Scary moment on Compass Tires

[Clancy]

I just finished building up a Norco Slate XR steel frameset and have taken it out on a few shake down rides. I mounted a set of Compass Barrow Pass Tires, 38c, on a set of Reynolds ATR wheels and was out on the second ride with these tires. Prior I had a set of Panaracer GravelKings SK’s in 35c. These are on a 700c set-up, not 650B. The Compass tires were dramatically faster and noticeably more compliant.

All was good until I descended a steep hill hitting maybe 25 mph. This is on my regular loop that I’ve ridden countless times on different bikes. The descent has a very slight S curve, very slight. I gradually leaned into the curve when very suddenly the front tire tried to wash out from under me. I managed to somehow instinctively straighten up and avoid the fall but doing so had me cross over into the other lane. Fortunately there were no cars, I was able to continue on.

From there I paid more attention to specifically how the front end felt and how weighted it was. If I leaned into a a turn just a touch too much the front would want to collapse on me or fall in.

I had the tires inflated to 42 psi. After the scare I adjusted my saddle a touch more foreward to load the front tire more and dropped the pressure a bit but it seemed to make no difference.

I’ve built up dozens of bikes and know my measurements by heart. The only difference is the Compass tires. I have had two other sets of Compass tires In smaller sizes, 28c and 32c. The 38’s measure 39.5mm and are almost perfectly round in shape. There is such a tendency for the tire to roll over and fall in on itself when turning that I do not feel comfortable on the bike.

Thoughts???

[fbhidy]

What are the internal and external widths of the Reynolds ? Are they set up tubeless? How much do you and the outfitted bike weigh together?


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[charliedid]

Try more air

[joosttx]

Try more air

+1 exactly what I thought.

[R3awak3n]

Was this on gravel or pavement?

[David Kirk]

What are the internal and external widths of the Reynolds ? Are they set up tubeless? How much do you and the outfitted bike weigh together?


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I echo this question - how wide is the rim?

A wide supple and soft tire on a narrow rim is not a good combo.

dave

[charliedid]

I echo this question - how wide is the rim?

A wide supple and soft tire on a narrow rim is not a good combo.

dave

ATR's are 23 internal and 32 external. "The 38’s measure 39.5mm and are almost perfectly round in shape"

It's a great question but I'm guessing it's not the issue?

[R3awak3n]

was about to say, non issue on that rim, its pretty damn wide, I have one and its the widest rim I have.

If I remember this is not the first time I have heard of this happen, but usually on gravel

[thwart]

Looks like the ATR's have a 21 mm internal width according to Reynolds... at any rate narrow rim is not the cause here.

More air seems the culprit, or much less likely a defective tire.

I have the same tires, and they've been great, including fast descents with turns. I weigh 155 and run around 40/45 F/R.

[David Kirk]

I agree - that rim should be more than wide enough.

dave

[JAGI410]

Tires new or used?

I love my Barlow Pass tires. I run them on H Plus Son TB14s. Never below 50psi though, usually 52f/55r

[Clancy]

Tires new or used?

I love my Barlow Pass tires. I run them on H Plus Son TB14s. Never below 50psi though, usually 52f/55r

here's answers to questions posted.

As mentioned, inner width 23mm
Tires new, second ride
Set-up tubeless
42 PSI front and rear
On pavement, semi-smooth chip seal
I weigh 155 kitted up.

And as mentioned, the front end feels as though it wants to flop over or fall in when I lean, even slightly.

I adjusted my saddle forward maybe 6-9 mm to put more weight on the front and lowered the pressure to mid 30's. No difference in the tendency for the front end wanting to wash out from under.

All of this was on pavement. I can't even imagine riding this sset-up on gravel.

[Pinned]

I've never understood all the praise for the Compass tires. The 28s I spent over $150 on for the set were one of my more regrettable bicycle related purchases - muted feel, slow, poor traction/grip, squared off seemingly instantly. I took them off the bike after a pretty squirrelly incident where it felt like I couldn't stop in traffic - a sensation I never experienced on any tire before or after.

