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Have Your Tires been Lying to you?

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Have Your Tires been Lying to you?

Old 01-08-20, 11:27 AM
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Ballenxj
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Have Your Tires been Lying to you?

An interview with Josh Poertner about tire pressure.
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Old 01-08-20, 11:52 AM
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Most tires anymore list a "max pressure" so, no they aren't really lying - technically. Just like a car tire, the max is the max for the tire and has nothing to do with the actual vehicle you are using it on. The video is very good however.

That said, I've been using the Berto tire pressure app and liking the results. It calculates a 15% static drop pressure for tires up to 60mm or something. Silca has their own website that does pressure recommendations also (it's a Beta, but was mentioned on the Marginal Gains podcast recently), but I find the weight distribution that they assume to be more front biased than what I've measured.
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Old 01-08-20, 12:13 PM
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Yeah, the Youtube kept shoving this in my face. Eventually, I watched it. It doesn't have anything new.

He does gloss over the nuance of larger tires have lower rolling resistance for a given pressure. (The reason is the more circular contact patch) But his point of the extra volume & lower pressure making for a smoother roll over rougher terrain does often end up being faster than the same wide tire at high pressure tire deflecting the forward inertia to the vertical plane as is the case with not smooth surfaces is not missed.

Yeah, we're balancing competing nominals. Smooth vs rough. Fat vs skinny. High pressure vs low. Light vs heavy. Supple vs not. That's 20 possible choice vectors to approximate a nominal pressure.

Our infrastructure is terrible. Wider tires at lower pressures is the response...Nothing new there.

FWIW: At 200 pounds, I run 23's at 110 rear, 100 front. I'd run 25's a bit less. By way of experiment: My 41's are noticably faster at 45-50psi than they are at 80psi. So he is right.

Last edited by base2; 01-08-20 at 01:27 PM.
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Old 01-08-20, 12:14 PM
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I'm a dinosaur, It would never occur to me to look at the sidewall (or anywhere else) to see what pressure I should ride. I choose pressure based on tire size, road ridden, tire grip, the bike it's on and how it's loaded. Jump on the bike and ride. Fine tune from there. Thumb squeeze when I like it for duplication, I usually never do know the actual pressure until I duplicate the thumb squeeze with my floor pump with gauge. (I rode sewups 20 years with nothing but frame pumps on all my bikes, no gauge at all. Sometimes I got to use a floor pump at races and actually knew the pressures but usually no.)

One thing I take real issue with is anyone telling me I should be using less than 5 psi lower in front than in back. That was what we all were taught back when we came from eggs (remember the dinosaurs). But there is a really good reason for so much pressure up front. There might just come a day when you have to slam on the brakes and do maneuvers to save your butt. At that moment, all your weight is on that front tire. Not 40-45% of your total. Not the 55-60% that the rear tire sees. No, that front tire is looking at 67% more weight than either tire sees normally. And at that moment, control is paramount. A blowout or rim failure disaster.

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Old 01-08-20, 12:44 PM
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I have problems with some of the points in this video. The first is the statement that tire pressure is set to 1/2 the blowoff value of a "standard rim". I've heard this stated in lots of places but I have yet to find a credible citation of where that comes from. Yes, it's higher than the maximum pressure but twice as much? I find that very hard to believe for all tires in all widths and all possible rim widths. I've had tires that blew off at the nominal highest pressure setting.

Going over to the Silca calculator, I have definite problems with their pressure. While some of the pressures would be valid for racing conditions, they become invalid for people who have to actually pay for their parts rather than have a sponsor. The suggested pressure for a mountain bike for me is 23 and 24psi front/rear, respectively. If I add a bikepacking load of 40 lbs, the pressure suggestion is 26 and 28 psi, front/rear. First, I've ridden tires that have that little pressure in them for off-road without a load and it wallows. It doesn't turn well. But, more importantly, that kind of pressure is going to result in the tires bottoming out on hard edges which risks not just a pinch flat but a damaged rim. I've seen some very nice rims ruined because someone was using the "optimum" ride pressure and completely ignoring the other function that tires serve...to protect the rims.
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Old 01-08-20, 01:04 PM
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Thanks! Definitely worth the time to listen.

