I totally agree with you. You also have to know that Berto's curve is an average of a lot of tires he tested. I have his raw data, and different tires deflect very differently. A supple tire will give you 20% tire drop for the same pressure where an ultra-stiff one gives you 5% or less. (In fact, one "critic" on rec.bicycles.tech complained that there was no way to get 15% tire drop, even with zero air pressure. I suspect he did something else wrong, but part of the problem was that he was using Specialized Armadillo tires!)
In the end, this brings up a much bigger question: Do we work from first principles forward, or do we work backward from our "subjective" experience of riding on the road? At Bicycle Quarterly, we start with the ride, and then devise tests to confirm that our "subjective" impressions hold up under rigorous scientific scrutiny. Bicycles are so complex that this seems to be the best approach. All attempts to work from first principles forward have not yielded useful results, as far as applicability to real-world conditions is concerned. So in the case of 15% tire drop, we tested tires at various pressures and looked at where the drop-off in performance occurred. We found that this roughly correlates to Berto's 15% tire drop. So we recommend Berto's chart as a starting point for experimentation. However, it's only a starting point... For example, I ride my wider tires at higher pressures than Berto's chart would indicate, if you extrapolate it. With the extremely supple casings, the tires otherwise tend to collapse under hard cornering... Jan Heine Editor Bicycle Quarterly www.bikequarterly.com On Tuesday, June 10, 2014 9:27:06 PM UTC-7, ted wrote: > > Which is an "appeal to authority" and a "seems to work ok where we have > used it". Which is fine so far as it goes, but its not much to go by for > guessing how applicable either extrapolations of Berto's curves or the 15% > rule of thumb are outside the domain where we have experience using them. > > I hope I don't come across as to critical. I have pulled the equation for > the regression fit of Berto's curves from the spreadsheet and like using > it. But I suspect I would do as well (practically speaking) if I just let > air out of the tires till they squished a good bit when I got on the bike, > and then just rode it and adjusted for feel. > > On Tuesday, June 10, 2014 8:57:33 PM UTC-7, Jan Heine wrote: >> >> Originally, the 15% drop came from the tire companies to whom Frank Berto >> talked. So Frank then just tried to figure out how to get that value. >> >> Our initial tire testing indicated that somewhere around 15% tire drop >> was the point where performance and comfort were optimized. Since then, we >> found that at least for supple tires, even lower pressures don't seem to >> slow the bike down, so it matters even less. >> >> However, even today, the 15% tire drop values from the chart are a good >> starting point for experimenting with tire pressure... >> >> Jan Heine >> Editor >> Bicycle Quarterly >> www.bikequarterly.com >> >> On Tuesday, June 10, 2014 4:19:49 PM UTC-7, ted wrote: >>> >>> But I think the real question is whats so special about 15% drop. I >>> can't recall seeing any supporting argument / evidence for it being optimal. >>> >>> -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/rbw-owners-bunch. For more options, visit https://groups.google.com/d/optout.