Surely someone will disagree with me, as by all accounts online these are simply the best tires available. I just didn't have that experience at all.

[Hilltopperny]

I've felt this before on my ATR wheels in both 700 and 650b. I found that I had too little air in the tires. I am also 50lbs heavier, but I run them tubeless with no less than 50lbs of pressure.

Scary feeling when you drift into the opposite lane to avoid an impending crash. Glad you weren't hurt and didn't trash your bike!

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[Tall]

Did your sidewalls maybe collapse? This blog post (and the comments) has some info on the topic: https://janheine.wordpress.com/2018/06/11/myth-11-rear-tires-should-run-at-higher-pressure/

During hard braking, the entire weight of the rider is on the front wheel. Now the front tire carries three times as much weight as it usually does. If this isn’t factored into the tire pressure, then the tire can collapse during hard braking: Suddenly, the sidewall no longer holds up the tire. The tire loses the ability to transmit the forces from the road to the bike – braking and steering are seriously impaired. It’s like riding with a flat tire.

In recent years, the potential for trouble has increased as we now run supple tires at lower pressures. Back in 2010, when we published the chart, we inflated our 23 mm tires to 120 psi. Running the fronts at 80 psi was fine, since the stiff sidewalls of the tires most of us rode helped with holding up the bike.

These days, many of us are on supple 42 mm tires inflated to 35 psi. Dropping the front to 23 psi is fine when rolling along, but during hard braking, the sidewall will collapse.

[hokoman]

And as mentioned, the front end feels as though it wants to flop over or fall in when I lean, even slightly.

I adjusted my saddle forward maybe 6-9 mm to put more weight on the front and lowered the pressure to mid 30's. No difference in the tendency for the front end wanting to wash out from under.


Wouldn’t this make it worse? I have had my rear tire fold over on my cargo bike coming up a cambered curb - both kids on the back. I check my tires more regularly and regularly air up, and never had that issue again. Kids are now 20 lbs more too. Raise your tire pressure. I have compass tires on 3 bikes and they have been great.

[sg8357]

I have the Compass 38s standard casing and the Gravel King 38s,
5psi makes a whopping difference when you are riding 40psi.
I started at 70psi and worked my way down to squirrel city at 40,
I'm 200lbs. now 62ish rear and 55 front. Too little psi in front led to
the front tire compressing during braking, not a good feeling.

Leaned over in fast down hill corners they feel great at the right pressure.

[Hindmost]

Did your sidewalls maybe collapse? This blog post...

Interesting Jan quotes. I've run some 26 inch 2.3 Compass tires on narrow mountain bike rims; on pavement at speed with low pressure the front was in a word: squirrely.

[weisan]

Clancy pal, glad you are ok.

Before Compass get thrown under the bus, I would say what you experienced is not normal and contrary to my experience with Compass tires, there might be other factors to consider, this needs further investigation.

[ergott]

My guess is 45-50psi would solve. Not positive though.

[Clancy]

My guess is 45-50psi would solve. Not positive though.

Consensus seems that I was running too low of pressure. For the GravelKings I run 24 psi when riding gravel, 40 psi when riding pavement. I started with 42 psi with the Compass tires mainly based on what I was doing with the GK's.

I'll try again with higher, not lower, pressure and report back.

[fogrider]

In recent years, it seems like everyone is saying how good lower psi is. I'm not a fan of lower is better. I'm also not a fan of higher is faster...every tire should have a correct pressure for a rider's weight for the road conditions. On my gravel bike, I run 38mm tires and run just under 50 psi on the road but drop it about 10 psi for the gravel.

[rwsaunders]

Serious question...don’t your tires feel sluggish on the road at those low pressures? I’m running 27mm tubulars at 80psi, weigh in at 175# and all feels good to me. I can’t imagine barreling down a hill and making a turn at 40mph at that low of a pressure, especially on clinchers.

