Part 5: Tire Pressure and Aerodynamics

July 12, 2016 18 Comments

Part 5: Tire Pressure and Aerodynamics

We will start this post with a quick refresh on the state of modern wheel aerodynamics.  

Rim Shape and Tires

In 1991 Steve Hed and Robert Haug patented a rim shape that would go on to be known as the 'Toroidal' shape.  The toroidal rim was unique in the it had no flat surfaces, a deep tire well for tubular tires and curved in such a way that the combined rim and tire formed an ellipse. 

SILCA Hed Bicycle Rim Patent Toroidal

Image from Steve Hed Toroidal Rim Patent #US5061013

In simplistic terms, this patent covers any rim that is wider than the tire and comprised completely of curved surfaces.  Now the problem with the patent was that actually making this rim turned out to be impossible with the technology of the day.  The Hed CX rim was roughly toroidal, but with an aluminum cap at the tire well and brake track could not make the beautiful curvature required for ultimate aero, and even at that, the angled aluminum brake track was problematic.

In 1997, Zipp (who owned the Haug half of the Toroidal patent, but had never made a rim of this shape) patented the 'Hybrid Toroidal' rim shape.  This patent took the concept of having the rim be wider than the tire in order to control the airflow, yet matched it with parallel brake tracks to make the concept more manufacturable. 

SILCA Leigh Sargent Hybrid Toroidal Rim Patent

Sargent/Zipp Hybrid Toroidal patent

The Rule of 105%

The rims of this era were all 19-21mm wide, and the Zipp and Hed rims were typically 23mm wide at the widest point, which was optimized for a 19-21mm tire.  During the early part of my tenure at Zipp 1999-2013 I noticed in the wind tunnel that any time the tire approached the rim width, the aerodynamics were compromised and from that formulated the rule of thumb we called the Rule of 105(%).  The Rule of 105 states that the rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and controlling it or smoothing it.  

SILCA Rule of 105 Tire Width Rim Width

Rule of 105 (%) Formulated in 2001 Based on Early Wind tunnel work with 21 and 23mm Tires

One of the most interesting aspects of the Rule of 105 is that before 2001, nobody was tunnel testing with 21 or 23mm tires.  The conventional wisdom was that you TT'd or raced Triathlon on 18-20mm tires and that was that.  However, I was at the Texas A&M Tunnel with US Postal in 2001 and Johan Bruyneel was talking about the amazing ride and grip of these new 21mm tires they received from the team sponsor.  He had made the decision to abandon narrower tires, even for the TT as the riders so preferred this new tire.  We immediately went about testing wheels with 21mm tires and found that the 21 and 22mm rims of the time just weren't wide enough.  

This would be the beginning of an amazing game of chicken and egg within the cycling industry as wheel manufacturers made rims that worked with wider tires and tire manufacturers and athletes kept pushing the limits by using tires even wider still.  In 2007 we struggled to convince cyclists at Paris Roubaix to use 27mm tires when they had always preferred the 'already very wide' 24mm tires by Paris Roubaix 2016 we had cyclists at Roubaix on 30mm front and 32mm rear tires with 25mm tires being used in TT's!

Why the Rule of 105

This CFD image from Bontrager does a great job in showing the 'Why' of the Rule of 105.  While the cycling industry has always liked to talk about aircraft wings, the reality is that no aircraft wing has ever had a bicycle tire as a leading or trailing edge.  This was the realization in the early 2000's that propelled Zipp, then Hed, then Simon Smart/ENVE, Bontrager and now many others to completely rethink the problem.  The real problem/opportunity is how to best take the dirty air off of the tire and smooth it with the rim in the front half of the wheel, and how to use the rim to impart some flow structures that will close up nicely around the tire on the rear half of the wheel. 

SILCA Trek Bontrager CFD Image

Image from Trek/Bontrager D3 Rim Shape White Paper Showing 25mm Tire

This is an image from the Trek/Bontrager white paper of 2011, you can see in the top image how the separation (in blue) is completely dominated by the tire as the rim is narrower.  The 'Zipp' image has the rim and tire at the same width as the tire, and the Bontrager at the bottom has the rim wider than the tire and able to 'recapture' the separated airflow from the tire.  These subtle differences can make for very large changed in drag, and even greater differences in handling.  Many brands have similar CFD imagery to this on their sites, the critical point is that subtle variations in rim shape can and will change aerodynamic drag as well as handling, but none of it is possible unless the rim is at least 105% of the tire width.

The Link to Tire Size and Pressure

So now that we understand how we got to now on this topic, let's revisit our Caliper Measured tires and rims from Part 1.

