rbrides

IMHO, the guidance on tire pressures is often inaccurate because it does not account for the total weight on the bike. With the prevailing science recommending lower tire pressures the metric for setting them is the rider's body weight. I have recently opted to Gross Vehicle Weight GVW, by including the bike, helmet, water bottles, Wahoo, tools, etc that I actually ride with. All of that equipment, including the bike weighs 24 lbs. I'm adding that to my body weight and using this gross total when referencing the many charts and recommendations for tire pressures.


I've ridden 4 times with this updated calculation and it "feels" good. I don't know what the performance detriment is for under inflated tires. All the research these days is about avoiding over inflated tires and the watts, power loss therein. If my logic is correct, I am under inflating my tires if I don't account for the additional 24 lbs of "stuff".


What do you folks think?

aclinjury

when reading charts, it's up to the readers to understand that the recommened psi is fo a particular "weight", and adjust upward for equipment.
The people who come up with these charts can't possibly know how much your bike or the accessories you are carrying can add up, so it's up to you to add in this weight on top of the "weight on the chart".

But I reckon you're right in your questioning, some folks forget or don't understand about this part so they just use their naked weight, which is wrong because nobody rides naked on an air bike.

wolfchild

It's only a simple bicycle tire and I try to keep science, math, astrophysics, chemistry and all other complexity out of it. I inflate my tires according to the conditions that I ride in... When I ride on pavement I put enough air to prevent pinch flats/rim damage and keep the rolling resistance good. When I ride off road I lower the pressure just enough to give me better traction and handling. That's it, I don't worry about any numbers or calculations.

cyccommute 

I'd say that you have it somewhat backwards. On pavement, the chance of a pinch flats and rim damage are minimal while low rolling resistance is what most people want to optimize. Off-road, the chances of a pinch flat and rim damage is much more likely so plan the pressure accordingly. I'm often amazed at the pressure that people use while riding off-road, especially with regards to tubeless. While you can't necessarily pinch flat with a tubeless, pinch flats do say that your tire pressure is too low and you are risking rim damage. This wheel is an example of someone not paying attention to tire pressure to the detriment of the rim

Untitled by Stuart Black, on Flickr

That's a $300 to $500 wheel that is useless because someone opted for traction over protection.

I agree that there's not much need for calculations but there is a need for a bit of common sense. A pinch flat is a cry for more pressure.

shelbyfv

Why would you not use the entire weight of bike, rider and accessories? Who would do that?

Flip Flop Rider

the ideal tire pressure will vary from rider to rider, so there is no hard and fast number to hit. Many factors go into what your tire pressure should be

wolfchild

If you read my post you will notice that I said I lower my pressure for off road riding " just enough "...meaning not too much not too little. I have never had a pinch flat or rim damage yet when riding off road...or on pavement ..Pinch flats can happen on pavement too when hitting a pothole or cracks or something with not enough pressure in the tire, especially with a heavier rider or a bike that's loaded.

79pmooney

Back in my car-less days, riding all winter on a fix gear and sewup cross-country tires, I ran whatever pressure was needed to keep my body off the road surface, Bottoming out on deep potholes happened. By March every year my wheels looked roughly like that bad section, but all around. (Nipples were frozen solid from road salt.. The worst dings would get pulled out., Bad lumps on the sidewall hammered and/or files.) Mafac brakes - the best caliper brake ever for really wonky rims! The ritual of spring - cutting out the spokes and rebuilding with new spokes and cheap 400 gm rims. The training sewups. New chain, cog and probably chainring. Freshly cleaned and greased bearings A new bike!

I loved that those tires simply didn't care how round the rim was. I've ridden home with sewup rims dented twice as deep as that one you show. (Bottomless potholes in deep puddles and loaded LowRiders do that! So does bunny hopping a routine crack on a 17,000 Pacific northwest miles old rim with no sidewall left from braking. That was mid-summer on my best rims and tires.

Sewups aren't pinch flat proof but pretty darn pinch flat resistant. (I did pinch flat on RR tracks in a race once but the year before I bottomed out and dented both rims on RR tracks a mile from the best finish I ever had in an important race. (Latex tubes. Start got delayed for two hours and I forgot to top off. Now miles 90 to 100 were going to be both very fast and on poor pavement so I didn't want my tires too hard there. Well, 6 1/2 hours after pumping up, not enough for those tracks! But those tires got me my most prized finish and were completely unhurt. Rims pulled out nicely later.)

Sewups. I"m going back to them. Just gotta wear out my current rims.

Ben

70sSanO

I’m thankful my Dad never talked about this when I was a kid.

John

genejockey 

Rider weight may vary from 100-300 lbs, whereas bike weight might vary between 15 and 30 lbs, and accessories between 1 and 10 lbs. So, even the heaviest (reasonable) bike and accessories (excluding bike touring) is a smallish fraction of the total weight on the tires.

Take me - I'm 220 lbs. So add the weight of the bikes, which varies between 18 and 22 lbs. Add 3 lbs for summer clothes and shoes, and +/- 3 lbs for water depending on how long I'll be riding. So the range for me is 241 lbs total up to 246. Say 250 in the winter with more clothing. All the variables add up to less than a 4% difference.

BTW, I think SRAM and Silca both ask bike weight, or total system weight.

ofajen

I would agree, but every discussion and chart I’ve seen was already clear that total weight of bike and rider was the relevant quantity as the physics dictates.

What I think is appreciated less often is that front tire pressure should not be significantly lower than rear pressure in many cases.

While the weight distribution for seated cruising may tilt to the rear wheel, weight will shift forward to the front wheel when braking and so maintaining reasonably similar pressure in front will make for good, controlled handling in hard braking, especially on roads. I usually run front pressure no lower than 90% of rear pressure.

Otto

cyccommute 

My post was more of an FYI for anyone else. Many people take the low pressure thing to an extreme. I see lots of tubeless rims that have been ruined because the person seems to be chasing optimal traction over protection. I’ve seen such silly suggestions at “take your mountain bike out and lower the pressure until the tire burps off the rim on a jump...then add 2 pounds”. Just because the tire doesn’t pinch flat doesn’t mean the rim can’t bang on something and dent it.

Troul 

i road ride, but I ride on surfaces that contain random potholes & debris scattered everywhere. I am not going to lower my speeds anymore than I already have to be watchful of hazards in my path, but I will inflate over the ideal psi that is suggested for my gross lbs. Riding anything less & I'd have an expensiver hobby replacing wheels all the time.

pdlamb

This. And I'm not going to stop and pump my tires up in the middle of a ride, so I start with the tires inflated to the worst conditions I expect on the ride.

Assuming I don't have a flat, that is. Then I'll pump.

aclinjury

liberal arts majors?

shelbyfv

Haha! I resemble that remark, History, 1971. Math phobic and senile but even I could figure that one....

bobwysiwyg

aclinjury

that's exactly right. Furthermore, some folks run their mtb tubeless tires so low a psi that the tires get lobsided (ovalized) due to carcass being constantly overstretched.