[Hilltopperny]

Serious question...don’t your tires feel sluggish on the road at those low pressures? I’m running 27mm tubulars at 80psi, weigh in at 175# and all feels good to me. I can’t imagine barreling down a hill and making a turn at 40mph at that low of a pressure, especially on clinchers.The larger volume tires are rated for lower pressure. Somewhere between road and mountain.
I find even my 48mm 650b gravel slicks feel fine when running 55psi on the road.
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[Marc40a]

You were running the pressure too low.

Those are pretty supple tires.

I keep notes on my tires pressures. I started off running the 38mm Barlow Pass at 50 front/55 back and ended up, many months later at 63 front/70 back. I’m about 180 lbs.

[NHAero]

Clancy, you and I are similar weights. I run Vittoria Voyager Hyper tires on my Anderson, 37-622, at 55 fr / 60 rr. I've fooled around with lower pressures but the front definitely wasn't confidence inspiring below 50 psi, definitely squirrelly.
BTW, these tires get no love here, but the bicyclerollingresistance web site tested them to require fewer watts than the Compass Bon Jon Pass. I see that Vittoria has discontinued them so time for me to stock up!

[Jan Heine]

Sorry to hear about your incident. I am glad you are fine. As many others have suggested, it appears that your sidewall collapsed – that's not a good feeling at all. If you run your Gravel Kings at 40 psi, you'll probably want to go at least 15-20% higher with more supple tires. The same applies to high-end tubulars - when I raced 'cross on FMBs, I had to run higher pressures than my friends on less-supple clinchers. I can run Schwalbes in the teens (psi), but even my Rat Trap Pass 54 mm tires need at least 30 psi on pavement...

Perhaps this is something we should mention in our instructions – in the past, most riders were running our tires at too high pressures, but not any longer. We'll work on that, because we don't want others to have that kind of 'moment'!

There is nothing inherently squirrely in supple tires – pro racers have used them for decades not just on mountain descents, but also in crits where the cornering loads are even higher.

Keep the rubber side down!

Jan Heine
Founder
Compass Cycles
www.compasscycle.com (https://www.compasscycle.com)

[m4rk540]

Jan, thanks for your response.

[eddief]

less squish?

[benb]

What's the point of the super supple tires if you have to put more air in them which counteracts their benefits? The whole point of these tires seems to be more comfort at the expense of tire life right? If you have to increase the pressure it seems to remove some of the benefits.

I ran thousands and thousands of miles on Challenge Gravel Grinders @ 38c and I was using 35psi in the front tire and 45psi in the rear and never once had a sidewall collapse or other issue. No flats either.

I'm running 45psi in the front and 65psi the rear on Panaracer Gravel Kings in 32c at the moment and have never had any problems. Heck the front is feeling pretty harsh if you sneak it up to 50psi!

I'm 175lbs and my bike is porky, about 25lbs, so 200lb total load, sounds like a good bit more than the OP.

Both sets of tires are/were a super cushy ride at these pressures for me.

Haven't tried the Compass tires (yet?) but I'm usually pretty good at being able to "feel" a tire so if I needed to put more air in it I'd hope I'd be able to tell.. I never go out and dive into the first corner under hard trail braking or anything on the first ride on a new set of tires anyway.

[shoota]

I adjusted my saddle a touch more foreward to load the front tire more


ohhh hell noooo. One wash out like that would have me selling them and putting the proper tires for the conditions back on.

[rain dogs]

This is the result of all the bollocks about "lower, even super low, tire pressure is always faster"

It's appropriate tire pressure is faster. Not so insanely high it's senseless, not so low that's it's sluggish or worse, and in this case, dangerous.

But you know what they say .... say it enough times and it becomes true.

[p nut]

I weigh the same, and have run 700x35mm Compass and 650x42 Grand Bois's at the same PSI as Clancy. Lots of steep descents and sharp turns and have never experienced what he is describing. 35's were tubeless on Crest rims and 42's were tubed on Pacenti. I know on my mountain bikes (tubeless), burping caused similar effects going around a turn.

[benb]

These are premium price tires and IIRC Jan has said they are made by Panaracer for Compass.

