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Camber, pressure and stability tech

4K views 40 replies 8 participants last post by  Bluebottle 
#1 ·
Tyres seem to be an emotional topic so I'm going to stick this on its own. It is purely about stability and unexpected factors that influence wobble, weave and dynamic stability (not traction and nothing on aerodynamic factors)

It won't interest many people but if you want more than google offers or just curious what a vehicle geek gets ip to, its here and there are individual bits/formulas etc that you can use to solve/explain other tyre issues you might run into.

There is a bit of maths

General tyre properties v cornering
http://sem-proceedings.com/02s/sem.org-2002-SEM-Ann-Conf-SEM059-Identification-Motorcycle-Tire-Properties-by-Means-Testing-Machine..pdf

Tyre pressure effect on stability
http://www.researchgate.net/profile/Matteo_Massaro/publication/261834253_The_effect_of_the_inflation_pressure_on_the_tyre_properties_and_the_motorcycle_stability/links/54b3ccec0cf26833efcf5396.pdf
 
#6 ·
After perusing through all of that I am still going to use the manufacture's recommended tire pressure posted under the seat. Then if I begin to have wobble or weave problem I will look to see if my suspension is set properly for the riding conditions. These papers almost could lead you to believe that tire pressure alone is the main control for wobble and weave. To me the tires only introduce the bad vibrations. I understand that you can limit some of the vibrations with proper tire pressure. Bike geometry and suspension can overcome a whole lot of this. A holistic overview somehow has to be considered.
 
#9 · (Edited)
You are absolutely right Gappy, use the manufacturers tyre pressure gappy and absolutely correct about the geometry.
The manufacturers have already done all the work.


I posted this as data for people who like to mess with tyre choices and go with options the suspension and geometry were not designed to cope with and those who are curious about kinematics or the design/research world.

Putting science in other people's posts seems to always get a bad reaction so I put it here instead.

--------------------------

On the question of what causes wobble and weave, its part of being on two wheels and there is another mode known as capsize.
It all gets a bit complex when you get into the detail of it and there are lots of different causes and combinations. Messing up your geometry is part of it but so is aerodynamics which is how I originally got involved.

Apart from the surprising effect of road camber and general curiosity, I wouldn't worry about it if your bike is standard.

I am glad some of you got some use out of it, I expected the post to disappear without trace :)
 
#12 · (Edited)
Not if they are taught properly
Position for Safety then view and grip

Camber angle was measured with a simple gizmo fitted to the back of a vehicle that was already surveying "actual normal roads" for another purpose. My judgement wasn't a factor.


Edit: there are 2 excellent publications that cover road positioning in great detail.
Advanced Motorcycling by the Institute of Advanced motoring
Motorcycle Roadcraft (police riders handbook) published by TSO
 
#14 ·
Nope
The curvature, cant/superelevation etc of the road can be modelled in a simulator and output as an actual route or a generic class/composite or altered to see what difference changes make to drive the test rig.
Modern roads are built using comparative geometrics and standard formulae so its the older ones that add most of the variation.

The simulation can also drive a motion platform to produce a bump road or it can reproduce aircraft runways, sidewinds and general test awesomeness.
 
#15 ·
The camber (transverse/cross slope) of the roadway has nothing to do with uneven motorcycle tire wear.

The uneven wear (left side in the US) is instead a factor of the way motorcycles lean in turns (in the direction of the turn), and that in countries where one drives on the right side of the road the left side of the tire has more miles on it than the right.

How can that be you ask? Simple: It's because left turns are longer then right turns.

Consider a 45° turn in a 20 foot wide two lane road, with a 100 foot radius at the center line. If I enter the turn turning right and stay 3 feet to the right of center the radius of my path will be 97 feet; however if I enter it turning left, now 3 feet to the left of center, my turning radius will be 103 feet. Therefore turning right, leaning right with the right side of the tires in contact with the road doing the work, I will travel 76.2 feet making the 45° turn,

However turning left, leaning left with the left side of the tires doing the work, I will travel 80.9 feet--4.7 feet and 6.2% further. Consider also that we tend to drive faster in left turns than when turning right.

That is where the rubber goes...

This is a wonderful article about motorcycle tire wear.

From that site:

 
#18 ·
No, but science does.

If the article is correct. I can ride just as easily around the outside of a wall of death as I can on the inside, that clearly isn't true.

While I totally agree that the major cause of laterally uneven wear is due to greater distance travelled etc., the angle of the road surface relative is also a factor. It is smaller but it is still there.

If you take the longer distance/speed out of the equation for a moment. , is there anything else to cause wear? Things like slip and relaxation length?

Of course there are. So are these things symetrically aplied?
The wall of death scenario says they aren't.
Consider an exaggerated camber of 45 degrees and follow it around an S bend at speed. When the road turns one way you are pressed against like a banked race track, grip is excellent.
When the bend turns the other way there are forces lifting you off the track and grip is less and the faster you go the worse it gets.

Which tyre wears faster, the one scrabbling for a grip or the one pressed firmly onto the road?


But the camber isn't 45 deg! True, but it isn't zero either so it hasn't completely disappeared.


The article also says camber can't be a factor because it doesnt contact the wheel at the point where the wear is - so does the wheel suddenly float in there at that point or something?
It assumes that the camber argument is something to do with the point of contact with an upright wheel. It isn't, it is about scrubbing in the corners - exactly the point he is trying to make but failing to follow to its conclusion.

Every biker should already know that a positive camber = more grip and negative camber = less grip. Every good rider should be using it to there advantage.

Not saying camber is "the" reason, only that it exists as a minor factor easily proved/disproved in a lab by making left and right corners the same length
 
#19 ·
I believe you have misread/misinterpreted the article, perhaps deliberately it might seem from your terse rant.

