Tire pressure - is this really right???

I think the article is pointing out a range to inflate your tire that balances out performance and comfort. For maximizing performance, from what I understand, the less deformation the tire endures the lower the rolling resistance (hence faster ride), but at the cost of comfort (harsher hard, the bumps will not be absorbed by the tire deformation (aka losing energy)).  This article seems to argue a point where you should stop inflating for a more comfortable ride with only sacrificing marginal performance.

Edited by ionlylooklazy 2010-03-23 8:51 PM

Looking at their chart, I should be running somewhere between 110 and 130.  I know I shift my weight around and probably put 60% of my weight on the rear tire.

I've found that I have to run at least 110 and seem to do a little better at 130, so their chart is pretty accurate for me.

No, this is a myth. Having a high tire pressure is only advantageous when on a perfectly smooth surface. Most of us ride on roads that are far from perfect. If you overinflate your tires your bike will literally bounce. Each time you bounce you lose contact with the road, and thus speed.

What you want is the minimum inflation that will still prevent pinch flats. This will not only be the fastest tire in most conditions, but provide the most comfortable ride.

scott

Completely lose contact with the road? Interesting, but it seems like even if it did, you are still putting energy into spinning the rim/tire and as soon as the tire "touched down" it would transfer that energy into forward motion (much like a kinetic energy recovery system used in some race cars). 

 Having minimum inflation will cause more deformation of the tire resulting in greater contact with road and greater rolling resistance. If you have a source that refutes this I would love to read it, it just doesn't seem to jive with my understanding of physics.

Edited by Synon 2010-03-23 11:41 PM

Uh.... the first link is from www.vintagebicyclepress.com.  Thus, my guess is that the page is taken from a book from the 1980s or earlier when it was believed that the highest possible pressure would give the best performance.

And the other link, from michelin.com, is probably providing far more up to date info.

You would probably see better comfort and better performance by deflating your tires just a bit. 

depends on the tire a bit, but as scott said, most people go WAY over where they should.

on normal road conditions with my vittoria tires i've found form e, at 150, 115psi gives the best results. over that the ride gets harsh and it slows down a bit.

The problem is that you are neglecting the increase in gravitational potential energy (which must be increased each time you go up) and the fact that although you put energy in to climb over an obstacle, it is almost entirely lost when the tire returns to the road as heat.  Further, while you can continue to spin up the rear wheel, very little of that energy will be useful as it will be lost (also as heat) as the tire initially slips on the road surface if it is travelling faster than the speed of the bicycle.



Here's a plot of Crr vs pressure; the two with trendlines are from roller data while the blue points represent what was found with Crr on the road.  You can see that the data points at ~85psi, ~100psi and ~115psi line up nicely with the Crr determined using a smooth surface.  However, beyond that (~130psi and ~145psi) the Crr increases dramatically on the road.


Note - this is not my data but from Tom Anhalt who posts on Slowtwitch and the Google Wattage forum and has spent countless hours testing Crr and Cda (with great real world results in his TT performances).

Shane

I weigh around 170 and run my tires 100/110. I've tried lower and higher pressures; this seems to be the sweet spot for me.

This user's post has been ignored.

Another +1 thanks for posting, I agree with the above. As a clyde I go max to avoid issues with pinch flats but this is inline with the above comment to go with the lowest possible pressure that you can sustain while avoiding flats

I found the Vintage Press article on a road cycling forum and read through their discussions there... I now ride with lower tire pressures.  I don't have any data or any hard facts but riding with lower tire pressure sure hasn't slowed me down any.  If anything the ride has got to be more comfortable and I've gotten somewhat faster... I like to think the faster part is mostly me though. 

Thanks for that Shane.  So it looks like the sweet spot (at least as regards crr) i somewhere between 115 and 130, roughly, depending on where the 'road curve' turns upward. 

Does the weight of the rider play a significant role in these numbers?  It would seem so, but I don't see any indication of that in the graph that you've posted.

Just because you are clydes doesn't mean you have to go to max. I have weighed up to 270 and still ran my tires at 110 to 115 psi. I've never pinch flatted a road tire.