I would give a good look at the Panaracer tires as they are generally 50% of the price of the Compass... maybe the Compass version has had the sidewall softened too much.

You *can* run the Panaracer versions at the pressures the OP mentions without sidewall collapse at a rider weight of 155 no problem.

Would love to see an article from Jan about what the amount of increased required pressure is with his sidewall design and why that design is still faster/more compliant/whatever. Along with things like how much softer/less stiff the Compass sidewall in the same tire size is compared to the Panaracer tire it's built off of.

[Jan Heine]

What's the point of the super supple tires if you have to put more air in them which counteracts their benefits? The whole point of these tires seems to be more comfort at the expense of tire life right? If you have to increase the pressure it seems to remove some of the benefits.

Something needs to hold up your weight. The extreme are airless tires like you find on the dock-less bikeshares, which use just rubber and foam to support the bike and rider. They are slow and squirm like crazy in corners. The opposite are supple tires, which rely mostly on air to hold you up. They are comfortable and fast.

The speed is easy to explain: Air has almost no hysteresis, so it forms a very efficient spring that consumes almost no energy. Rubber has a lot of hysteresis – flexing it takes a lot of energy. The less rubber you flex, the faster your tire.

As far as comfort is concerned, the tire consists of two springs, one a rubber spring and the other an air spring. Each have their own spring rate. Combined, they determine the tire's comfort. If you reduce the rate of one spring, you need to increase the rate of the other.

The problem with the rubber spring is that, as the tire flexes, it bulges. The sidewalls become less vertical, the spring rate is reduced – but a decreasing spring rate under load is the opposite of what you want. The more you flex the tire, the weaker it becomes. So you need to run a higher total spring rate on a stiff tire.

The air in a supple tire has a rising spring rate – it becomes harder the more you compress it. This means you can run a lower total spring rate, hence more comfort.

From the perspective of tire drop (how much the tire deforms under the rider's weight), a supple tire has more tire drop, even though it runs at higher pressure, than a stiff tire. Hence it's more comfortable.

Jan Heine
Founder
Compass Cycles
www.compasscycle.com (https://www.compasscycle.com)

[chrisroph]

Your pressure was too low for the amount of force you put into the tire. How accurate is your pressure gauge?

How fast were you cornering? I don't corner as hard on pavement when riding my gravel bike with 40 lbs in 40mm schwalbes as I do a road bike with 27mm tires at 90lbs.

[bikinchris]

This is the result of all the bollocks about "lower, even super low, tire pressure is always faster"

It's appropriate tire pressure is faster. Not so insanely high it's senseless, not so low that's it's sluggish or worse, and in this case, dangerous.

But you know what they say .... say it enough times and it becomes true.

^^This ^^

[romalor]

Scary post on instagram about someone training for Transcontinental Race and crashing on brand new tires . Now I know you need to be careful the first 50 100 km when the tires are still a bit greasy from factory etc but it seems it had the same issue as you

Resulting in a life changing crash 2,5cm shorter leg now

Don't know if it's really related to the tires
He seems to think so
https://www.instagram.com/p/Bp9RpJyhU2_/

[Jan Heine]

This is the result of all the bollocks about "lower, even super low, tire pressure is always faster"

It's appropriate tire pressure is faster. Not so insanely high it's senseless, not so low that's it's sluggish or worse, and in this case, dangerous.


In the days when people ran their 28 mm tires at 120 psi, the advice was "When in doubt, let out some air." Fortunately, those days are over, and the advice needs to be modified. As you say, the appropriate tire pressure is best.

By the way, the speed of supple tires doesn't change with pressure, at least on smooth roads, so you don't gain anything, nor give up anything, by going a little higher.

Jan Heine
Founder
Compass Cycles
www.compasscycle.com (https://www.compasscycle.com)

[Clancy]

I really appreciate Jan jumping in with his advice. Based on what he said as well as others I am more than certain that I was running much too low of air pressure. Unfortunately the riding conditions in the Texas Hill Country aren’t the best for trying out the theory. A nice cold front is blasting through with winds/rains. But I will hopefully have a chance tomorrow and will report back.

