General Discussion Triathlon Talk » Optimal Tire Pressure Rss Feed  
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2016-06-30 8:25 PM

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South Windsor, CT
Subject: Optimal Tire Pressure
I rarely venture into TT and it's always fun to see all the experts tell us all what's right. Personally, I try to test and see what works best for me.
I've been riding Conti 4k's on my Zipp 808 race wheels using the 23width for the past few years and have been experimenting all year with 25's.
I'm finding my optimal pressure seems to be ~80 in the front and ~90 in the back. If it's a training ride, using 70/80 for the front/back is awfully comfy.

There are many variables which include: size of rim, weight distribution on the tire (front/rear), surface (smooth/rough), surface (dry/wet), course (hard vs gentle corners and long descents or not), temperature (hot or not)

It should start with knowing your own weight, weight of rider + bike and all gear-ideally measured with front then back tire on scale and other on block. Then you can guess what a good PSI would be and test it. See how it feels, how it rides-especially in various conditions. Test using loops at various pressures trying to keep other things constant and see if you are faster at 80 or 90 or 100. This can be repeated to fine tune.

Some charts.
Tire pressure vs Rider Weight:
http://www.epictri.com/wp-content/uploads/2013/12/TirePressure2.png
http://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Fo...

Flo Chart:
http://flocycling.blogspot.com/2014/09/flo-cycling-tire-pressure.ht...

Tire/Wheel Rims and Actual Measurements:
http://www.slowtwitch.com/Tech/Rim_Widths_2014_4370.html

Various Tire Height/Width at Different Pressures:
http://forum.slowtwitch.com/forum/?do=post_attachment;postatt_id=16...

Other stuff:
http://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Fo...
http://www.crw.org/safety/12safetyPgs/12nov-pressure.php
http://www.dorkypantsr.us/bike-tire-pressure-calculator.html
http://www.huntbikewheels.com/pages/recommended-tyre-pressures

Tire Rolling Resistance Reviews:
http://www.bicyclerollingresistance.com/

Josh's Recent Article:
http://trstriathlon.com/talking-tires-with-joshua-poertner/

ST's Treatise Thread:
http://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Fo...



2016-06-30 9:23 PM
in reply to: dtoce

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Regular
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Tucson, AZ
Subject: RE: Optimal Tire Pressure
What's your weight? What kind of RR differences are you getting between your various pressures?
2016-07-01 8:35 AM
in reply to: ThomasGerlach ProTri

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South Windsor, CT
Subject: RE: Optimal Tire Pressure
Originally posted by ThomasGerlach ProTri

What's your weight? What kind of RR differences are you getting between your various pressures?


My weight is ~155, with bike/gear/fluids on pre ride ~178#'s (=81 kg)

I've weighed myself using blocks so my weight distribution, in aero, on my Cervelo P2 (2012) is 80/100#'s=45/55% front/back distribution

The charts are quite variable and I've always used thinner tires and am still not used to lower pressures, fearing pinch flats. Certainly there is a push to lower pressures on wider tires. There does appear to be a 'cutoff' above which there is sharply decreased return for the benefit of the higher pressure.

The roads here in New England are quite variable. Temps are also pretty variable.

One chart suggests 65/80 psi but I felt like it was slow, comfortable but slow. It was hard to know if my form from aero (CdA) changed or the rolling resistance (Crr) changed. I've not consistently tested a lot lately as I've been changing lots of things and there are too many moving parts. I tried 80/100 and that was too much for sure. It was a rocky ride and form suffered a bit for sure.

I do know, that a recent Oly bike PR happened (with an estimated CdA of 0.26 by aerolab (I am still working on my lack of aeroness but this was a really good day for me), using my race wheels/conti4K 25's front and back at a pressure goal of ~80/90. I pumped to 77/87 and expected the heat of the day to raise the pressure a tiny bit (FWIW, no data to support this or the actual number post race and I didn't check it, but I do find the tires are a couple pounds more after going out in the 90 degree heat of the day here, but this is not a consistent finding)

I'll let you know more as I discover more...
2016-07-01 2:23 PM
in reply to: dtoce

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Exton, PA
Subject: RE: Optimal Tire Pressure
I did some basic testing with tire pressure a couple years ago, and using 23's front and back I use 85(f)/100(r)psi on my tri bike.

I do think I'm a little high at that, now I have switched the rear tire to a 25mm(keeping a 23 on the front) I am planning on doing 80(f)/80(r)psi. I would like to do some testing as low as 70/70 and as high as 90/90, but that route I have used in the past as been compromised(Road torn up). It was a nice 12 mile loop from my house. Not sure where to do this at now
2016-07-01 7:49 PM
in reply to: mike761

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South Windsor, CT
Subject: RE: Optimal Tire Pressure
so, I have some data from tonight
I did loops of ~4 miles trying to hold my planned IM RP watts (170). stayed aero, wore my race kit and only varied the tire pressure. Of course, in real life, my CdA is likely highly variable due to fatigue, head position etc. I set a constant for CdA in aero lab to see if there was a trend with tire pressure. The problem is consistency and it's quite hard to be reproducible, lap after lap and hold exactly 170w. I'd give myself a strong B- for a grade but it's initial data and I'm looking for trends. I tried to do two each for reproducibility but had to fix my aerobars and only did one each of the final ones. I'll test again next week.
Very windy today, I noted a tornado warning on my phone when I took a pic of the temp for calculating the air density later for aero lab. Yikes!

