Mtn bike puntures

[paulvs]

should we make a couple of dents in the drum to prove this theory or use  MTB slicks?

[madcyclist]

Johan, if you have ever driven a 4x4 in deep sand you will know that reducing the pressure will reduce  rolling resistance AND increase traction. The Scwalbe study therefore makes perfect sense to me - specially if you read the reasoning behind it.

[Thug]

 

Why is that Johan ?

 

According to a graduate dissertion on rolling resistance utilising Schwalbe tyres' date=' tyres at a lower pressure reduces rolling resistance.

 

http://www.bicicletta.co.za/Downloadable%20docs/Rolling%20Resistance%20Eng%20illustrated.pdf

 

 

[/quote']

 

That study wasn't controlled well enough to tell us anything meaningful. To say that he rode over the same grass or gravel patch in each leg of the test is wishful thinking. Futher, how do you maintain a constant speed on a bike? You cannot.

 

The only way to create a perfectly controlled study is on a drum in a lab. That way you can control the surface, speed, input power and of course, measure the resistance. If you look at the figures in the study, they're so close together that the slightest bit of noise skews the data. For instance, the tested said he maintained a constant speed to eliminate wind resistance. Well, what about wind. How did he control the elements? How did he control the deformation of the grassy strip after ten passes?

 

It's a nice attempt, but like with so many of these things, the most obvious variables were not controlled.

 

As to your question why a lower pressure produces a lower RR? When a tyre is deformed, energy is lost in reforming it at the exit end of the cycle. The more rubber in there, the more energy lost. Air also loses energy, but very, very little inthe case of pneumatic tyre.

 

 

 

 

 

Sorry Johan, you misunderstood my question, I was asking why you thought the statement by paulvs was nonsense ?

 

In my opinion, your statement that the lower pressure the lower the rolling resistance

is nonsense is as you so normally quaintly point out,  "anecdotal"

 

Have you ever done a controlled study that proves otherwise?

 

 

 

 

 

 

 

[Johan Bornman]

should we make a couple of dents in the drum to prove this theory or use  MTB slicks?

 

I think this is a good idea. A drum is a controlled environment and if you could Pratley some gravel and grass to it, even better. As long as you can control the path, you're improving accuracy and eliminating noise. 

[Christie]

Im not a tyre technology expert, but as I see it, there are 2 things to define:

 

rolling resistance - I would describe this as the loss of energy of a moving tyre. I'm guessing most of th energy lost is due to friction. 

 

traction - the reaction force the tyre feels when rolling, coming from the friction between the tyre & the surface it rolls on.

 

As the friction increases, the tyre has more traction, but also more rolling resistance. This is a fixed relationship, both RR and traction are influenced by friction.

 

 

What seems to be confusing the issue is speed. Lots of factors influence speed, especially on a MTB, traction & RR can not be directly related to speed.

 

[dirtrider]

What is the point of doing a test like this in under LAB conditions ? If you want to test something then you do the test where the application will take place . I think that report on tyre pressures/sizes is very well done and has a copy behind my toilet door .

[RockCoach]

 

What is the point of doing a test like this in under LAB conditions ? If you want to test something then you do the test where the application will take place . I think that report on tyre pressures/sizes is very well done and has a copy behind my toilet door .

Spare loo paper?  sorry couldn't resist.

 

Anyway, the point is to try and control as many variables as possible: mainly wind and the randomness of the surface. A rider will never be able to take the exact line multiple times and any deviation will create discrepancies in the data.  In a lab you can you can control the tests using surfaces with varying coefficients of friction. On a hard surface, a hard tyre will have a lower rolling resistance than a soft one. Things I suspect become a lot more complicated when the ground is soft.

 

In the real world not only does the tyre deform momentarily, the terrain can also deform under the pressure of the tyre (riding in mud or thick sand or even loose rocks) and I'm not so sure that can be accurately simulated on a drum in the lab.

 

 

 

[Johan Bornman]

In the real world not only does the tyre deform momentarily' date=' the terrain can also deform under the pressure of the tyre (riding in mud or thick sand or even loose rocks) and I'm not so sure that can be accurately simulated on a drum in the lab.

