I don't know the scientific answer to this question but I did ride a Trek Yfoil for years. In fact, I still have it at my daughters across the country and ride it when I visit her. One of the hardest things to get used to was the sensation that the rear tire was underinflated. I would get off the bike, check the pressure (I never go more than 105-115) which would be fine, but the sensation would be that of a squishy underinflated tire.

depending on how high HIGH is..I typically can run an additional 10psi on the rear with my Softride and still not notice the kind of road shock I do on a standard road frame. Running Dugasts at 200psi..I cannot say for certain.

I run my rear tire on the Titanflex about 10psi higher than on my rigid bike.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

just thinking back to the roller data, higher psi = less rolling resistance on the rollers, no matter how high.

on the road, there was a breakpoint at around 120psi, where the bounciness sapped power.

but with a beam bike, you now have a suspension, perhaps you should run far more than just 10psi extra?

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

That sounds like a fun thing to test for someone with a beam bike and a power meter...

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http://bikeblather.blogspot.com/

It's got to help.

FWIW, I have not found huge pressures to slow me down...however I don't have a PM to prove that. I just know that when I race my non-aero wheels (Bontrager Race X Lites) with TUFO tubular clinchers at 200psi I fly. I'm not joking.

       

Not necessarily...there's something to be said about having the surface irregularities being absorbed in the tire contact patch and returned nearly completely in the trailing portion. I don't think a beam will necessarily provide such a benefit. It might be more comfortable over the really large hits, but it may not be "faster" overall.



...at least it "feels" that way, huh? Human perception is a funny thing...

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The forces on the tires are going to be the same if the position of the seat relative to the contact points is the same- regardless of how its all connected.
The vibration- and what you feel- is going to be the difference. because it feels squishier, or softer, does not mean that there is less weight on that tire- its all about how the shock is absorbed through the frame and the beam.
So like everyones been saying- you can jack up the pressure to a certain extent beyond a UCI shaped bike, and you wont notice as much- but if your 200lb body exerted 130lbs on the rear tire and 70 on the front on the same position on bike X, then its still gonna have 130 rear and 70 aft on your softride.

But its not weight on the tires that causes speed loss its the tire casing flexing. If a beam soaks up some of the shock vs the tire then it may be better to go with a higher pressure ( I said may).

styrrell

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Styrrell

You're right about the static weight on the wheels but are missing an important point.

Let's say you have 100 lbs on the rear wheel as an example and you hit a 1G bump. The weight on that wheel momentarily becomes 200lbs and forward momentum is lost because that 100lbs has to be lifted upward.

With a beam, that 100lbs on the 1G bump becomes something much less than 200lbs, closer to 100 than 200. Because of the suspension, the rear wheel lifts, but the rider doesn't, or does much less and the forward momentum that's lost is less because it may only lift 40lbs upward.

The assumption there, though, is that less energy will be lost in flexing the beam than in flexing the tire. If it's a crappy tire, maybe...if it's a good tire, I'm thinking it's unlikely...

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See my response to styrrell above. Your assumption is that the flexing of the beam is less lossy than the flexing of a tire casing...





Do you "accept the truth" of actual on road measurements? You've seen my plot of "resistance to forward motion" (lumped as Crr) vs. tire pressure, right?

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What about using a suspension seatpost?

Perhaps the root of my problem lies in for some reason thinking that the rolling resistance 'truths' imply too much of a real world difference. Perhaps these differences (say between 100psi and 200psi) only account for 20 seconds and when I don't see myself a minute or two slower I dismiss it.
As I did admit earlier, it would take careful field testing for me to know either way. I wonder if it's not time for me to enter the world of Power Measurement?

That thought sort of puts things in perspective, huh? :-)

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I'm not sure I understand...suspension seatpost >> damping than a beam? (Not the one I have.)

Anyway, here was what I was thinking: a suspension post would allow you to measure on-road Crr as a function of tire pressure while keeping CdA and mass essentially constant, versus having to worry about those additional variables when testing using different bikes. (Indeed, the best experimental design would be to always test using the suspension post, once when it was active and once when it was locked up.)

Probably the best post for that would be a cane creek thudbuster which does not change the ST length much over its travel. In addition, you can put in some wooden dowels in place of the elastomers so that it removes the suspension...Not that Im volunteering.

Stephen J

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I believe my local reality has been violated.
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Happiness = Results / (Expectations)^2

ive got a thudbuster

someone pass me a powertap

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

No...it just got me thinking that it's not so much the LARGE deviations that we're concerned with when investigating pressure vs. "resistance to forward" motion...instead it's the higher frequency, small deviations that are the "energy sinks".

Remember, even with AFM's "bumpy rollers" there was extra energy required to "roll" the tires at all pressures, even though the small wires weren't actually lifting his entire mass. Also, in my on-road testing, the "breakpoint" of pressure I saw wasn't at an extremely high pressure and it wasn't on a super rough road surface either. I still think it's far better to have all that higher frequency energy being used to flex a tire casing (where most of the energy can be returned to the road surface with small losses) than to transmit it through a tire and have it absorbed somewhere else.

I don't think the response of a suspension beam or post is fast enough to be able to handle these high frequency inputs...they'll be great for absorbing the larger amplitude, lower frequency stuff, but that's not really what's going to be slowing you down on a "normal" road surface.

The beams or suspension posts are really more about comfort over "big hits" and taking the edge off of them than they about a suspension being used to "follow" the very small road surface roughness deviations. They just can't respond fast enough...there's probably too much unsprung weight.

Anyway...my though of a test would just be to do the Crr vs. Pressure test with a beam bike and the same tires and wheels as a rigid bike and just see if there was the same "breakpoint pressure" for both. My suspicion is that the beam bike's "breakpoint pressure" may not be any higher...

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Want to test the theory Andrew? I could lend you my Powerwing for a while to see.