The Importance Of Tyre Pressure.

 How to get the correct tyre pressure for bike racing.

Josh Poertner, Technical Director at Zipp Speed Weaponry, was in Australia recently. I spoke to Josh about the technology deployed in the creation of high end bicycle components, like the range of wheels manufactured by Zipp. What stood out was the outstanding array of technologies which Zipp tap into, while researching better ways to improve their products. If you buy a pair of Zipp wheels, it's not just the wheels they've tested, it's the entire system, including bikes, tyres, specific environments where the wheels will be used and so much more. It's Zipp's clever approach to researching and leveraging off diverse new technologies, that positions the company as market leaders.

Just check out the 2012 Kona Bike Count to see how popular Zipp wheels are.

Josh began discussing the importance of feedback from racing teams as part of the process in product development. Zipp worked with Omega Pharma Quickstep in the lead up to the Paris Roubaix this year.

Here's PART ONE of an interview with Josh Poertner - THE IMPORTANCE OF TYRES.

I think that for us it was really great for us to start with them, us and Specialized. Rolf Aldag he's been really amazing to work with with that team, taking that scientific approach with 'what is the right wheel, what is the right tyre, what is the right tyre pressure for this day for this stage, for this race.' Really you know getting that perfect tyre pressure, it's critical.

Do you know what tyre pressure they were running this year? 

They tend to start the race with five or five and a half bar, it's low and they're latex tubes so by the end of the race they are half bar lower. It's dangerously low, it's one of those things we really had to work to perfect with the 303 tubing with the whole rim edge geometry, we did a lot of high speed film trying to understand the way the tyre bottoms to the rim and kinda folds around, it's really complicated and we tried to give it extra surface area to bottom out. We found that by the time you make the rim strong enough to not break from that impact you were likely to pinch flat the tyre, even in a tubular they are paper thin tubes and paper thin casings on those tyres, you know it's that fine balance and of course they want to run them lower which is really what you need, it's a lot more efficient on the cobbles, plus it's just that much less fatiguing with the arms the upper body and everything else but you know again it's one of those numbers you have to optimize you can't maximize it you can't minimize it , you've got to find the right sweet spot and that's where you make your money because things that are maximizable or minimizable well that's pretty obvious to everybody but things that are optimized, you know that's like Formula One and you have to have the right mix on the right day. That's a real quantifiable advantage.

Does that come down to the tuning of the frame if you're building a bike that has a certain amount of flex, or is just the wheel, tyre pressure and tyre combination taken into consideration?

It would …. you're order of magnitude above the little thing with the way springs add. So springs added in series are like one over A plus one over B plus one over C equals one over your total. The way they add in series the system is always driven by the softest spring, which in our case is almost always the tyre. Which is why we always try to quantify ride characteristics back to tyre pressure. You know when these magazine guys who write about bikes, you know it rides so harsh or it's so this or so that. Typically when you look it it's almost always quantifiable back to PSI.

You can do a ton of engineering on a frame, a ton of design, a ton of layup work for optimization of one part of a system at great expense, so you can have something to talk about and at the end of the day the gain might be equivalent to 15 percent of tyre pressure or psi. It's not like mountain bike suspension where you have these tuneablities where they're just entirely different orders of magnitude.

How does flex from a seatpost, say putting a 27.2, long post, into a bike so you have that flex that aids ride comfort, which can make you think that the bike is more compliant?

Yeah you know a seat post is huge, right, relatively you can get a few millimetres of flex out of a seatpost, that's where stuff starts to get additive, I guess the same is true with the frame. If you have a good wheel, our Firecrest rim shape and the whole 303 super toroidal rim shape you can get almost a millimeter of compliance within the rim section within the radial deflection of the rim. You know that millimeter of deflection is a couple of psi 3 or 4 psi just with that. You add that with a more compliant frame and a more compliant fork and you have a 27.2 seatpost instead of a 31.8 seat post an all that stuff adds up and you have ten - twelve millimetres of total combined compliance. You know the thing that you feel, the seat of your pants in your hands is more the natural frequency stuff, the harmonics. That's a whole lot more difficult to quantify, that high frequency buzz, to some people is the feeling of fast and to some people the feeling of discomfort. I always call this the "jeep" problem. You know you go a hundred miles an hour in a Jeep and open top Jeep on the highway and you feel like you're going to explode right , like you're on the edge of death, but you go a hundred miles an hour along the highway in an S Class Mercedes and you don't even realise you're moving, you could just take a nap right. But they are both a hundred miles an hour.