 
[/quote']

 

You are right, it cannot be accurately done in the lab, but then again, neither can it be done in the veld. What we do know is that a smooth steel drum will give an accurate figure and then any other irregularities will just add to the "smooth drum" RR. Therefore, a tyre that has the least RR on a smooth drum, will also have the least on any other surface (provided that it can float on top of loose sand, something I'll address in one of the other posts) and vice versa. Although a smooth drum doesn't give the absolute RR for a non-smooth surface, it does point out the tyre with the least RR.

 

 

[Johan Bornman]

Im not a tyre technology expert' date=' but as I see it, there are 2 things to define:

 

rolling resistance - I would describe this as the loss of energy of a moving tyre. I'm guessing most of th energy lost is due to friction. 

 

[/quote']

 

Not quite. There is no friction involved in a bicycle tyre. If there were friction, it would squeal on a parking lot garage floor like a car tyre, where there is friction due to deformation and squirm.

 

QUOTE=Christie] traction - the reaction force the tyre feels when rolling, coming from the friction between the tyre & the surface it rolls on.

 

Traction can be nicely defined as the static friction (stiction in shock absorber terms) between the two surfaces. As we know, the static friction exists up to a point where the two surfaces slide against each other and then reduces dramatically, as in a tyre skidding and which is preventable with ABS. ABS on cars keep it within its static friction range.

 

 QUOTE=Christie]

As the friction increases, the tyre has more traction, but also more rolling resistance. This is a fixed relationship, both RR and traction are influenced by friction.

 

 

Just to reword it slightly: As the stiction increases, so does traction. However, RR stays constant unless something else has changed. RR is not influenced by friction.

 

 QUOTE=Christie]What seems to be confusing the issue is speed. Lots of factors influence speed, especially on a MTB, traction & RR can not be directly related to speed.

 

 

They are not related to speed. The only time speed will come into play in RR is when the wheels turn so fast that centripetal forces deform the tyre and hence its profile on the road.

 

[Johan Bornman]

snip snip snip

 

In my opinion' date=' your statement that the lower pressure the lower the rolling resistance is nonsense is as you so normally quaintly point out,  "anecdotal"

Have you ever done a controlled study that proves otherwise?


[/quote']

 

No I haven't done such a study, but others have.

[Johan Bornman]

Johan' date=' if you have ever driven a 4x4 in deep sand you will know that reducing the pressure will reduce  rolling resistance AND increase traction. The Scwalbe study therefore makes perfect sense to me - specially if you read the reasoning behind it. [/quote']

 

This is an interesting analogy, but unfortunately flawed.

 

Rolling Resistance in a pneumatic tyre is caused by hysteresis, a phenomena where energy is lost when rubber is deformed and allowed to spring back. A superball is a great example of that. If you drop it from 1 meter, it bounces back only 980mm (say), the rest is lost to hysteresis. On a car tyre, this is one of the many causes of energy losses due to moving formward at low speed before wind resistance kicks in. There is drivetrain friction, deforming road surface etc etc.

 

On a 4x4 in deep sand, the wheels experience two types of resistance:

 

a) the standard rolling resistance mentioned above and

b) another type of resistance which is equal to the car having to constantly climb uphill.

 

 The "uphill" is physical depth of wheel sunk into the sand. As the car moves forward, it constantly has to do the equivalent of going uphill, although the earth keeps on collapsing and it never quite gets to the top. The energy expenditure is the same though. A rail car works the same. The steel wheel dents the rail and the rolling resistance is equivalent to the uphill journey of the wheel up the dent. Like a donkey with a carrot on a stick, it never quite reaches the top of the hill and  therefore there is always resistance untill you reduce the hill to zero by making the carriage weightless.

 

On the 4x4 going through sand though, the solution is to make the tyres float on top of the sand, avoiding the uphill battle - pardon the pun.

 

Therefore, a 4 x 4 with soft wheels gains a bit of RR because the tyres are softer, but gains a lot because the tyres now float on top and doesn't have to constantly ride uphill.

 

Strictly speaking, you have not reduced the RR, just improved the conditions.

 

 

Bring that back to a mountain bike and we now have to decide of Scwalbe discovered this phenomena or not. Like I say, the study is a bit iffy.

 

 

[madcyclist]

Johan, you are playing with semantics. Try the following for yourself.

 

1) Find a patch of soft sand

2) Approach it at a given speed (constant momentum) with tyres pumped hard.

3) Measure the distance you coast into the sand before you bog down.

4) Do the same thing with tyres at a reduced pressure.

 

I will bet my titanium Merlin XLM against your grandfather's penny farthing that as pressure reduces (up to a point - probably at around 0.8bar) you will coast further and further into the sand. Same energy - more distance.

 

This will apply the same to a 4x4 in soft sand, and the only reason for this (nevermind hysteresis or constantly climbing uphill) is that the actual resistance to moving forward is reduced by floating OVER the obstacle (the sand) instead of fighting it. 

 

The same will apply on soft grass (to a lesser degree)

 

[Imtb]

I will bet my titanium Merlin XLM against your grandfather's penny farthing that as pressure reduces (up to a point - probably at around 0.8bar) you will coast further and further into the sand. Same energy - more distance.

Agreed!

Having done Day 1 of the Imana Wild Ride (beach sand) on hard tyres and found it really difficult, I  changed to softer tyre pressure on Day 2 and it made a huge difference, my bike did seem to float over the sand rather than sinking down like it did on day 1...on beach sand - whether one can say the same for soft grass, I am not sure

[Johan Bornman]

Johan' date=' you are playing with semantics. Try the following for yourself.

1) Find a patch of soft sand
2) Approach it at a given speed (constant momentum) with tyres pumped hard.
3) Measure the distance you coast into the sand before you bog down.
4) Do the same thing with tyres at a reduced pressure.

I will bet my titanium Merlin XLM against your grandfather's penny farthing that as pressure reduces (up to a point - probably at around 0.8bar) you will coast further and further into the sand. Same energy - more distance.

This will apply the same to a 4x4 in soft sand, and the only reason for this (nevermind hysteresis or constantly climbing uphill) is that the actual resistance to moving forward is reduced by floating OVER the obstacle (the sand) instead of fighting it. 

The same will apply on soft grass (to a lesser degree)
[/quote']

 

I can't take you up on that bet, unfortunately. A penny farthing easily sells for $25000 on e-bay and  Merlin....well....

 

I grudgingly concede that semantics are involved. But I would still like to separate environmental contributors to RR from those inherent in the tyre itself. Only then can we say X-tyre is "better" than Y-tyre.

 

 

[iluvatar]

nice article thug. interestingly they DID control the rolling resistance variables by referencing them from a lab test.... (par 1 page 2)

 

i am satisfied that they controlled the variables effectively - how else to test the effects but by riding? On the surfaces to be investigated? I will say that the author could have discussed means to control the speed - i would hazard a guess that he did many runs and took those closest in speed, but that's guesswork and so easy to criticise that he would have conformed it with his supervisors.

 

I would go with his statement that the rider must balance suspension setup - hard/soft, riding conditions - south african single track is hard-packed earth, mostly - and application - single-track/downhill/pure cross country where there IS grass.

 

I agree with the soft sand principles - the author is showing this in his paper, because, whenever i've ridden on grass, it is equivalent to riding on soft sand, since the grass is like a springy carpet. i would say that soft sand or mud is perhaps even more giving than grass.

 

praps a good read of the paper is required again, as the paper presents a separation of environmental factors and lab-tested RR.

 

[iluvatar]

another point - take a drop with 1.5 bar and you'll break rims... going over general single track probably needs a littlemore than 1.5 bar..... also suspension and tyre-pressure combinations are important. too little and the suspension will not be properly activated - all the gibve is in the tyre. too much and there's not enough grip and youmay get skittish, imprecise steering.

 

the suspension manufacturers recommend pressures, do they not? as do the tyre manufacturers.

 

what about damaging the sidewall with too low a pressure?