# Thread: Measuring tire deflection vs measuring psi's...a better way?

1. ## Measuring tire deflection vs measuring psi's...a better way?

Random thoughts for a Monday morning after spending a weekend riding with friends and family, and setting up and adjusting 5 different bikes consisting of 3 fatbikes, all with different rim width/tire combos, a full suspension 26er and a full suspension 27.5er. I am the de facto fleet mechanic, and am no longer capable of remembering what everybody prefers for their setups. I'm seriously considering putting up a dry erase board charting each rider and bike combo with shock/fork and tire pressures specific to the bike for that rider.

As we all know, a few psi's +/- completely changes the behavior of fat bikes. Measuring psi's, however, is a completely inconsistent metric to use across the mtn biking world. Different tires sizes with different casings, on different width rims, tubeless vs tubed, all set up in that "sweet spot" at different psi's. Drastically different psi's when you start talking about 26er's, vs fat, mid fat, fat plus...ad nauseam. And disregard nailing down the definition of "sweet spot", lets just leave it at what you like and works for you in given conditions on a particular wheel/tire combo. So, that psi that works for you on a given wheel/tire combo is a totally useless value to apply to a larger or smaller tire/wheel combo. Isn't what we're really looking for is a measurement of tire deflection (harness)- which is what we do when we do the squeeze test? Wouldn't a measurement of deflection be a significantly more consistent metric to apply across various sized setups than trying to compare psi's?

Is there even a way to measure deflection on bike tires? Would a simple compression force meter work?

2. With the different casing types and broad range of tire widths, I doubt it would work well. Probably have to include calculations based on tire width. The perfect deflection for a road tire vs. cyclocross tire in dry vs. cyclocross tire in wet vs. fat tire vs. 29er is too wide a variation. IMO, most people who care about bike performance have bike pumps, and if it's my pump and my bike and my tires, I know what I need for pressure. For you, chalkboard/dry erase board. Or tell your friends to air up their own tires.

3. I think the calculation you need is to do with casing tension

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4. My fat bike and 29+ method is pretty simple. I take both thumbs and drive them into the tread.

This approximates landing square on a sharp rock on a downhill.

I use a little fudge work but it gets me pretty close to what's needed and not too far in the other direction.

5. I don't know if there's a simpler way. I replaced my tire gauge with a digital one with higher precision and found that I was still seeing big traction improvements going from 3.5 to 3.25 psi. I probably use tfinator'e method for summer riding, and minor adjustments based on psi for winter riding.

6. Important to remember, when talking about pressures folks are using w/ fat bikes, that, especially with low pressure, high volume, "high floatation" tires, what works for one person, may be different for another. And, what worked one day (dry sand/wet sand- hard snow/soft snow) might need to be adjusted another day because these types of surfaces change day to day too. Also, my low pressure gauge is pretty "constant" or reads accurate as per itself, but at super low psi's like 1.5 or 2, each gauge, especially dial gauge's, may read a little different. So, one person's 2.5 might be another person's 3psi. And for sure, with high floatation tire/rim set-ups on any vehicle, small amounts of adjustment make a big difference... worth taking the time to adjust.

7. I wrench on lots of peoples wheels/tires. No way to remember them all.

So I engage them in conversations about what's working, what isn't, and why, after each ride. Eventually I write their base pressures right onto the rim with a paint pen. When doing this I make sure that they understand it is a base pressure -- and I also make sure they understand when to raise it and when to lower it.

I like basic, consistent dial gauges like the Meiser.

8. If I bounce too much, I bleed pressure out. If I spin out, I bleed pressure out. If I'm too slow I put pressure in. If I bottom out I put pressure in. Same advice I give out to those that ask the pressure question

Wish there was an universal answer, but there doesn't seem to be one except carry a low pressure high volume pump. 20 to 25 pumps with a lezyne makes it a different bike.

https://www.amazon.com/Lezyne-Micro-...ds=lezyne+pump

9. Originally Posted by mikesee
I wrench on lots of peoples wheels/tires. No way to remember them all. ......Eventually I write their base pressures right onto the rim with a paint pen. ......
I'm in the same boat. I've got scraps of note paper floating around everywhere; on my bench, in my work bag and office desk that I eventually try to get in a spreadsheet file. But that does little good in the field. I've taken to a small strip of masking tape somewhere on the frame with fork/shock/F/R tire pressures.

10. Originally Posted by Bumpyride
If I bounce too much, I bleed pressure out. If I spin out, I bleed pressure out. If I'm too slow I put pressure in. If I bottom out I put pressure in. Same advice I give out to those that ask the pressure question

Wish there was an universal answer, but there doesn't seem to be one except carry a low pressure high volume pump. 20 to 25 pumps with a lezyne makes it a different bike.

https://www.amazon.com/Lezyne-Micro-...ds=lezyne+pump
Yep same here. No need to overthink it.

11. Originally Posted by watermonkey
Random thoughts for a Monday morning after spending a weekend riding with friends and family, and setting up and adjusting 5 different bikes consisting of 3 fatbikes, all with different rim width/tire combos, a full suspension 26er and a full suspension 27.5er. I am the de facto fleet mechanic, and am no longer capable of remembering what everybody prefers for their setups. I'm seriously considering putting up a dry erase board charting each rider and bike combo with shock/fork and tire pressures specific to the bike for that rider.

As we all know, a few psi's +/- completely changes the behavior of fat bikes. Measuring psi's, however, is a completely inconsistent metric to use across the mtn biking world. Different tires sizes with different casings, on different width rims, tubeless vs tubed, all set up in that "sweet spot" at different psi's. Drastically different psi's when you start talking about 26er's, vs fat, mid fat, fat plus...ad nauseam. And disregard nailing down the definition of "sweet spot", lets just leave it at what you like and works for you in given conditions on a particular wheel/tire combo. So, that psi that works for you on a given wheel/tire combo is a totally useless value to apply to a larger or smaller tire/wheel combo. Isn't what we're really looking for is a measurement of tire deflection (harness)- which is what we do when we do the squeeze test? Wouldn't a measurement of deflection be a significantly more consistent metric to apply across various sized setups than trying to compare psi's?

Is there even a way to measure deflection on bike tires? Would a simple compression force meter work?
I ride on soft beach sands on my Minn 3.0 with shaved BFLs. I find that 1.5 psi front and 2.5 psi back pretty ideal. Of course, on hard surfaces there's a lot of self-steer but when crawling along in 22/32 at say 3.5-4.0 mph I could care.

Best I could come up with is a static squash print. Basically sitting the bike, balanced with a hand on a piece of drift wood and then dismount, measure area of front and rear squash. If you know front and rear weight distribution, this would give you an idea of the front and rear footprint. That plus weight should give you an idea of effective footprint at X pressure and it should equate closely to tire psi. It doesn't tell you the edge loading of tire though. Weight distribution side to side not front to back is not linear as the tire is not square nor perfectly flat.

I did this with 4.0 H-Billies and came with about 30 square inches front and rear. Total of 60 square inches. This is at 4-5 psi and as I weigh 195 and bike is say 35 pounds that 230 total divided by 60 is darn close to 4 psi average though I'd guess it's higher in the center and lower on the perimeter.

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