But to clarify and answer the questions that popped up, the best way is to describe the sensation. I’ve ridden motorcycles for years and one quickly learns when the front tire is worn when leaning the bikes wants to dive in or fall in on its’ Self. When the tire (front and rear) become worn and flat, turning becomes very unpredictable and scary because of the squared off corners. That’s what the front Compass tire felt like which a collapsing sidewall would mimic.

Curious Jan, does the advice on adding 15-20% more pressure over what I ran hold true for both pavement and gravel?

Eg, I ran 42 on pavement and 24-26 on gravel. Is the general idea to start at 15-20% higher on both?

[Jan Heine]

Curious Jan, does the advice on adding 15-20% more pressure over what I ran hold true for both pavement and gravel?

Eg, I ran 42 on pavement and 24-26 on gravel. Is the general idea to start at 15-20% higher on both?

I'd say yes, that is a good starting point even on gravel. You don't want to bottom out excessively –*even when running tubeless. From there, it takes experimenting. On gravel with evenly-sized aggregate, you can go quite low to gain speed and comfort, but on really rough stuff, you need a bit more pressure to prevent bottoming out.

Jan Heine
Founder
Compass Cycles
www.compasscycle.com (https://www.compasscycle.com)

[Ken Robb]

I really appreciate Jan jumping in with his advice. Based on what he said as well as others I am more than certain that I was running much too low of air pressure. Unfortunately the riding conditions in the Texas Hill Country aren’t the best for trying out the theory. A nice cold front is blasting through with winds/rains. But I will hopefully have a chance tomorrow and will report back.

But to clarify and answer the questions that popped up, the best way is to describe the sensation. I’ve ridden motorcycles for years and one quickly learns when the front tire is worn when leaning the bikes wants to dive in or fall in on its’ Self. When the tire (front and rear) become worn and flat, turning becomes very unpredictable and scary because of the squared off corners. That’s what the front Compass tire felt like which a collapsing sidewall would mimic.

Curious Jan, does the advice on adding 15-20% more pressure over what I ran hold true for both pavement and gravel?


Eg, I ran 42 on pavement and 24-26 on gravel. Is the general idea to start at 15-20% higher on both?

It seems to me that you could run lower pressure on gravel because you wouldn't have enough traction there to experience the same tire collapse you did when cornering on pavement.

[unterhausen]

I like to run low pressures in my 38mm gravel king sk's. I have let them get down to about 20psi. The only problem with that is during cornering on pavement. I have had some scary moments. Once I get to gravel it seems okay. But I also have a ding in the rear rim due to that, so now I usually pump them up more. I'm a little surprised that 40 isn't enough.

[Waldo]

I've had multiple vague-wandering experiences with large volume Compass and Grand Bois tires while riding them at low pressures. Higher pressures cured the problem. Not sure if it's the tires or us being unaccustomed to riding high volume tires at low pressures.

[benb]

Something needs to hold up your weight. The extreme are airless tires like you find on the dock-less bikeshares, which use just rubber and foam to support the bike and rider. They are slow and squirm like crazy in corners. The opposite are supple tires, which rely mostly on air to hold you up. They are comfortable and fast.

OK, that's super interesting, I have yet to try a bikeshare bike, we have them outside my office and all over my hometown at this point, but 99% of the time where they are present it's easier for me to walk. For some reason I thought they usually had normal tires as I've seen maintenance guys checking them outside our office. (Docked bikes). I haven't seen what's up with the Dockless ones in town.

But in any case you're comparing extremes.

How about a comparison between another highly regarded performance tire in the same category as the Compass tires?

For example.. the Panaracer comparison since they manufacture the Compass tires (right?). They have a lot of tires that are very similar. If your tire has a softer sidewall and requires increased pressure, does it actually flex appreciably differently than a tire with a slightly stiffer sidewall that can handle a lower pressure? What is the effective difference?