Some take-aways:

- Smooth is fast
- The higher the tension in the casing, the easier it is to cut.
- If you can hear it, it's slowing you down.
- Tire pressures on sidewalls have next to nothing to do with anything.
- For optimum, do laps, decreasing pressure a few psi at a time (road). When you feel squishy cornering or tire bob while pedaling, go back up a few psi.
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Old 01-08-20, 11:38 PM
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Same info we've heard for the past few years, but perhaps a bit more authoritative considering the source and his experience.

Pretty much matches my own experience. Particularly tailoring tire pressure to suit the road conditions. I'll often stop to adjust pressure slightly for a Strava segment to optimize for that particular type of road. Asphalt, striated concrete, chipseal (and there are many variations in chipseal) and just plain busted up pavement all have different feels. One particular loop segment I ride often has chipseal on one side, striated concrete at the turnaround point, on a new bridge, and hitting that sets up a weird chattering vibration that feels like it slows the bike to a crawl. If I get the tire pressure just right it floats over that stuff, and doesn't feel mushy on the new smooth asphalt on the other side of the loop.

But each new type of tire needs its own tweaking. And I usually avoid going as low as I could for my weight. I'd rather put up with a slightly harsher ride than risk pinch flatting. That's happened to me only once, but it was pretty extreme -- a couple of construction trucks bottlenecked me and I had to run over some busted up bricks or cinder blocks to avoid the trucks. Just a slow leak from a classic snakebite, didn't actually go flat for another couple of miles. It's likely if I'd been running maximum pressure I'd have lost control running across the bricks, instead of just a somewhat hard bounce.
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Old 01-09-20, 05:24 AM
  #8  
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I don' t think the video ever said "setting tire pressure to 1/2 the blowout value is standard" He said the standard for those ranges printed on the sidewalls is that the upper value represents 1/2 the blowout value. The sidewall values have always ranges with a max and by setting the max at half the blowout pressure, the tire manufacturers are looking to avoid liability (and avoid the inaccuracies of the ways people measure their tire pressure) if an over-inflated tire blows out and there is an accident.

The cool takeaway for me in that video is that humans don't really have good internal mechanisms for judging how fast they are moving! We've had bicycles for about 200 years, cars for about 100 years - but for all of mankind's time before that, humans really couldn't go much faster than running speed or I guess trotting speed on a horse... We didn't really need much speed sense.

So we tend to use noise as a proxy for speed and it was only recently that science was added to the mix to show how width/pressure on real world surfaces really influences speed vs. people feeling faster with certain combinations.
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Old 01-09-20, 06:01 AM
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I watch Josh on YouTube, and along with all these others who say lower my tire pressure, it seems like a bunch of bunk. Having tried around where people recommend I ride, it is to low, and I swear I feel it dragging. Currently riding my 700x28 at no less than 95 psi for a 80kg rider, and loving it (except that bit of a belly I need to lose).
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Old 01-09-20, 06:18 AM
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It's interesting how ideas about tire width, and pressure have changed over the years, Fifteen or twenty years ago the idea that wider tires were better for the average rider was ridiculed. People like Grant Petersen , and later, Jan Heine were outliers. The big manufacturers were equipping bikes with extremely narrow tires. I found a bike that had 700x20mm tires.; it must have been uncomfortable
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Old 01-09-20, 06:25 AM
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Originally Posted by base2 View Post
Yeah, the Youtube kept shoving this in my face. Eventually, I watched it. It doesn't have anything new.

He does gloss over the nuance of larger tires have lower rolling resistance for a given pressure. (The reason is the more circular contact patch) But his point of the extra volume & lower pressure making for a smoother roll over rougher terrain does often end up being faster than the same wide tire at high pressure tire deflecting the forward inertia to the vertical plane as is the case with not smooth surfaces is not missed.

Yeah, we're balancing competing nominals. Smooth vs rough. Fat vs skinny. High pressure vs low. Light vs heavy. Supple vs not. That's 20 possible choice vectors to approximate a nominal pressure.

Our infrastructure is terrible. Wider tires at lower pressures is the response...Nothing new there.

FWIW: At 200 pounds, I run 23's at 110 rear, 100 front. I'd run 25's a bit less. By way of experiment: My 41's are noticably faster at 45-50psi than they are at 80psi. So he is right.

