsters rang up people but if they answered the phone in Spanish, they hung up. Naturally, the poll was way inaccurate because it did not represent the true population. Reminds me of the BREXIT polls.

An advantage is an advantage. If your FTP and VO2max are both astronomically high, you'll still be faster on equipment that's faster under controlled conditions. Were this not the case, Road bikes and TT bikes would look the same.

I used to love reading Frank Berto and sort of enjoy seeing references back to what he wrote BUT, it seems odd to me that Jan and others always refer back to Berto as the source for this 15% guideline. Where did Berto get it from? If from his own testing, has anyone tried replicating those tests? Beyond that, how useful is this rule of thumb to the average cyclist anyway? How in the world am I going to measure that? If one tries to chart it, isn’t one going to need to do measurements for every single weight on every single model tire with every single inner tube (or lack thereof) combination on every single rim to get anything very meaningful? All this before one even thinks about the accuracy of the average pump gauge or how much pressure one loses removing the pump from the valve. With a floor pump, I pump to about 5 psi over what I think I like best and ride until it seems like things are a bit soft, which might be 10-20 psi or more below what I think I like best but, I really don’t know because I don’t know how much pressure I lose connecting the pump or how much pressure goes into back feeding the pump. With the hand pump, I use a thumb test combined with how energetic I’m feeling. Given all this precision, I think I prefer a 14% drop.

Frank Berto said "It was based on discussions with the bicycle tire experts at Michelin, National, IRC, and Continental. They agreed that "Tire Drop" is the key criteria in matching tire size and inflation pressure to rider weight."

But to his and Jan's credit, they have always suggested this as a starting point -- if you like a little more or less pressure, go for it. :thumb: