Suspension power losses/savings

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Brucey
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Re: Suspension power losses/savings

Postby Brucey » 6 Feb 2019, 12:53pm

squeaker wrote:
Samuel D wrote:Our bodies bounce badly by comparison and that is the source of Heine’s suspension losses.
But how does he measure these internal losses?


IIRC he inferred them by coasting downhill over rumble strips and observing the speed change vs riding on the smoother surface alongside. US-style rumble strips are pretty lumpy so it represents an extreme case, but it shows the principle.

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Samuel D
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Re: Suspension power losses/savings

Postby Samuel D » 6 Feb 2019, 1:06pm

I don’t know the details because I haven’t read the relevant Bicycle Quarterly issue, but I presume Heine logically deduced that the main losses were in the cyclist’s body after observing speed differences on different surfaces. Where else might the hundreds of watts go without being easily observed?

No doubt there are small energy losses in the bicycle itself, clothing, etc., too. But most is wasted in heating the human body. Heine has mentioned a US Army (or similar) tank study that attempted to measure these heating losses in more detail and found that humans in vibrating tank seats could dissipate something like 2 kW for short, agonising periods.

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Mick F
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Re: Suspension power losses/savings

Postby Mick F » 6 Feb 2019, 2:38pm

I lose one minute per mile riding Moulton compared to Mercian. Ride for ride.
What's the difference?
Some weight penalty and some aerodynamic penalty, but they wouldn't add up to one minute per mile.

I can only suggest that the suspension saps the speed.
Mick F. Cornwall

thelawnet
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Re: Suspension power losses/savings

Postby thelawnet » 6 Feb 2019, 2:56pm

Mick F wrote:I lose one minute per mile riding Moulton compared to Mercian. Ride for ride.
What's the difference?
Some weight penalty and some aerodynamic penalty, but they wouldn't add up to one minute per mile.

I can only suggest that the suspension saps the speed.


Weight penalties are not significant, but aerodynamic penalties, beyond very slow speeds, are large. An effective increase in your surface will make significant differences to speed.

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Mick F
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Re: Suspension power losses/savings

Postby Mick F » 6 Feb 2019, 3:04pm

Yep.
Same bloke riding in the same clobber, but on different bikes.

The plus side of a Moulton's aerodynamics are the small wheels, but the minus side is the space frame.
You could argue that they'd cancel out, but we'd need a wind tunnel to find the truth.
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squeaker
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Re: Suspension power losses/savings

Postby squeaker » 6 Feb 2019, 3:50pm

Brucey wrote:
squeaker wrote:
Samuel D wrote:Our bodies bounce badly by comparison and that is the source of Heine’s suspension losses.
But how does he measure these internal losses?


IIRC he inferred them by coasting downhill over rumble strips and observing the speed change vs riding on the smoother surface alongside. US-style rumble strips are pretty lumpy so it represents an extreme case, but it shows the principle.

Ah, thanks :) I do hope he assessed the effect at a range of speeds to minimise resonant effects.
"42"

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Re: Suspension power losses/savings

Postby Samuel D » 6 Feb 2019, 6:26pm

Mick F wrote:I lose one minute per mile riding Moulton compared to Mercian. Ride for ride.
What's the difference?
Some weight penalty and some aerodynamic penalty, but they wouldn't add up to one minute per mile.

I can only suggest that the suspension saps the speed.

I wouldn’t be sure of that. The suspension may be a net gain for speed. On the other hand, the small, narrow tyres have higher rolling resistance than 700C tyres – probably much higher in most typical cases.

If I were designing a small-wheel bicycle today, I’d skip the suspension and give it the fattest tyres I could reasonably source and fit. The aerodynamic penalty is minor because the tyres are not tall, and the benefit in rolling resistance and comfort is significant. But high-performance, fat tyres were neither commonly available nor popularly accepted when Moulton was designing his bicycle.

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Mick F
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Re: Suspension power losses/savings

Postby Mick F » 6 Feb 2019, 6:45pm

I've tried to do roll-out tests but it's difficult to get meaningful facts.
I had a thread on here about it.

It seems that the acceleration from rest is less with a small wheel than with a big wheel. Despite trying and trying, I seem to be going slower at the same point only a few yards (five or six) from starting off.

Gentle slope, starting from the exact same point, it seems that Moulton is sluggish even at very slow speeds.
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Re: Suspension power losses/savings

Postby Scunnered » 7 Feb 2019, 11:17am

Mick F wrote:I lose one minute per mile riding Moulton compared to Mercian. Ride for ride.
What's the difference?
Some weight penalty and some aerodynamic penalty, but they wouldn't add up to one minute per mile.

I can only suggest that the suspension saps the speed.


I'm not familiar with the details of a Moulton, but suspension usually has a spring component and a damper component. The spring component is designed to be elastic and the damper is in-elastic. The damper therefore absorbs energy.

In a tyre, the spring component is the air chamber and the damper is the tyre casing, which is designed to produce minimal damping for low rolling resistance.

A suspension with damping may allow a higher speed over a rough surface due to improved control, but will still require more propulsive effort.

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Re: Suspension power losses/savings

Postby Samuel D » 7 Feb 2019, 11:38am

Mick F wrote:it seems that Moulton is sluggish even at very slow speeds.

That’s consistent with tyre rolling resistance being the problem. Small-wheel bicycles need tyres of especially low rolling resistance to be efficient.

Scunnered wrote:A suspension with damping may allow a higher speed over a rough surface due to improved control, but will still require more propulsive effort.

I believe that statement is too categorical. Without damping, a bump will set the bicycle oscillating in perpetual motion. All damping is provided by the rider in that case. Properly tuned damping in the suspension system would reduce oscillation and therefore energy lost in the rider. I’d be surprised if this didn’t provide a net saving of energy and therefore higher speed for a given pedalling power.

More power is required on rough surfaces than smooth even with suspension. Actually, re-reading your statement, perhaps that is all that you meant.

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Re: Suspension power losses/savings

Postby Brucey » 7 Feb 2019, 11:46am

one way of looking at it is that damping is really there to quell resonances in suspension systems; there are two such resonances to worry about

a) the sprung mass (mainly you, on a bike) bouncing up and down on the springs and

b) the unsprung mass (the moving parts of the suspension) flapping up and down uncontrollably even if the sprung mass isn't moving.

Engineering a) properly is expensive and difficult, and is one of the main reasons why most (bicycle especially) suspension is a bit crap. Dealing with b) is also non-trivial eg because the spring rate is different from that which applies to a); effectively the spring rate of the tyre is in series with the suspension spring rate in a) (which means that the tyre rate can be ignored in many cases) but it is in paralell with b), which means it is very important.

On bumpy surfaces it is usually pretty obvious that the energy absorbed by the suspension is small by comparison with that absorbed by a rider on an unsuspended bike; freewheeling down a bumpy track on two different bikes will soon tell you this.

However what is less obvious is that just pedalling hard tends to excite a type a) resonance and this tends to absorb energy all the time, even when the suspension has little or no work to do. Hence my earlier comments about needing to pedal in a specific fashion. I have yet to meet a rider that could pedal smoothly at 10/10ths aerobic effort up a draggy climb without bobbing like crazy on a suspension bike, which is why lockouts were invented. On many suspension bikes without lockout, trying to ride out of the saddle is basically a complete waste of time.

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Mick F
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Re: Suspension power losses/savings

Postby Mick F » 7 Feb 2019, 3:17pm

Mick F wrote:It seems that the acceleration from rest is less with a small wheel than with a big wheel.
Samuel D wrote:
Mick F wrote:it seems that Moulton is sluggish even at very slow speeds.

That’s consistent with tyre rolling resistance being the problem. Small-wheel bicycles need tyres of especially low rolling resistance to be efficient.
Just going back to my youth here!

We lads used to make go-karts using pram wheels and various designs of wooden arrangements - with steering of course.
Mine at one time had four wheels maybe 8" diameter, and my mate's one had big wheels from a big Silver Cross type of pram. His wheels would have been two feet or more in diameter. Both carts had solid tyres and good free wheel bearings.

Same slope, from rest, his would leave mine standing within the first couple of yards.

Only difference, was size of wheels.
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Re: Suspension power losses/savings

Postby andrew_s » 8 Feb 2019, 9:44pm

Brucey wrote:
squeaker wrote:
Samuel D wrote:Our bodies bounce badly by comparison and that is the source of Heine’s suspension losses.
But how does he measure these internal losses?


IIRC he inferred them by coasting downhill over rumble strips and observing the speed change vs riding on the smoother surface alongside. US-style rumble strips are pretty lumpy so it represents an extreme case, but it shows the principle.

cheers

The method, according to my memory, was to ride the same length of road repeatedly at a set speed, on and off the rumble strip, and compare power meter readings.
IIRC, the speed was around 17-18mph, the tester being unable to consistently hold higher speeds for long enough when on the rumble strip.

He used downhill coasting for real road (rather than drum) tyre rolling resistance testing.

US-style rumble strips are pretty bad, but not really any worse than the rougher varieties of pavé. The extra 200 -300 W required to maintain speed shows why the cobbled sectors on the Flanders classics are as effective at breaking the aero tow as climbing.

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Re: Suspension power losses/savings

Postby PDQ Mobile » 8 Feb 2019, 10:11pm

^^
Yes, so a bike with decent damped suspension will travel faster over such a surface as the unsprung weight is much less?
The wheels and bits move but frame and rider glide forward level and serene! Well not quite but that's the theory.

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Re: Suspension power losses/savings

Postby atlas_shrugged » 9 Feb 2019, 11:11am

I ride a recumbent (~20mph) and in winter a conventional hybrid (~15mph). I have noticed on my recumbent that it significantly slows down on rough road. My explanation for this is that the moment of inertia is significantly higher on a recumbent compared to an upright bike where the bike pivots on the BB without 'rotating' the rider. I noticed this especially on a ride from Oxford to Cambridge. On smooth flat bits of road or on downhill sections I was faster than riders around me (on road bikes). However on rough bits of road I was much slower and on uphills much much slower.

This makes me think that any light, frictionless, and simple technique for making a bike glide over the boulders that our UK roads have become will provide a big speed advantage. Maybe some kind of camera/computer rock-hopping capability. It would make it feel like riding in the Netherlands he dreamed.