I like everything in your explanation I'm just curious what the % difference is between a Compass tire in terms of spring rate of the sidewall vs air spring when you compare it to a Panaracer, or a Specialized, or a Challenge, or whatever.. let's talk tires in the same category, not comparing a gravel racing tire with a bike share tire that isn't even pneumatic.

The bike with the more sidewall material would sure seem to have some other advantages like puncture resistance.

[MikeD]

There's something else going on here because 42 psi is more than enough pressure at the 38 mm width of the tires and the OP's weight of 155 pounds + ~ 25 pounds for the bike. Plugged it into a spreadsheet based on Frank Berto's 15% tire drop and got 34/41 psi F/R recommended pressure.

[Jan Heine]

There's something else going on here because 42 psi is more than enough pressure at the 38 mm width of the tires and the OP's weight of 155 pounds + ~ 25 pounds for the bike. Plugged it into a spreadsheet based on Frank Berto's 15% tire drop and got 34/41 psi F/R recommended pressure.

Tire gauges can vary quite a bit. I have two floor pumps. For the same tire, my Blackburn shows 25 psi, but my old Silca shows 38 psi. That seems to be extreme, but it gives you an idea.

This is one reason why we say that Berto's Bicycle Quarterly chart is a starting point for experimenting. Once you've found the right pressure, you can easily replicate it with your floor pump, no matter what the actual pressure is. Frank Berto actually bought a $ 250 precision gauge before he established those original measurements, but most of us don't have one (and don't need one).

Jan Heine
Founder
Compass Cycles
www.compasscycle.com (https://www.compasscycle.com)

[benb]

Tire gauges can vary quite a bit. I have two floor pumps. For the same tire, my Blackburn shows 25 psi, but my old Silca shows 38 psi. That seems to be extreme, but it gives you an idea.

This is one reason why we say that Berto's Bicycle Quarterly chart is a starting point for experimenting. Once you've found the right pressure, you can easily replicate it with your floor pump, no matter what the actual pressure is. Frank Berto actually bought a $ 250 precision gauge before he established those original measurements, but most of us don't have one (and don't need one).

Jan Heine
Founder
Compass Cycles
www.compasscycle.com (https://www.compasscycle.com)

Thanks for this, that is a great reminder.

I use the Berto/Bicycle Quarterly chart and it has worked really well for me, but the pressures are always ever so slightly off... must be my pump(s).

I am almost always amazed when I hear about people running the same pressure in the front and rear tires at this point.

[MikeD]

Tire gauges can vary quite a bit. I have two floor pumps. For the same tire, my Blackburn shows 25 psi, but my old Silca shows 38 psi. That seems to be extreme, but it gives you an idea.



This is one reason why we say that Berto's Bicycle Quarterly chart is a starting point for experimenting. Once you've found the right pressure, you can easily replicate it with your floor pump, no matter what the actual pressure is. Frank Berto actually bought a $ 250 precision gauge before he established those original measurements, but most of us don't have one (and don't need one).



Jan Heine

Founder

Compass Cycles

www.compasscycle.com (https://www.compasscycle.com)



Can you comment on the minimum tire pressure on the sidewalls of most tires? Vittoria said this to Velonews "According to ETRTO, bicycle tires are allowed to deflect 30% of its height at maximum load only. We respect the ETRTO, but we do not limit the body weight of our customers. 115PSI minimum air pressure is the consequence for our high-end 25mm tire with its very flexible casing; the minimum air pressure is related to the worst case: heavy load, rear wheel, aged tire.
We will address this matter more precisely on our MY16 new models.
— Christian Lademann – Product Manager
Vittoria S.p.A."

115 psi minimum pressure on a 25mm tire seems ridiculously high to me. That's even higher than the maximum pressure rating stamped on 23 mm tires I used to run.

[Lionel]

I have bombed down dirt and road descents with my Compass UL 38mm on Bora 17mm inside wheels (with tubes). Not sure what pressure I am putting in there as I mostly go by feel but I never had a scary moment, quite the opposite. These tires feels ultra safe to me. I probably run them higher han the OP I guess.

