We just had a chat with Josh Poertner aka JoshAtSilca in our forum about his new adventures with Silca.
http://www.slowtwitch.com/Interview/The_Silca_man_Josh_Poertner_4340.html
Should you however have other questions for Josh, you may post them in this thread or within the article.
Cool! Josh is one of the most articulate and interesting reads in the cycling world. Thanks for doing this!
Hate to do this to you, but tell us more about pro rider fails, Josh. 
I’ll tell you a good pro-mechanic fail story.
When we first developed the 808 in 2003 we had CSC, and it took us much of the year to convince anybody to even ride the prototypes…they were really skeptical and somewhat afraid of it. Tyler Hamilton was a huge fan and when he went to Phonak in 2004 he basically told team management that Zipp would be the wheel ‘sponsor’ and he saw huge potential in the wheel, especially after he went to the wind tunnel with us. However, the Phonak structure was very old-world, where Riis had developed this amazing culture, structured the team like a company, was very open to data and science, asked us to work ‘with’ the mechanics, etc. Phonak was essentially half German and half Spanish, with old-world mechanics and soigneurs from the old world. So we were like aliens when we went to work with the team. It was a really completely different experience than what we had at CSC, and not in a good way.
So we have the big official 808 launch before the Tour, and my wife and I decided to make an extended trip out of it. So I’m showing her the Tour, from the inside, and as we’re in amongst the team busses on the morning of the prologue, I hear my name being called, and it’s Tyler at the Phonak bus. The mechanics are having this shouting argument amongst themselves, riders are involved, it’s a mess! I get there to find that the 808 rims are blown up like blow-fish and jamming forklegs of the new BMC Time Machine which was released to the public maybe the day before.
So everybody is freaking out, and I’m trying to figure out how the 808’s became all distorted and blown up…the mechanics are saying something about air from the tire inflating the rim…nothing makes sense. Finally, we get to the air pressure, and they are using helium from a tank, and have the 20mm Conti track tires at something like 280psi. Essentially, the bulging toroidal shape of the rim was swelling outward (as it was designed to do) making the 27.5mm wide rim something like 35mm wide from the insane pressure squeezing the thing inward! After about 30 minutes or arguing, we get them to agree to reduce the pressure to ‘only’ 220psi and ultimately it all settled down. Some of the riders were actually riding to the start house and bleeding off some air.
Sadly, the director they had at the time was pretty adamant about this pressure thing as well as those particular track tires, and if you look back at that entire Tour, that team was plagued by punctures and crashes in the TT’s…the riders were downright afraid of the TT bikes by the end of it.
It was ultimately the impetus for us to start a real educational process with the mechanics and directors on not only what wheels for what days, but also tire choice and the importance of pressure.
It was ultimately the impetus for us to start a real educational process with the mechanics and directors on not only what wheels for what days, but also tire choice and the importance of pressure.
Ha – thanks for that! It’s neat that you managed to loop it back to inflation. Inflation seems to be an extremely important and overlooked part of racing.
I wonder how many watts are lost with 280psi on the road.
holy shit
I wonder how many watts are lost with 280psi on the road.
holy shit
OF HELIUM FOR CHRISSAKES
I wonder how many watts are lost with 280psi on the road.
holy shit
OF HELIUM FOR CHRISSAKES
Well, if you are going to that pressure, at least they are being careful about adding too much mass.
AndyF should test that with an aerostick! I have no idea what the losses are, but I’ll tell you that the handling must be unthinkable as I watched VERY seasoned pros roll across the finish line with absolute fear and loathing in their eyes!
If nothing else, you are pretty much entirely eliminating any steering damping effects as the contact patch becomes more or less a tiny little circle, so not only do you feel every little bump and seam, but the wheels track in the direction of any crevice. The tires will literally begin to flatten in profile due to the localized rubber breakdown at those pressures on the road.
In a nutshell…don’t do it!
Excellent point.
The other thing this story started, was a study of the effects of inflation pressure on wheel aero. It isn’t very intuitive, but at high pressures, the tires become larger in diameter and also become much more round in overall shape as the tread becomes stretched and distorted to accommodate the casing growth.
You can literally see the drop off in aero performance begin at around 100-110psi as both total drag increases slightly and stall angle begins to decrease. A tire like the GP4000s will change in stall angle from 17-18 degrees at 100psi to only 12 degrees at 140psi, and in that space it will increase in drag by 15-25grams in the 0-12 degree range. On a wheel like an 808, difference is worth more than 1 depth class…so an 808 with GP4000 at 140psi is slower than a 404 with the same tire at 100psi…that’s a big deal!
I was going to request a valve extender but looks like you are already working on one. I don’t care what it costs, I just want one that I dont have to worry about.
My LBS thinks I am crazy because I keep wanting to buy stuff they don’t sell. My best experience there was trying to explain to the manager what a crack pipe was and why I wanted it. Took about 10 minutes before he had it figured out.
I LOVE customers telling shops what to buy! In the early Zipp days, we would have shops call in and say ‘This guy is standing here with a credit card telling me he wants a ‘four-hundred and four’…no wait, what, oh sorry, a 4-oh-4, and that it is a pair of wheels, do you sell those?’ It’s fun to remember when the internet wasn’t really much of a thing and if you didn’t have a catalog or magazine ad, then it pretty much didn’t exist. Though likely, calling it a’ crack pipe’ adds some significant confusion!
Please have your shop email us sales@silca.cc or call us 317.964.0592
Thanks for the support!
So one of the ultimate questions: What is the optimum tire pressure for speed.
A lot of us have been basing that guess of a field test by Tom A:
But of course that optimum value should vary based on tire/tire size/road surface/rider mass etc.
So what are your observations on this and do you have any rules of thumb that you use to advise pro teams?
