And in the end, it's a matter of taste. There's a width and a pressure I usually find most comfortable, which others will not agree with.

It's your bike, and your choice. There's no reason tell anyone the details. If they don't know, they can't disagree.

Huh? Are you saying I'm being cryptic? I'm just speaking generally. On my racing bike, I like 25mm wide tires at 90/100, and on my commuter, I like 32mm at 60/70. I weigh 170, and my commuter bike weighs 40 lbs. My racing bike weighs about 25 lbs.

No, I was being straight. Every aspect of one's bicycle is a personal choice, frame material, component choices, gearing, position, even bar tape. Tire choice and pressure isn't any different.

Years ago I stopped riding with local clubs because people knew I was in the business and asked me too many questions, including why I preferred this or that. In the end it became annoying so I steered clear. These days I still don't discuss bikes, mine or theirs unless I'm asked. Any time I'm asked about why I do something, I keep it vague. I don't feel I need to justify my choices, nor do I feel anyone else does either.

As an engineer... I can relate.

15% tire drop is a good starting point; Berto and Heine have given us a good tool towards finding what works best for each of us. Kudos to the OP and others who have built on B&H's original work.

Conventional Tire Wisdom for cycling on the road as many of us understood it prior to the popularization of 15% drop: Maximum pressure for least rolling resistance is best. Very clear now that is most often not the case.

I just read the original article, and while it was interesting, I didn't see anything about how the author (Frank Berto) actually determined that the 15% drop was an ideal starting point. He mentioned in one paragraph how the experts agree that tire drop is important. Then in the next paragraph he stated that the magic number is 15%. He took some measurements to determine the necessary pressure to achieve the 15% for various tire sizes and bike & rider weight (credit to him for that, it does give us something). I understand the author is giving the value as a place to start, but where is the evidence/experiments/calculations, anything to convince a curious reader/rider that there is validity in 15% tire drop. No insult intended to Berto, the OP or anyone else, 15% may be the correct value but its hard to tell based on the article.

The original experiments were some time ago, but as I (vaguely) remember Frank Berto first ran trial runs to determine tire drag at various combinations of load, width and pressure. Then found that the most efficient pressures tended to cluster along lines that corresponded to roughly 15% drop.

Please don't hold me to this, it was a while ago. However, similar experiments have been run since, and the general consensus is above a set pressure (related to width and load) rolling resistance on good pavement flattens and doesn't improve meaningfully with more pressure. However the drawbacks of excess pressure begin to be more significant. So it's not like there's a magic number or tire drop.

It's that the best balance of low rolling resistance, and handling properties tends to be in that area.

BTW- it's not like this is new information, except that Frank B. put numbers to what experienced riders managed by rule of thumb. Before people started applying "if more is good, most is best" approach to all things bicycle, from tire pressure, to tire section (narrower), to frame stiffness, & spoke tension, cyclists got help and the benefit of experience passed on from older riders through the club system.

In my cycling career, I've seen fashion trends for extremes in just about every aspect of the sport, only to pull back to "normal" as the next hot thing came along.

Whether you use these tire drop numbers, or your own subjective judgements, or some combination of both, you have to find what works for you, and learn which changes will improve or worsen a property.

Agreed. I think many of us look at the recommended front tire pressure given by those tables for narrow tires and instinctively feel that they are too low.

And I think our instincts are correct in that. The figures are based on static weight distribution on level ground and no acceleration. Now consider the circumstance where I'm on a fast descent of a steep hill and see a rough patch of pavement ahead or a bunch of rocks that have been washed onto the road. My weight has already been shifted toward the front wheel by the grade and now is shifted even more when I hit the brakes - in fact if I hit the brakes hard almost the entire weight of bike+rider will be on the front wheel. This is also a circumstance where I really don't want to have loss of control from a fast pinch flat of the front wheel - but there's a big risk of that if I inflated the front tire based on the assumption that only 40% of my weight will be on that tire.

Or you could do like I do. Inflate the tires to about 10 pounds less than the max listed on the sidewall, and eliminate the need for a mainframe computer. While that formula may work, I put it in the same box as paying $200 for a RD that weight 4 grams less than the one you have. Just ride and have fun, dont try to overthink everthing.

bad advice

this works for you because you are a certain weight and ride in a certain way

i promise you that if many riders took your advice they would have problems

for instance
i am 260 lbs
and i have a bike with 700 x 23 tires with a max pressure listed as 100 psi

at 90 psi those tires allow the rim to hit the ground
every time i ride over something larger than a cigarette butt

That's a good point. No matter how hard any tire is pumped up, there is some combination of riding speed, rider stance/weight distribution, and especially sharp road defect, that will result in a pinch flat. And there is also the matter of different tolerance for risk; a racer going down a windy mountain road faces different risks than a commuter pootling along a suburban artery.