SILCA Continental Tire Measurements Actual

Actual Measured Widths of Tires On Various Bead Width Rims

The link between pressure and aero starts to become clearer as you look at the chart above.  Between 87 and 115psi most of these tires will grow by nearly 1mm in width.  In strict aerodynamic terms, this added width comes at a cost of roughly 1watt per 2mm of tire at low yaw angles.  However, the big penalty can come at moderate yaw angles as the tires approach the width of the rim.

First let's look at a Zipp 404 Firecrest, a rim with 26.5mm outer width and 16.5mm Bead Width.  With a 23c tire, we see less than 10 grams of drag difference between 6Bar, 7Bar, and 8Bar.  However, with a 25c Tire, we see some significant effects to the aerodynamics of the wheel with changing pressure as the tire growth over those pressures takes rim from being 102% of Tire Width to only 98% of Tire Width.SILCA Zipp 404 Firecrest 25mm Tire Data

Effect of Pressure on Zipp 404 Firecrest with 25mm Continental GP4000sII 25c Tire

All 3 pressures on the 23mm tire made a difference roughly equal to the margin of error of the wind tunnel (A2 Wind Tunnel), so 6Bar, 7Bar, or 8Bar would all be within 10 grams of the blue line .  However, the 25mm Tire is approaching the threshold of aero efficiency due to the inflated width of that tire on a 16.5c bead width rim, and at this tire width, your pressure can make a relatively large aero difference.  

At yaw angled between 10 and 20 degrees, the difference between 7 and 8 Bar tire pressure (100.5 and 115psi) in this instance would be between 1 and 9 watts.  When you consider that a full ceramic bearing upgrade for this same wheel set represents a savings of 0.8-1.0 watt it becomes clear that these aero differences related to tire pressure may be small, but are most definitely non-zero!

Recommendations

For setups where the rim is 105% of the measured tire width or greater, tire pressures will have very small aero effects.  Our friends at FLO Cycling recently completed a very detailed study on 23mm tires on one of their wheels in 5psi increments (rim was 105-108% of tire) and found 0.5-2.0 Watt Difference.  You can read the results HERE. For their setup, the optimal pressure turned out to be 95psi for the 23c Tire on 17.5c Rim.

Again, these are very small numbers, however, at the margins of performance, they may be critical to performance, and best of all, these gains are free of charge to those willing to experiment.

We continue to recommend measuring your tire width and carefully logging your tire pressures to help you better understand these effects.  The thinking should be that wider tires require lower pressures, and if you are violating the Rule of 105 for an Aero Critical event, then perhaps consider downsizing your tire or try and see if a slightly lower pressure may be the solution.

BONUS: Tire Wear and Aerodynamics

As an added bonus we've decided to thrown in a fun graph showing a new 23c GP4000SII and one that has seen 1000 miles of use as a rear under 175lb athlete.  The effect of tire wear was something I first noticed in the wind tunnel 10+ years ago and have been interested in ever since.  While your tire wear will vary based on surface conditions and rider weight, we can unequivocally say that tires with visible center tread wear or flat spotting on the crown of the tire are costing you time out on the course.  

This is logical if you think about it, the crown of the tire will wear flat, and flat, is a terrible aerodynamic shape!  For the sake of our limited data collection time and money, we have used a USED Rear Tire.  Front tires will wear more slowly, but remember, the aero performance of the tire will be slowly degrading every time you use it, so for 'A' races we recommend tires will low mileage!

SILCA Tire Wear Aero Study



18 Responses

Antonio Martinez
Antonio Martinez

September 18, 2017

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Steve
Steve

June 15, 2017

Thanks for an amazing set of articles Josh. I hope to see many more.

Would love to know what protocols you would suggest for us part time athletes to carry out the testing you suggest for finding the optimal pressures for our own racing.

Hxs
Hxs

May 19, 2017

Does this apply only to U-shaped rim that is widest in the middle of the rim?

What if the rim is a V shape, angled brake track?
Would that design expect brake track to be the widest part or the tire itself to be the widest part?

SpencerThusy
SpencerThusy

May 15, 2017

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Grant
Grant

November 19, 2016

My tire of choice are Compass 38’s and unfortunately I’m not aware of any road rim that is 105% wider. Should rim aerodynamics even be a consideration given my tire choice?

Justin Green
Justin Green

September 13, 2016

So how would this change say Tubular vs Clincher? Say I am running a set of Enves 6.7 tubulars. They were designed and tested best with 23mm tires. Lets say I run Conti GP 4000s IIs or Comps on them. Typically tubies would run higher pressure no?

