Scary moment on Compass Tires

[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]

Originally Posted by 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.

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

[benb]

Originally Posted by Jan Heine

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

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]

Originally Posted by Jan Heine

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

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]

Originally Posted by 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.







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 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]

Originally Posted by spoonrobot

I use a small scissor stand (looks like this 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]

Originally Posted by rwsaunders

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.

Originally Posted by Clancy

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]

Originally Posted by HTupolev

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]

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.

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..