That's one of the problems we have even at the pro level with our athletes, that's why the data is so important. One of our pro teams a couple of years ago were running 160 psi in their time trial setups, and for god's sake it's slower than the lower pressure, they were like 'oh no we can feel it, it feels fast.' When you really go out there and take a look at the data, it's not faster. But it feels fast, right? There's a lot of interesting stuff that John Cobb  did years ago, talking about the helmets that cover the ears and how mentally riders with those types of helmets tend to feel like they're going faster than the riders with the open ear helmets, and there's a real mental thing there. You're perceiving your own reality all the time but it may not actually be borne out in the data. The cool thing about what we have now is we can actually collect that data, talking about the probe that we have that collects air speed. That takes the last of the variables out.

We've got this pitot tube we work with a company that's developed this pitot tube and we're helping them with refining and the mounting plus how to use it. Essentially it's like aircraft airspeed technology for bicycles. We can measure true world air speed and yaw angle. It's about fourteen inches long and sits off the front of the head tube. So it actually sits out the front of the handlebars, you get instantaneous 60 hertz - that's sixty samples per second of what the airspeed and the yaw angle is. That can interface with your ant plus device with your power and ground speed data that we can calculate your cda which is your drag, or drag coefficient times area. We can calculate your cda straight out of that, and you can also find through some testing that's called the Chung method, we can actually determine rolling resistance. So it's pretty cool, you can actually send people out there and you can determine through riding a series of speeds what the rolling resistance and the cda of the rider are. You now have all the data, it's like riding in the wind tunnel but in the real world.

This is like the sanity check, like the frontal air speed was say four kilometres an hour higher in the second test and it netted similar data to test one. What would have been an unknown test result two years ago you can now calculate that back , and that shows that the cda was down or you have this cool thing where your aerodynamic drag rises at the square of velocity and the power to overcome that rises at the cube of velocity but your rolling resistance in your tyres is linear to velocity and so when you do your testing you can do it at a series of different speeds and then you can split what's drag and what's rolling resistance. You can parse that out mathematically and that's really powerful. There's some interesting stuff going on with track racing in that field.

Is that tyre pressure scenario as critical on a velodrome?

The velodrome is more interesting in that we are learning how rider position change with fatigue can dramatically affect aero. Typically we were seeing the power is relatively constant, say in a teams pursuit. Yet from lap eight they were gradually getting slower, but you look at the power only slightly starting to dip in those later stages of the race. What you start to find out of that is that they are making more drag typically because there's some sort of contorting, you can't hold that tight aero position and you're moving and rocking. Twenty years ago you would have looked at that and said oh well that's fatigue. We had heart rate twenty years ago but that means nothing, you look at the heart rate and go well you were pegged for four minutes, you were on the limit but that doesn't tell you anything. Then we had power and you were slower on this lap than that lap but your power wasn't really all that different … but it's track it's indoor it's low yaw angle. So you can try and take those variables out. Well now with this you can take that out. We started to take strobe photography images to try to capture what that movement was like. Here's you crossing a timing strip at one, two and three and four and five. As you're slowing down you're twisting and go sideways. So the question is how do we change that, is it core work, upper body work, position training - is it mental.

Another question you start to look at is this and we've looked at this with time trial finishes when you're that shredded and that fatigued your natural inclination is do whatever you have to do to get that power down but at those high velocities you're power requirement is the cube of the velocity change. So if you're distorting yourself aerodynamically to get down that last fractional amount of power is it really worth it? In a lot of cases you find it's really not. Ninety nine times out of a hundred times, these guys in the last two hundred popping out of the saddle and trying to put a sprint down to get to the line, I would guarantee they're going slower than if they'd remained in a perfect position with a declining power. That goes back to the perceived reality in that moment, when you're absolutely shredded you've got nothing left, we've all had that feeling. You feel like you're not putting anything down and I think a lot of that is just training with that athlete to say no if you can stay completely dialed. In the end you're only making say 400 watts if you can stay completely dialed it's better than 450 watts and flailing.

Position holding can be really intense on the track. Pilates and yoga really works your core, it's mentally focused, yoga can be very very difficult both physically and mentally and it works a whole bunch of things that we as cyclists don't tend to work on.

On the track what about rolling resistance and tire pressure given that it's a different surface to road?