[R3awak3n]

I pretty much always feel super safe doing crazy corners on tires that measure 35mm up. Gives you so much confidence. I do run my tires, even supple ones, higher than most people (I am a big guy though)

[spoonrobot]

Berto's chart is an interesting artifact from another time and doesn't really apply to modern demi- and balloon tires. The difference in suppleness as well as tire height due to rim width over the range of modern tires/rims, has rendered most tire pressure charts pretty useless. For demi and balloon tires a difference of 2-3-4 psi can often represent 10-20% of the overall pressure change. This is significant and is a blind spot when riders change from higher pressure tires where small numerical pressure changes are much less significant.

Food for thought: Panaracer Gravel King slick 650bx48 require 18 psi to reach 15% sag under 210 pounds of rider+bike.

Compass Switchback Hill 650bx48 require 27 psi to reach 15% sag under 210 pounds of rider+bike.

Buy a good pressure gauge and measure everything yourself.

I am almost always amazed when I hear about people running the same pressure in the front and rear tires at this point.

It's possible to collapse the front under very steep or very hard cornering. Easier on singletrack or dirt but still possible on pavement. Front tire can often hit higher peak load than the rear under braking as well. Some courses I run my gravel bike or mountain bike f/r equal. On my road bike I have added air to the front before a very technical paved descent.

[MikeD]

Berto's chart is an interesting artifact from another time and doesn't really apply to modern demi- and balloon tires. The difference in suppleness as well as tire height due to rim width over the range of modern tires/rims, has rendered most tire pressure charts pretty useless. For demi and balloon tires a difference of 2-3-4 psi can often represent 10-20% of the overall pressure change. This is significant and is a blind spot when riders change from higher pressure tires where small numerical pressure changes are much less significant.



Food for thought: Panaracer Gravel King slick 650bx48 require 18 psi to reach 15% sag under 210 pounds of rider+bike.



Compass Switchback Hill 650bx48 require 27 psi to reach 15% sag under 210 pounds of rider+bike.



Buy a good pressure gauge and measure everything yourself.







It's possible to collapse the front under very steep or very hard cornering. Easier on singletrack or dirt but still possible on pavement. Front tire can often hit higher peak load than the rear under braking as well. Some courses I run my gravel bike or mountain bike f/r equal. On my road bike I have added air to the front before a very technical paved descent.


How are you measuring tire drop?

[spoonrobot]

I use a small scissor stand (looks like this (https://images-na.ssl-images-amazon.com/images/I/71zb0U8PpgL._SX425_.jpg) but smaller) with a level on the platform. Put platform to rim+sag height and lay level bridging platform and rim.

Sit on the bike fully loaded and have someone let air out of tire until the level is level. Measure pressure with Accu-Gage.

[MikeD]

I use a small scissor stand (looks like this (https://images-na.ssl-images-amazon.com/images/I/71zb0U8PpgL._SX425_.jpg) but smaller) with a level on the platform. Put platform to rim+sag height and lay level bridging platform and rim.

Sit on the bike fully loaded and have someone let air out of tire until the level is level. Measure pressure with Accu-Gage.


Nice!

[HTupolev]

Serious question...don’t your tires feel sluggish on the road at those low pressures?
A given tire pressure has very different implications for tires of different sizes. The contact patch size is similar, but the way the tire's overall structure deforms against the road is totally different. And in particular, given otherwise-similar construction, a narrower tire will deform more. A simple example of this is tire drop (i.e. how much % of the tire's height is lost when under the weight of the bike+rider, versus when the tire is totally unloaded).

If I pump my 25mm road tires to 50PSI, they feel flat and squirmy. If I pump my 53mm Rat Trap Pass ELs to 50PSI, it's like riding on a pair of basketballs.

I had the tires inflated to 42 psi. After the scare I adjusted my saddle a touch more foreward to load the front tire more and dropped the pressure a bit but it seemed to make no difference.
That sounds backwards. If the tire is squirming and folding over, you want to put less load on it, or inflate it stiffer.

