Scary moment on Compass Tires

[shoota]

Originally Posted by Clancy

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]

Originally Posted by 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.

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

[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]

Originally Posted by 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.

^^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]

Originally Posted by 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.

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

[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]

Originally Posted by Clancy

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

[Ken Robb]

Originally Posted by 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?

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]

Originally Posted by Jan Heine

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.