#1/2-Tires 70-90psi (my usual race plan for pressure)--Fr/Rear
AP-168/NP-170; time=12:33 ( I rode from set start and stop places so Garmin data was off a tiny bit by satellites but the distance was constant)
AP-168/NP-172; time=12:24

#3/4-Tires 65-82psi
AP-166/NP-173; time=12:18
AP-169/NP-174; time=12:12 (second fastest time in a tie)

#5/6-Tires 60-77psi
AP-173/NP-176; time=12:00 (fastest of the day, but power is a bit higher)
AP-170/NP-173; time=12:22

#7-Tires 60-70psi
AP-170/NP-173; time 12:17 (fourth fastest of the day with lowest power of the top 3)

#8-Tires 60-65
AP-171/NP-175; time 12:12 (second fastest time in a tie but power is higher)


Overall, the workout went well.
Av HR-125/peak 132
AP-170/NP-173
VI=1.02 (nice and efficient)
IF=0.74, right at goal for the day
C=91

Looking at aero lab and holding the CdA constant, there is a definite trend to improved Crr as the PSI goes down. Of course, assuming a constant CdA is rather unreasonable...Still, it's a real life experiment.
speed 19.0mph
2016-07-02 8:08 AM
in reply to: dtoce

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South Windsor, CT
Subject: RE: Optimal Tire Pressure
I have been reading Jan's blog. What a wonderful read. I loved his summary and share it below.



What does it all mean? Here is the take-home summary:

Stiff casings always will be slow. They are even slower at lower pressures.
Supple casings are fast, and pressure doesn’t matter.
On smooth roads, tire pressure is a matter of personal preference (at least with supple tires). High and low pressures offer the same performance.
On rough roads, lower pressures are faster. So if you want to optimize your speed on all roads, including rough ones, go with a relatively low, but safe, pressure.
Your tire pressure needs to be high enough to avoid pinch flats. If you get pinch flats, increase your tire pressure, or better, choose wider tires. Pinch flats are rare with wide tires.
On pavement, your pressure needs to be high enough that the tire does not collapse during hard cornering.
The minimum safe pressure is higher for more supple casings. Stiff casings hold up the bike more, and thus require less air pressure.
On gravel, you can run lower pressures than on pavement. On loose surfaces, the tires don’t collapse as easily, because the cornering forces are much lower.
Don’t run your tires so low that the casing cords start to break. That happens only at very low pressures, but if you start seeing multiple lines across the casing where cords have broken, inflate the tires a bit more.
Berto’s chart still is a good starting point. Inflate your tires to the pressures it recommends, then experiment by adding or letting out some air.
See what feels best to you. That is the optimum tire pressure for you. Don’t worry about tire pressure any further! At least on paved roads, you won’t go faster or slower if you change your tire pressure.

Even simpler, here is a summary in two sentences:

Ride the tire pressure that feels good to you.
When in doubt, let out some air.

It’s really that simple!


2016-07-05 7:31 AM
in reply to: dtoce

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Exton, PA
Subject: RE: Optimal Tire Pressure
Originally posted by dtoce

so, I have some data from tonight
I did loops of ~4 miles trying to hold my planned IM RP watts (170). stayed aero, wore my race kit and only varied the tire pressure. Of course, in real life, my CdA is likely highly variable due to fatigue, head position etc. I set a constant for CdA in aero lab to see if there was a trend with tire pressure. The problem is consistency and it's quite hard to be reproducible, lap after lap and hold exactly 170w. I'd give myself a strong B- for a grade but it's initial data and I'm looking for trends. I tried to do two each for reproducibility but had to fix my aerobars and only did one each of the final ones. I'll test again next week.
Very windy today, I noted a tornado warning on my phone when I took a pic of the temp for calculating the air density later for aero lab. Yikes!

#1/2-Tires 70-90psi (my usual race plan for pressure)--Fr/Rear
AP-168/NP-170; time=12:33 ( I rode from set start and stop places so Garmin data was off a tiny bit by satellites but the distance was constant)
AP-168/NP-172; time=12:24

#3/4-Tires 65-82psi
AP-166/NP-173; time=12:18
AP-169/NP-174; time=12:12 (second fastest time in a tie)

#5/6-Tires 60-77psi
AP-173/NP-176; time=12:00 (fastest of the day, but power is a bit higher)
AP-170/NP-173; time=12:22

#7-Tires 60-70psi
AP-170/NP-173; time 12:17 (fourth fastest of the day with lowest power of the top 3)

#8-Tires 60-65
AP-171/NP-175; time 12:12 (second fastest time in a tie but power is higher)


Overall, the workout went well.
Av HR-125/peak 132
AP-170/NP-173
VI=1.02 (nice and efficient)
IF=0.74, right at goal for the day
C=91

Looking at aero lab and holding the CdA constant, there is a definite trend to improved Crr as the PSI goes down. Of course, assuming a constant CdA is rather unreasonable...Still, it's a real life experiment.
speed 19.0mph


I assume you were running 25mm front and back?

I think the only time you can do this kind of testing is on a calm day( almost no wind). Just a little variation in wind conditions could change the result more than what you are seeing. So if you had some gusts of wind or stronger steady wind on any trial it could have thrown the results off.

You may have seen more of a trend if you went with a larger variation. Example 110psi, 100psi, 90psi, 80psi, 70psi, 60psi, 50psi

Also would have been interesting to run the test a second time replacing the front tire with a 23mm.
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