For some reason in the world of cycling, people often think ‘mechanic’ and ‘engineer’ are synonyms.
Oh and another one:
What have you observed to be the typical margin of error of your run of the mill consumer pumps? +/- 1psi? +/- 10psi?
Hi Josh,
I’m always interested in the educational background and history of the industry big names so…
I would like to know what did you studied, what’s your work history in the bicycle industry and what skills do you think helped you get your first opportunity at Zipp
Thanks
Jack, I can always count on you to bring some awesome to the party!
The thing I’m most obsessed with in regards to this question at the moment is how we can measure surface roughness in a meaningful and repeatable way to create an algorithm for this.
Unfortunately surface roughness is the variable that changes everything in the real world, if not for that, then you really would just have decreasing Crr with increasing pressure. On top of that you have rider mass, weight distribution, rim width, tire width (which isn’t standardized between manufacturers), etc…
Honestly, for the pros, we try to strip out any ambiguity or thought and make it as simple as possible. The mechanics have a hard enough time prepping the bikes without having to calculate air pressures, and in fact, most teams will not even do customized air pressure by rider, much less differential front/rear pressures, and certainly not pressures calculated based on conditions.
I recommend a trial and error approach for the individual rider as much of what you have to do is balance the vertical compliance desired with cornering performance. I’ve found that going out and cornering on the tires at various pressures will pretty quickly lead riders to a good number. Especially for criterium racing, I recommend starting at 100psi and going out and railing a few laps, if you are completely comfortable, then push the pressures up to 105 and try again. As soon as you begin to feel a bit uncertain or bounced around in the corners, stop and go back. If you start at 100 and feel bounced around, try 90 and so on. As the tire is providing significant cornering stiffness as well as cornering compliance, it seems that riders are better able to find a balance there than in standard riding conditions, where you really might not notice 10psi at all in flat riding conditions, you might find that same 10psi might make the bike either skittish or vague compared to a baseline.
In general I recommend 5% lower front pressures than rear, even in triathlon where weight distributions are closer to 50/50 you can still benefit from the added steer damping effects of the slightly longer contact patch while not markedly increasing the likelihood of pinch flatting or rim damage.
One thing to note from the TomA curve you posted…other Crr data I’ve seen from real world testing is pretty similar in that it the curve is U shape, but rather asymmetric in favor of being under pressure. That is to say that you are penalized less by being 5psi under pressure than you are for being 5psi over pressure. So when in doubt, better to drop a few psi than to add them.
Any new on the on the bike pumps in the works? I like to carry a mini pump in my jersey on long rides.
I remember when Silca (full length) frame pumps with Campy pump heads were required equipment, but as a broke student I had a Zefal HPx.
The industry standard ‘gauge’ is actually a bourdon tube mechanism that screws into the pump base and has a needle and dial placed on top. They are generally all uncertified, but likely specified in the +/-5% range. From my testing, I’ve seen generally +/-6% on new pumps and +/-10% or more on used pumps. From a precision standpoint, they are generally +/-3% or worse.
For the non-engineers, accuracy is how close to the bullseye you are, and precision is how accurately to each other the darts land. So you can be very precise, yet inaccurate if your pump reads 98psi every single time, but that is actually 104psi when tested against a reference.
The replacement gauges that I just did for the older SILCA pumps are ASTM classified at +/-2% with 1% precision/repeatability. So if you pump to 100psi, that may actually be 98-102psi (vs 95-105 for most)and the reading will repeat better than 1% of itself. This was a huge problem when we were working on the Roubaix wheels all those years ago as the pumps on the truck were off by about 10% to each other. As the pressure gauge mechanism isn’t THAT robust, a year of use and abuse in the team truck will knock it out of whack by more than you might expect.
This is really critical on mountain and cross tires, where you are trying to repeat low pressures and 1psi, may be a big deal. 24psi on one pump may be 26 on another, and that 24 might vary by 1 or 2 psi each time you inflate to it. Some close friends raced Cape Epic last year and had to completely re-calibrate their tire pressure strategy after forgetting the pump and buying one locally. They had been in the 22-26psi range between the two of them and on the new pump were using more like 18-22 to get what they thought was the same actual pressure.
So, what are your goals regarding these metrics for the new Silca floor pump that you’re hoping to release in July?
The industry standard ‘gauge’ is actually a bourdon tube mechanism that screws into the pump base and has a needle and dial placed on top. They are generally all uncertified, but likely specified in the +/-5% range. From my testing, I’ve seen generally +/-6% on new pumps and +/-10% or more on used pumps. From a precision standpoint, they are generally +/-3% or worse.
For the non-engineers, accuracy is how close to the bullseye you are, and precision is how accurately to each other the darts land. So you can be very precise, yet inaccurate if your pump reads 98psi every single time, but that is actually 104psi when tested against a reference.
The replacement gauges that I just did for the older SILCA pumps are ASTM classified at +/-2% with 1% precision/repeatability. So if you pump to 100psi, that may actually be 98-102psi (vs 95-105 for most)and the reading will repeat better than 1% of itself. This was a huge problem when we were working on the Roubaix wheels all those years ago as the pumps on the truck were off by about 10% to each other. As the pressure gauge mechanism isn’t THAT robust, a year of use and abuse in the team truck will knock it out of whack by more than you might expect.
This is really critical on mountain and cross tires, where you are trying to repeat low pressures and 1psi, may be a big deal. 24psi on one pump may be 26 on another, and that 24 might vary by 1 or 2 psi each time you inflate to it. Some close friends raced Cape Epic last year and had to completely re-calibrate their tire pressure strategy after forgetting the pump and buying one locally. They had been in the 22-26psi range between the two of them and on the new pump were using more like 18-22 to get what they thought was the same actual pressure.