The author is asserting the road camber does not contribute to uneven tire wear when traveling in a straight line, which is the most common "conventional wisdom" explanation for left side (or right side in the UK) tire wear.

This photo illustrates his point regarding the relativity of road camber, lean angles and tire wear when traveling in a straight line:



Road camber does of course affect wear when cornering, however the effect of a 1° (mild) to 3° (rather steep) transverse slope is just a minor contributor as compared to that of 20° to 45° lean angles and the added distance traveled on "outside" turns...
 
#20 · (Edited)
If the article is correct. I can ride just as easily around the outside of a wall of death as I can on the inside, that clearly isn't true.
English is my second language, please point out where it says so in the article.

Just like Helium and the other noble gasses are part of the atmosphere, road camber contributes to tire wear. Their presence can even be measured.
 
#22 ·
English is my second language, please point out where it says so in the article.
You are very good with it, and I admire your ability to be fluent in more than one!

Other than non-spoken languages (C++, Pascal, COBOL, TCL, Java, VB and many others) English is my only language; I too found no reference to any thing that stated or implied some ability to ride around the outside (or for that matter the inside) of a "wall of death"...
 
#21 ·
Terse rant is a bit steep for simply explaining why a statement is wrong.

We seem to be in complete agreement, road camber does of course affect tyre wear when cornering to some unspecified degree.

That being the case, how can the previous statements be true ?
ie "it is never the camber" ,
"The camber (transverse/cross slope) of the roadway has nothing to do with uneven motorcycle tire wear"
"ROAD CROWN HAS NOTHING TO DO WITH LEFT SIDE TIRE WEAR!"

I'm perfectly happy consider I that I misread the piece but all these statements are absolute.
 
#24 ·
Yes Erik, your English is superb.

Your comment appeared as an edit after I had begun my reply.
The wall of death has a very exaggerated camber at its base, if tyres do not wear due to camber they must not be slipping, if they don't slip I can make the camber as steep as I like in either direction without ever dropping off.

This doesn't happen so somewhere along the line camber must be having an effect on grip and therefore wear.

This is not compatible with any of the statements I listed
 
#25 ·
We are cross posting

I didn't know that was a popular opinion Cliffy, but now you have said it I can see why he mentioned upright tyres ( which seemed completely irrelevant until now)

However the author has just replaced one popular misconception with another and it is getting repeated and believed

"Never" and "nothing to do with" are clear cut and untrue, they aren't things I can misread and if camber is a factor, just in a different way to popular opinion, isn't it a simple matter to say that?
 
#27 · (Edited)
The same principles - if you get pushed against the slope your grip improves, if you get pushed away from the slope your grip reduces.

The wall of death has a 90ish degree bank and a road averages about 1 in 10 depending on where you live. In the middle is the banked turn used in racing.
 
#28 ·
The same principles - if you get pushed against the slope your grip improves, if you get pushed away from the slope your grip reduces.

The only difference is the wall of death has a 90 degree bank and a road averages about 1 in 10 depending on where you live
Try 1 in 50 on asphalt

http://www.cdeep.iitb.ac.in/nptel/Civil Engineering/Transportation Engg I/12-Ltexhtml/p5/p.html

Table 1: IRC Values for camber

Road surface________Heavy rain Light rain
Concrete/Bituminous_____2 %____1.7 %
Gravel/WBM____________3 %____2.5 %
Earthen_______________4 %____3.0 %
 
#30 ·
What would be quite interesting to me would be to select comparable sized motorcycle and automobile tires, mount them on your test rig and record the performance characteristics of each in simulated motorcycle use. I believe the comparison would put a big dent in the "darksiding" community's arguments...
 
#32 · (Edited)
It is definately an acquired taste Bolzen.

What would be quite interesting ...automobile tires, mount them on your test rig and record the performance characteristics.....
That rig wouldn't be right, though everybody seems to assume that is where the science/manufacturers "argument" will come from.
On different project it was the geometry changes that caused the biggest differences between the two types of tyre not traction or any of the things that get argued about.


People really do get emotional about darksiding so before all that I'd like to explain why the "negligible" 2deg. camber is a far bigger factor on grip and wear then people think and it multiplies the "distance travelled" idea about wear.
The car tyre stuff will then make much more sense.

Edit: Got a job to finish first
 
#31 ·
My rear tire is more like a car one now due to mileage. I don't know how folks can drive "darksided" all the time. I really don't. I don't have that much skills.
 
#34 ·
I want to put the camber thing to bed first.

We seem to have settled that camber is a factor after all, but the belief now seems to be its only 2 deg. difference in lean so it isn't significant (using Eric's 1:50 figure)

It isn't 2 deg.

The lean advantage is 4 deg. It is the lean angle from verticle and the camber together, like this -

Using a 20 degree lean toward the road centre
20 + 2 = 22

20 degree lean to the outside
20 - 2 = 18

22 - 18 = 4

There is 4 deg. difference between the lean angle on left and right bends, that is quite a big chunk out of just 20 in total. A 20% advantage is enough to win a race or change wear.

But it doesn't stop there and this difference in angle leads to several other factors that have been completely ignored. I'm going to pick a simple one that doesn't need any maths.

When a bike tyre leans it runs on rubber that is nearer the centre where the circumference is shorter. That means it turns more times to cover the same distance and therefore wears more.

Now with the camber making the lean angle different depending the direction of the turn - it means that the wheels make more rotations on a (US) left bend than a right bend. More rotations = more wear.

So yes, turns are longer/shorter - but you are also doing them on smaller wheels and with more scrubbing of the tyres and other complications.

Got to go again ( just been handed the keys to a 700 horsepower monster :p )
 
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