I never knew I had a single "choice vector" let alone 20. As soon as I figure out if a "choice vector" is good or bad I'll come back either really happy or seriously pissed.
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Old 01-09-20, 08:47 AM
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edit: my comments below really are about where I thought this was going, but hasn't.
To the tire pressure ratings on the sidewalls of your tires, they should only be relevant to a new rider or a low pressure tire like on a vintage hookless rim, or maybe on a kid's bike. Once you ride the bike/tires you should know where your preferred inflation range sits. If you ride at MAX pressure, you have probably done this before. There is no single ideal pressure, given all the variables.
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I find all this talk about wide tires at low pressure giving the lowest rolling resistance a bunch of confusion.
The rolling resistance of a tire can only be measured on pavement or very hardpacked surfaces. On gravel, the rider who picks the best 'line' will have the most efficient forward motion.

Originally, the tests of real world rolling resistance proved that going from a 23mm to a 25mm to a 28mm at equivalent pressure, reduced the road surface contact patch, thereby lowering rolling resistance. Then the conversation went toward comfort and by lowering the tire pressure in a 25 or 28 so that the contact patch equaled a 23mm tire, the rolling resistance was the same but slightly more compliant and perceived as more comfortable. From that point the conversation devolved. To the ridiculous statements like - "on my 50mm tires at 25psi, I'm faster than on 35s at 60psi, and the rolling resistance must be better because I feel less beat up at the end of my ride." Jan & Grant espouse a particular type of cycling and both make their livelihood from the products they sell into that cycling niche. Fat tires and low PSI have a place in cycling; they are great for off-road, loaded touring, cargo bikes, mountain biking, etc. Tires with lighter sidewalls make a smoother softer ride. But the hyperbole about fat&soft having lower rolling resistance than narrower and harder needs to be taken as 'alternative facts', otherwise known as bullpucky.
:

Last edited by Wildwood; 01-09-20 at 03:26 PM.
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Old 01-09-20, 08:51 AM
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Originally Posted by jpescatore View Post
I don' t think the video ever said "setting tire pressure to 1/2 the blowout value is standard" He said the standard for those ranges printed on the sidewalls is that the upper value represents 1/2 the blowout value. The sidewall values have always ranges with a max and by setting the max at half the blowout pressure, the tire manufacturers are looking to avoid liability (and avoid the inaccuracies of the ways people measure their tire pressure) if an over-inflated tire blows out and there is an accident.
That's exactly what was said and you just repeated it. I don't happen to believe it and I don't think anyone is likely to admit it if it were true. My point is that the maximum pressure is set based on a blow off pressure but I really doubt that it is set to half of the blow off pressure. There's a bit of wiggle room but not twice as much. A tire that can be pumped to 130 psi can't really be pumped up to 260 psi. At that pressure, the rim itself becomes a problem because aluminum is makes for a really lousy pressure vessel.

And, as I said above, some tires...supposedly good quality tires...can't even take the nominal high pressure.
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Old 01-09-20, 11:14 AM
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Originally Posted by Wildwood View Post
edit: my comments below really are about where I thought this was going, but hasn't.
To the tire pressure ratings on the sidewalls of your tires, they should only be relevant to a new rider or a low pressure tire like on a vintage hookless rim, or maybe on a kid's bike. Once you ride the bike/tires you should know where your preferred inflation range sits. If you ride at MAX pressure, you have probably done this before. There is no single ideal pressure, given all the variables.
______________________ I find all this talk about wide tires at low pressure giving the lowest rolling resistance a bunch of confusion.
The rolling resistance of a tire can only be measured on pavement or very hardpacked surfaces. On gravel, the rider who picks the best 'line' will have the most efficient forward motion.