Wide supple tires like those sold by Compass can be incredibly sensitive to tire pressure, since so much of the tire's stiffness is coming from the air spring and so little from the casing. It doesn't take that much underinflation for the handling to get weird. I generally pump my Rat Traps to a significantly higher pressure than most other tires of similar size.

There was one time that I had to pump my front Rat Trap on the road, after riding through some glass. Because there was a bike shop only ~5 miles away, I figured I'd only bother inflating to about 15PSI. Even though I was riding along a straight flat MUP at low speed, this turned out to be a scary experience: the bike was handling like a boat, and little bowl-shaped dips in the pavement were suddenly like ship-eating whirlpools of myth and legend.

[gpendergast]

My general rule of thumb for the first ride on a supple/tubeless tire is to inflate to the max rating on the sidewall and let them stretch overnight.

In the morning, reduce pressure to ~20psi higher than I think my ideal riding pressure will be, and go spin around the neighborhood. I'll do some really sharp s-curves, dive in to some corners, get a feel for the tread at low speed. Then let out ~5 psi at a time, test riding the same way after each reduction in pressure.

Eventually, the pressure gets low enough and you feel the tire start to collapse. It's a very distinct feeling, as the OP noted. Now I have a benchmark range (precise to the gauge on my pump, not necessarily an accurate pressure reading as others have said) and I can start to dial in the sweet spot for each wheel on familiar shorter rides.

I also find it interesting to use vernier calipers to measure the width ~24 hrs for the first week or so.

[rkhatibi]

Wide supple tires like those sold by Compass can be incredibly sensitive to tire pressure, since so much of the tire's stiffness is coming from the air spring and so little from the casing. It doesn't take that much underinflation for the handling to get weird. I generally pump my Rat Traps to a significantly higher pressure than most other tires of similar size.


This has been my experience with 35c Bon Jon extra lites as well. I started around 45-50psi, but preferred 55-60psi on the street. Still felt as nice, but much better in corners and under acceleration. Changes of 5psi were really noticeable. 90kg on Velocity Ailerons (20mm internal)

[unterhausen]

some pressure gauges have a 1/4 npt threaded gauge attached. So you can retrofit a pressure gauge with a known accuracy. Even the least expensive industrial pressure gauges have pretty decent accuracy.

[oldpotatoe]

This is the result of all the bollocks about "lower, even super low, tire pressure is always faster"

It's appropriate tire pressure is faster. Not so insanely high it's senseless, not so low that's it's sluggish or worse, and in this case, dangerous.

But you know what they say .... say it enough times and it becomes true.

Agree..when I 'forget' to put air in my Vittoria Pave tubulars, I can tell they 'feel' crappy, squirmy, particularly on the front, immediately. Put more air in 'em and if they still feel 'sqirmy', get better tires that don't.
The differences in rolling resistance, etc, are there but like most things 'compared' on bikes, the differences are teeny, tiny..Great for a chart but means not much in actual riding..unless, they 'feel dangerous..:eek:

[joshatsilca]

The Berto charts are a great starting point but important to measure your weight distribution. Also remember that descending, or descending/braking quickly creates situations of heavy front biased weight distribution that should be accounted for.

Also, pump gauges are typically +/-5% gauge accuracy when new and are dying slowly with every inflation. We recommend replacing your gauge every 5 years with moderate use if you are seeking accurate pressures. When we were developing the 303 for Roubaix we found that the 3 Specialized pumps on the Saxo truck had a 12psi variance between them at '70psi' and they were all just a few months old but clearly also got a lot of use. For that particular use case, the difference between using one pump and another was a set of broken wheels in the Arenberg, after that we managed all tire pressures with a $500 0.1% Ashcroft digital bleeder setup we called 'The Truth'

In the new SuperPista Digital (https://silca.cc/products/superpista-digital) we're using a very high end Wika industrial process pressure sensor and processor that allow us to do a custom curve fit resulting in +/-0.5psi accuracy between 8 and 100psi then drifting to 2% accuracy at 200psi. This sensor is tested to 10,000 inflations with less than 1% accuracy drift.