Patrick
Patrick

August 22, 2016

Josh,

Should the 105% be the brake track width or the max of the rim? I thought it was meant to be brake track width so the tire/wheel system doesn’t go tire/narrower/wider/narrower, but rather more like an ellipse.

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July 19, 2016

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John
John

July 14, 2016

“While your tire wear will vary based on surface conditions and rider weight, we can unequivocally say that tires with visible center tread wear or flat spotting on the crown of the tire are costing you time out on the course.”

I can see this being true for a front tire. I’m less sure on the rear, particularly on a TT bike where the leading edge is mostly shielded. The aero hit would seen to be minimized, a rear wheel overall is more shielded, and in general used tires have lower Crr. Lots of stuff to consider!

rct
rct

July 13, 2016

Is the data above based on static profiles or does it take into account the effects of rotation on airflow around the tyre and rim?

Josh at SILCA
Josh at SILCA

July 12, 2016

So when considering the Belgium + rim, I would not consider it to be an aero rim in the same way as the deeper rims we are discussing. For any non-aero rims (which I would consider to be less than ~30mm) the aero performance is basically completely dominated by tire shape and size as the tire has more than half of the surface area of the rim/tire system. I will amend the article to point this out, but the Rule of 105 really is about ‘aero’ rims which I would define as rims that have enough surface area to change or control the airflow onto or off of the tire. For rims like this, the tire is THE driver of aero performance and measured width is going to drive the primary drag and rim shape is going to have nominal effects around the margins. Realistically, choose the tire with the volume you need (harsher conditions need the 25, 26, 27, 28mm etc and smoother can use the 23, which will run about 25mm on this rim) The 23c will be more aero, but the wider tires are going to give you a more comfortable ride and greater margin of safety for rim impact damage, pinch flats..etc.

zak
zak

July 12, 2016

23c tires on the Belgium plus rims still run 25mm wide and IMO have a better profile which the above data seems to indicate in from an aero perspective, i think the same leads to a better handling setup and yea you can run lower pressure as you still have a massive increase in air under the tire compared to a narrow rim

Chris
Chris

July 12, 2016

I’m loving this research series. Thanks so much!

Matt Surch
Matt Surch

July 12, 2016

Cory, I too would like to see what Josh says in response to your question, but since I’ve been in your position – roughly – I’ll provide my ratonale for using 26mm tires. I use 25mm external-width toroidal Woven carbon rims in 55mm depth. In 2015 I used GP4000s in 23mm at around 95-100psi. I pinch-flatted more than once, and cut their shoulders more than once. In aero terms, they were fast. In rolling resistance terms, they were fast (based on results of testing from Josh and Tom A). The issue was that they were too shallow on the wide rims, and that meant part of the casing was not covered by tread on the shoulders where I was cornering during criteriums. Compounding this situation, I had to run pressure high enough to prevent pinches, which increased ‘casing tension’ (Josh’s term) to the point that an offending object could too easily cut the casing’s chords and create a tear. So in practive, these tires didn’t work well for me (165ish lbs). In contrast, my smaller team-mate runs them at a lower pressure on a similar rim, and doesn’t have the same issues. So, one has to factor how much pressure they can run on the low end of the spectrum to get low rolling resistance, good handling, and good puncture protection. For me, the answer is the Compass Extralight 26mm clinchers, which are extremely supple (and I assume would test well for rolling resistance), grip well, and allow me to run 80psi, which makes my Cervelo S5 a much more pleasant bike to ride, gives me better grip in the turns, and reduced punctures (I’ve had zero in a couple thousand kilometres). Yes, I am taking an aero hit – a wider rim to get me to 105% would be better – but with caliper brakes it’s not even presently possible to improve on this front. So there might be an argument to be made for disc bikes using rims scaled to their tire widths for different applications, and cambered brake tracks….

Cory
Cory

July 12, 2016

Also, in regards to tire pressure on the Belgium + with 23mm & 25mm tires. I know that the actual width on the tire ends up being around 27mm when running 25mm tires. So I’ve been trying out just running 80psi rear and 78psi in the front. It’s going well. If I ran 23mm based on aerodynamic recommendations of this 105% rule, then should I just bump it up to 85psi and 83psi? Again, this is if you all would recommend running 23mm over 25mm on a Belgium + rim at all. I like the feel of the 25s for everything I do, but I could learn to like the 23s.

I only ask these questions because these diagrams aren’t very clear.

Cory
Cory

July 12, 2016

So if I’m riding a Belgium + clincher (rim width allegedly at 25mm) then running a 23mm tire is more aerodynamic than a 25mm tire because of this 105% rule?

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