So at some point it becomes an actuarial exercise; do the levees need to be built to withstand a 20-year flood or a 50-year flood?

However, I think the point of the original chart authors stands; most people tend underinflate skinny tires, and overinflate fat tires. (Not to mention, most people choose inappropriately skinny tires for their application, on the illusion it will make them faster). And this information helps to solve that.

This was Jobst Brandt's objection to the figures when they started making the online rounds ~15 years ago -- that when climbing or descending a steep enough hill, nearly all of the weight is on one of the wheels. That's not only a good argument for keeping the tire pressures over a certain amount to prevent pinch flats, but to bump up the tire size if you find yourself needing to use high pressures to avoid them.

IMO, if pinch flats at reasonable pressures are any worry, it means that you don't have enough tire section. Some more pressure will reduce the odds of pinch flats, but at a cost in handling. As for the descending issue, I'd much rather have the traction that would be lost with higher pressures, so I can descend with enough cornering traction, than be forced onto the brakes.

Obviously this is a judgement call, but it's rare that I do any hard braking on descents, except maybe for full stop situations, and that isn't on turns. If I see a bump that has me worried, I can let off the brake for a second, to float the front wheel over it, then resume braking.

I'm not defending 15% as the final word, simply as a starting place for those who don't have one.

Yeah. I haven't had any pinch flats with 15%, and I'm a couple years into that experiment. Maybe I don't ride aggressively enough, or my tires are too big, or I'm just lucky. I'll stick with it until I have a reason to do otherwise.

I ride tubulars on the road, so have a better ratio of rim freeboard to tire width, and like you have never bottomed the front wheel. Having developed most of my riding habits when there was more art than science, I still use my rule of thumb of high enough that I don't notice tire drag, and low enough that I can corner with confidence.

The reality is that my tubulars might lose 5psi or more in the course of an all day ride, so it's hard to get worked up over any degree of precision.

bump!

I was wondering about the old "15% tire drop" chart recently and googled it up, only fo find this post I forgot I had written!

The original links are dead, but here's a Rene Herse article with the chart in it.

Nowadays my commuter is wearing 2 inch ThickSlicks, and the past week I was finding they seemed a little too soft. The pump gauge this morning told me they were both down to 20. I increased the rear to 35 and the front to 30, and the ride in to work felt fine, probably I didn't need to go that high (but they'll find their way back down over the coming weeks).

So the formula which would require 150*600/(50*50) + .75*50 - 25 = 48.5 (plus or minus, depending on uneven front/rear load split of my 260lb+heavy bike) is way too high.

It must be that, since the chart stops at 37, the extrapolation of what the chart is showing is not really valid all the way out to W=50. I'm virtually certain that the 30/35 I rode this morning, was not dropping 15%. I don't think 20/20 was even dropping that much.

The basis of it is pretty iffy to begin with, but with 3 inch tires you are way off the correlation

no this is not the Krampus 29x3, this is my CrossCheck 29x2

With the Krampus I know my optimal pressures are more like 12/10 -- 15/12, even with my fat azz. And with tubeless removing the possibility of pinch flats, the limiting factor there is wallow/cornering stability

We could debate about what is ideal, but the principle behind the chart makes sense. We used to think that more pressure is always better, and now we know that's not true.

This is the graph from Rene Herse article (since the OP is from 12 years ago I guess this is for tubes):



Riding with these pressures is quite comfortable, but...

The problem I see is too much difference between the front and rear tires, eg 40% - 60% for a road bike.

Also if I compare with the data engraved on my tires, these results are much too low (front pressure is well below minimum, rear is slightly above).

So usually I choose pressures that remain compatible with those coming from the tire and wheel manufacturers... less comfort but also less pinched tires.

I ride tubeless Rene Herse tires and always found the minimum value their calculator recommends to be both comfortable and fast:

https://www.renehersecycles.com/tire...re-calculator/

They do say in the previous article it doesn't matter much for modern supple tires anyway, though:

Science weeps every time someone quotes a JH experimental result or conclusion.

Yeah, look at all the real science being done by bicycle marketing departments

The existence of a million marketing departments does not make JH any less of an experimental hack.