Originally, the tests of real world rolling resistance proved that going from a 23mm to a 25mm to a 28mm at equivalent pressure, reduced the road surface contact patch, thereby lowering rolling resistance. Then the conversation went toward comfort and by lowering the tire pressure in a 25 or 28 so that the contact patch equaled a 23mm tire, the rolling resistance was the same but slightly more compliant and perceived as more comfortable. From that point the conversation devolved. To the ridiculous statements like - "on my 50mm tires at 25psi, I'm faster than on 35s at 60psi, and the rolling resistance must be better because I feel less beat up at the end of my ride." Jan & Grant espouse a particular type of cycling and both make their livelihood from the products they sell into that cycling niche. Fat tires and low PSI have a place in cycling; they are great for off-road, loaded touring, cargo bikes, mountain biking, etc. Tires with lighter sidewalls make a smoother softer ride. But the hyperbole about fat&soft having lower rolling resistance than narrower and harder needs to be taken as 'alternative facts', otherwise known as bullpucky.
:


He is talking about what is faster. You're the one talking about rolling resistance, and so confusion. Spending much time thinking about contact patches and steel

drum tests seems to lead to that IMO.
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Old 01-09-20, 11:26 AM
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I'm almost to the point of not correcting misinformation about low pressure and big tires being faster simply for the fact I'll be up 20w over the schmuck who shows up to the race.

Oh, cute, 28's at 40psi. Have fun!

Now, comfort, do whatever floats your boat. Sure.

Oh, and the evils of latex. Tell me how evil it is. Or how it isn't any faster. Right.
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Old 01-09-20, 11:51 AM
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Originally Posted by Paul Barnard View Post
I never knew I had a single "choice vector" let alone 20. As soon as I figure out if a "choice vector" is good or bad I'll come back either really happy or seriously pissed.
Yeah, I'm super dum-dum.

Vector: The line on the graph that goes through the point you want from where you are.
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Old 01-09-20, 11:53 AM
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Originally Posted by base2 View Post
He does gloss over the nuance of larger tires have lower rolling resistance for a given pressure. (The reason is the more circular contact patch)

To really cover that would have been a sidebar as long as the video. But they did mention the greater casing tension at the same pressure in a larger tire, which is the root cause of the rolling resistance you mention. Just don't forget that you'll get a worse ride quality from the wider tire at the same given pressure for the same reason. There is still no free lunch.
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Old 01-09-20, 12:00 PM
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People have been misinterpreting "max pressure" to mean "recommended pressure" forever. See it all the time now, particularly newer/inexperienced amateurs. They see a number on the sidewall and think that is the pressure to ride at.

In response...some bike tire brands have started printing "recommended pressure" on the sidewalls...which are just about as bad a guess. Specialized recommends their 700Cx42mm Sawtooth tire be run 50-80PSI...which even the low-end rating is lauguably high even for a loaded-touring load.
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Old 01-09-20, 12:12 PM
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Originally Posted by Caliper View Post
To really cover that would have been a sidebar as long as the video. But they did mention the greater casing tension at the same pressure in a larger tire, which is the root cause of the rolling resistance you mention. Just don't forget that you'll get a worse ride quality from the wider tire at the same given pressure for the same reason. There is still no free lunch.
True & +1 The tire casing/mop threads bit is a genius bit of explaining.
That the forward direction of travel is being constantly directed some direction other than forward by every inconsistancy in the pavement is energy lost.

Energy we care about in this case being forward inertia.

The highest pressure that preserves the most forward inertia in a given set of variables (Terrain, width, load, etc...) is the "nominal pressure."
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Old 01-09-20, 03:40 PM
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Originally Posted by woodcraft View Post
He is talking about what is faster. You're the one talking about rolling resistance, and so confusion. Spending much time thinking about contact patches and steel

drum tests seems to lead to that IMO.
My earlier reply didn't post. So this time with brevity.

Then - 'He is talking about what is faster' - means someone is dreaming about going faster over XX miles on a rough road or off-road or gravel road (how about CX, too? - that's a natural follow). And I would offer that there are so many variables to FASTER as to be a subjective and highly variable answer, requiring far more analysis than tires.



edit: As an example of fastest tire for an off-road situation - and we are not talking mtb situations - take an off road race like is run every year in Italy, 'The White Roads' race Strada Bianca (or something). What's the typical tire width of the top10 riders over the past few years? 28mm@80psi? 30mm@75psi? 32@70psi 35@65? 40@50psi? 48mm@35psi?

I don't know the answer but it's likely much closer to 28/30mm firm than 40/48mm soft.