You go to a world class event, world championships or Olympics. The tyres are so minimalist and so thin they're good for kilomteres, they're as minimal as they possibly can be. The harder the tyre cement the lower the rolling resistance right. So there's so many clever, interesting gluing techniques that people work on and that's super secretive. An indoor wood track, that really is the place that favours high pressure, there's not a lot of penalty to it.

The stuff we look at on the road is much harder because there's real penalty to those high tire pressures in that environment. The human penalty of a rider getting shaken or struggling to maintain control, there's handling, how much grip do you have how much grip do you need and then you also have the rolling resistance effect. If you've got a tyre at say 200 psi on the road you're having to elevate the entire system over every bump, there's a lot of energy required. You're literally lifting the tyre off the road hundreds of thousands of times, so how do you dial that in to a place that's more efficient. Then at the molecular level the rubber begins failing in 'shear', because the tyre has to make the contact patch that it has to make, if the surface is jagged and it only has so much area to make that contact patch, the tyre wants to fill over all the bumps and into the crevices you get these localized shear effects and you start breaking down the cross linking of the rubber and the tyres start getting hot, that energy's gotta come from somewhere, so that's a loss. I can tell when we go with the teams which mechanics and which guys have been doing that. You can see it in the tyres, the tyres that have been run at 160psi or 180 psi on the road they look like it, you get a lot of that micro cracking, micro circumferential cracks that start to run around the tyre and it almost looks like an old tyre that's been in your garage for two years.Yet it's a month old tyre that's been used twelve times, that's all that micro shear.

Wheras if it had been run at a lower pressure you wouldn't see that as much!

Yeah you don't see that nearly as much because the tyre can easily make the contact patch by compression of the air and not by manipulation of the tread surface. The thinner the tread the worse that gets.

You're putting a lot of effort into creating world class wheels, yet when you talk to anyone who's been around bikes a long time and ask them what the most critical component on a bicycle is, they'll always say the tyres.

Look at auto racing. Talk about car racing, our racing in the States they don't allow tyre warmers, vs say F1, it's hard for people to get that … that thing can be damn near un-driveable in the first laps. All of the technology in the world doesn't really overcome the tyre, 'cause the tyre's the thing that's touching the ground, you know all of the forces have to go through the contact patch of the tyre.

In cycle racing we don't have a system to gain an advantage from heat in tyres to gain grip.

We don't have a system that needs the heat, what that shows is the criticality of getting tyre pressures right. The grip, the lateral grip of a tyre system is dictated totally by tyre pressure.

Then there's the tyre construction, threads per inch, pliability and other factors.

We always assume that some things are fixed. That's one of the things companies have been doing lately is selling wheel tyre kits - combo's. That was a big question for us at Eurobike this year. The phrase I keep using is design a rim that's tyre 'agnositc', who am I to tell you what tyre to ride for your event.

Personal perception of a tyre's strengths come into play here too.

Like what's the road surface. For some races and some conditions a 19mm tyre or a 20 mm tyre will work, say on the track. With the 303 we really optimized that for 27's for like Roubaix type tyres, there are conditions that require tyres like that. At the same time we're not crazy enough to think that that's what everybody's going to ride on that wheel, and it certainly isn't what you need to ride all the time. If you want to buy that wheel and put 23's on it then I think it works great. It's a lot harder but we spend a lot of time and a lot of money trying to develop rim shapes and tyre bed shapes and all these geometries that will work well with the wide variety of the potential use cases of the product. What are the twenty most widely selling tyres worldwide and they're pretty much the same in every country. When you look at the data you go well that's for a good reason, they're good tyres and they work. There's always specialty cases out there, 'oh and this is the lowest rolling resistance clincher you can buy.' I go well that's great but I need you to know what that means, there's no puncture resistance in that tyre. When you're buying that tyre you need to understand the puncture situation, if you're good with that, well if you're going for your forty kay national championship. Risk vs Reward. There are some tyre tube combinations out there that are worth two or three watts of savings per tyre, that's a lot you know. If you lost your nationals last year by four seconds, that might be worth the risk to you. But you need to understand going in, that you're gonna take those watts, or you might not finish.

Thanks to Josh Poertner from Zipp Speed Weaponry for his assistance in compiling this article. Also a big thanks to Echelon Sports for providing a venue and access to Zipp products.

All photographs by Robert Cobcroft

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