On my mountain bike I run between 25psi and 35psi in tubed tires depending on tire and conditions. Occasionally I will pinch flat, but the sacrifice in traction is not worth putting in a higher pressure. Plus, in the instances where I do pinch flat I doubt a higher tire pressure would have saved me.

Not sure about the weight of the rider - I think this was just Tom's data from his own riding and testing so I'm pretty sure rider weight was relatively constant.

As for the upturn, I think Tom posted another graph (that I couldn't find) with a little more resolution on the pressure and it seemed the sweet spot was closer to 115 than 130 but that is just going from my memory.

FWIW - I train in the 95-105psi range and usually race around 110-115psi.

Shane

Another Clyde here.  I tried running at 110 for a long time and found I pinch flatted way too often.  When I increased my std pressure to 120 I now almost never pinch flat.   I also believe that the tire you run makes a significant difference here.  I've tried many of the popular race tires.  I feel that especially ones that are noted for thier ride quality (Vittoria) pinch flatted more due to ther more "supple" side walls just weren't robust enough for my weight.  My GP4000's, altough alittle harsher just don't pinch flat at 120psi.

Interesting stuff.
I weigh about 170 and pump it up @ 110- the chart says I should be about 95-100.
I guess I will give it a shot.

Just another n=1 observation ...

When I was really heavy in the States (closer to 200 lb than 150), I needed to run the tyres between 110-120.

When I was lighter in the States (closer to 150), 90-100 would do.

When I was heavy in Indonesia where the roads are crap, I needed over 120.

Now, light in Indonesia, I've got to run around 110 and I do have a really sore rear (but then again I need to ride a lot more, too) afterward, but it's better than having the tyres flat all the time.

Another option for heavier riders - use a wider tire, at least in the rear. I weigh 170 and use a 25mm; if I were over 200, I'd definitely be using a 25, or even a 28.

Edited by feh 2010-03-25 1:22 PM

I'm seriously considering that when my current set of training tires wear out.  Especially if I switch to the HED C2 rims.  But with std width rims i don't think I'd be able to get an inflated 25 past the brakes, a 23 barely squeezes by.  Yes I know, inflate after mounting.

Yess! Rider weight is the single largest factor in determining your optimal pressure. It needs to be higher for higher riders.

Also, as has been said, CRR is a measure of how much energy it takes to deform the tire around bumps in the road. On a perfectly smooth road, it will take much less energy than a chipsealed road - which is but everyone already knows this: It's more comfortable to ride smooth road than chipsealed road.

Two things to keep in mind: the CRR chart is optimized for a high end race tire (Open Corsa EVO is the third best tire ever tested, according to BTR). These are not Gatorskins or Vittoria Rubinos - these tires will have much lower differences in optimum pressures. Also, he's a lean, mean racer, whose position is a bike racer position - not a triathlete's. A competitive tri position will have much more weight on the front wheel, so numbers should be closer to equal. 

So:
-Larger = more pressure
-More aggressive position, closer to equal f/r
-Your tires are likely less supple than these $110 tubulars

I did a CRR measurement in the kinesiology lab recently. On my setup (Vittoria Diamante Pro II, 144 lb, Easton EA90SLX wheels, in aero), I had the lowest CRR at 117f/119 rear. I usually run them at 112/115 for bouncy reasons. 

Your brake pads are too tight. I run "wide" rims (similar to C2), and 25 tires during the winter, and don't have any issues. The point of having C2 is to have a wider tire and contact patch - trust me, they run noticeably better. Don't short yourself because you're afraid to loosen your brakes a bit. 

Thanks.  That's helpful.  Just to be sure I'm clear:  the bolded part means that the lower-end racing tires (I currently race on Conti Grand Prix 4000) need less difference between front and rear.  Is that right?

I didn't make that clear. The bolded part means that there will be a slightly less dramatic difference between 115 PSI and 130 psi. Tires like Gatorskins are much tougher than the one shown (they weigh about twice as much, as well), so they are harder to deform.