Last edited by Wildwood; 01-09-20 at 03:57 PM.
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Old 01-09-20, 05:07 PM
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I tried their tire pressure calculator, and it recommended a psi that I had tried in a race last summer -- and my rear tire burped out most of it's air and left me standing on the side of the trail while all of the other riders blew past me.
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Old 01-09-20, 05:20 PM
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Originally Posted by 79pmooney View Post
I'm a dinosaur, It would never occur to me to look at the sidewall (or anywhere else) to see what pressure I should ride. I choose pressure based on tire size, road ridden, tire grip, the bike it's on and how it's loaded. Jump on the bike and ride. Fine tune from there. Thumb squeeze when I like it for duplication, I usually never do know the actual pressure until I duplicate the thumb squeeze with my floor pump with gauge. (I rode sewups 20 years with nothing but frame pumps on all my bikes, no gauge at all. Sometimes I got to use a floor pump at races and actually knew the pressures but usually no.)

One thing I take real issue with is anyone telling me I should be using less than 5 psi lower in front than in back. That was what we all were taught back when we came from eggs (remember the dinosaurs). But there is a really good reason for so much pressure up front. There might just come a day when you have to slam on the brakes and do maneuvers to save your butt. At that moment, all your weight is on that front tire. Not 40-45% of your total. Not the 55-60% that the rear tire sees. No, that front tire is looking at 67% more weight than either tire sees normally. And at that moment, control is paramount. A blowout or rim failure disaster.

Ben
YES. Quoting you in hope of getting more eyes on something important.

Actually it is worse than all weight on front tire. Bicycle brakes create g loads. Any bike at all that is functioning properly will pull 0.5g in a hard stop. Which means that 1.5 times the static weight of entire loaded bike is all on front tire. That's before you hit the bump. We all use more pressure than is really necessary for just riding along. And a good thing we do because JRA can become quite exciting at any moment.
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Old 01-10-20, 10:24 AM
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Originally Posted by 79pmooney View Post

One thing I take real issue with is anyone telling me I should be using less than 5 psi lower in front than in back. That was what we all were taught back when we came from eggs (remember the dinosaurs). But there is a really good reason for so much pressure up front. There might just come a day when you have to slam on the brakes and do maneuvers to save your butt. At that moment, all your weight is on that front tire. Not 40-45% of your total. Not the 55-60% that the rear tire sees. No, that front tire is looking at 67% more weight than either tire sees normally. And at that moment, control is paramount. A blowout or rim failure disaster.

Ben
I run my 25mm front tire around 45psi. Rear around 55-60psi. I weigh about 140 in race shape.

I have had to endo my bike at least twice while braking (Iíve probably done it more often but didnít notice as much). Once while sprinting out of a corner in a 100-field crit, and once when an SUV from the oncoming tried turning left across me and nearly hit me head on.

My tire didnít explode.

Iíve run my front tire at 35 psi in a wet, ultra technical crit where 9/25 people crashed. Horrible course, but even though two people crashed directly in front of me as I was entering a corner, I was able to slow down and tighten my exit. So, the lowest pressure you can safely run is 100% dictated by how willing you are to pinch flat, imo. If youíre running tubeless and you donít care about cracking rims, you can go very low and be fine. Though I was running tubes in all these scenarios.

EDIT: Though I could see burping being an issue with lower pressures. I havenít tried TL yet because it really feels like the bike industry has yet to figure that out.

Last edited by smashndash; 01-10-20 at 10:27 AM.
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Old 01-10-20, 11:54 AM
  #24  
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Originally Posted by Koyote View Post
I tried their tire pressure calculator, and it recommended a psi that I had tried in a race last summer -- and my rear tire burped out most of it's air and left me standing on the side of the trail while all of the other riders blew past me.
That really has an impact on your overall speed, doesn't it? The number of bent rims I see at my co-op because people are running low pressure on tubleless is staggering. This is one of the more egregious examples I've run across

Untitled by Stuart Black, on Flickr
Untitled by Stuart Black, on Flickr

That's a Hadley thru-axle hub on a downhill wheel. It's a $400 to $500 wheel that is just beat up. We got it because the idiot who did the beating didn't realize the value. Unfortunately, it's mostly scrap. I salvaged the hub but it's not something our clientele really ever uses.

If the idiot had run the tires at 35 to 45 psi, he'd still have the wheel.
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Old 01-10-20, 12:49 PM
  #25  
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me and my tires do not talk to each other
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