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Discussion Starter · #1 ·
I'm entering the V-strom world coming from a background of sport bikes, cbr's, yzf's, vfr's, one nighthawk...

I'm trying to understand the logic, and physics behind having a larger, skinnier front wheel, to a smaller, fatter rear wheel. most of the sportbikes I've ridden have had similar wheel sizes (rear and front, though standard skinnier-front than rear).

So in just putting on my first thousand or so miles on the V-strom, I'm able to feel for myself, experience the difference in handling; a bit slower to enter turns, have to lean it over a bit more to attain the same turning radius at mid-speeds. Also when I have the weight loaded to the rear, the small contact patch of the front becomes more noticeable, requiring more effort on my part to keep the front wheel in a straight line.

So, in order to understand the wheel physics of the V-Strom, I'm thinking in reverse; I present the question:

How would a motorcycle (bike) differ in handling if it were to have a smaller, fatter FRONT tire, and a smaller, skinnier REAR tire (rear-wheel driven of course)?

Would this (theoretical) bike need to have a longer wheelbase and lower center of gravity (or different mass distribution/bias) to be as stable as the larger-front, smaller-rear counterpart (aka V-strom like)?
 

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How would a motorcycle (bike) differ in handling if it were to have a smaller, fatter FRONT tire, and a smaller, skinnier REAR tire (rear-wheel driven of course)?

Would this (theoretical) bike need to have a longer wheelbase and lower center of gravity (or different mass distribution/bias) to be as stable as the larger-front, smaller-rear counterpart (aka V-strom like)?
I can say that I wouldn't want a smaller fatter front tire or a skinnier rear tire in mud or loose gravel.
 

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have to lean it over a bit more to attain the same turning radius at mid-speeds.
Don't all 2-wheelers lean at exactly the same angle in the same radius turn at the same speed? I'm talking about the angle from the tire contact points to the combined center of gravity of bike & rider. If you're sitting taller on a strom it may feel like more angle.
when I have the weight loaded to the rear, the small contact patch of the front becomes more noticeable, requiring more effort on my part to keep the front wheel in a straight line.
Check your preload, and you many need to change springs to suit your loaded riding weight.
Suspension Adjustment Many of us find good results with a fork brace, and a few stroms have steering stem bearings that are either loose from the factory or work loose; correctly tightening these stabilized the bike tremendously. Lowering the front by raising the forks helps with stability, contrary as it may seem.

I'd like to see the answers to the questions you ask about wheel diameter and tire width. Also include tire profile.
 

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Don't all 2-wheelers lean at exactly the same angle in the same radius turn at the same speed? I'm talking about the angle from the tire contact points to the combined center of gravity of bike & rider...
Nope, longer wheelbase requires more lean angle.
 

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I'm entering the V-strom world coming from a background of sport bikes, cbr's, yzf's, vfr's, one nighthawk...

I'm trying to understand the logic, and physics behind having a larger, skinnier front wheel, to a smaller, fatter rear wheel. most of the sportbikes I've ridden have had similar wheel sizes (rear and front, though standard skinnier-front than rear).

So in just putting on my first thousand or so miles on the V-strom, I'm able to feel for myself, experience the difference in handling; a bit slower to enter turns, have to lean it over a bit more to attain the same turning radius at mid-speeds. Also when I have the weight loaded to the rear, the small contact patch of the front becomes more noticeable, requiring more effort on my part to keep the front wheel in a straight line.

So, in order to understand the wheel physics of the V-Strom, I'm thinking in reverse; I present the question:

How would a motorcycle (bike) differ in handling if it were to have a smaller, fatter FRONT tire, and a smaller, skinnier REAR tire (rear-wheel driven of course)?

Would this (theoretical) bike need to have a longer wheelbase and lower center of gravity (or different mass distribution/bias) to be as stable as the larger-front, smaller-rear counterpart (aka V-strom like)?
Good questions, but it's a big, complicated subject, well beyond a simple internet post to answer. If you're really interested in the topic get a copy of Tony Foale's Motorcycle Chassis Design book.
 

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Its probably tradition and marketing but...

A larger diameter rolls over obstacles easier

A larger diameter generates greater centripetal force than smaller at the lower speeds

Lighter is better to lift over things and un sprung weight
 

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I would also think a larger wheel has MORE contact to the road-surface... and likewise would need more lean than a smaller-diameter tire to get the same turning radius of that contact-patch.

As mentioned above - This is a very complicated subject which has many variables.... for example, if the rear tire was larger-diameter, this means the center-to-center distance from the rear-axle to the front sprocket is longer forcing a longer swingarm. A larger-diameter rear also means the seat must be higher forcing a higher center-of-gravity.

Also the WIDTH of the tires adds yet another variable into the mix.
 

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@ PTRider ... Great link. Everyone should take the time to set their bike up properly to get the most out of what the designers had in mind, suspension being one of the most important aspects of bike setup.

Thanx! :thumbup:

Now, back on topic...
 

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Wheel Psychics?

Do they predict your mileage for a particular tire?

Next you'll be telling us about Engine Astrologers...

Sheesh. :yesnod:
 

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As mentioned

Motorcycle vehicle dynamics tend to be much more influenced by chassis geometry and weight distribution than wheel size. Obviously if you go to extremes (like the superfat rears on choppers) the wheel dimensions can have a significant effect. Note that the contact patch area is mostly a function of tire pressure (tire stiffness enters in somewhat) although contact patch geometry is related to width and diameter. Having a wider (heavier) tire on the front, rotating faster (smaller diameter) would require more force on the bars to turn the tire, and a wide tire would move the center of gravity farther outside the contact patch when leaned, presumably also making the steering feel heavy. While factory engineers sometimes reach a different compromise point than we might choose, they are not generally stupid. You can bet they tried different dimensions, and decided that what we got is in the ballpark of optimized.
 

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...Note that the contact patch area is mostly a function of tire pressure (tire stiffness enters in somewhat) although contact patch geometry is related to width and diameter...
This is an important point, and one that most people don't realize. A wider tire has no bigger contact patch than a skinny one if the pressures are the same.
The other thing is that a bigger contact patch doesn't, in and off itself, give you any more traction. What it does do is spread the mechanical and thermal loads over a larger area, which allows a softer compound to be used while giving the same life. It's the softer rubber that gives more traction, not the size of the contact patch.
 

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Discussion Starter · #13 ·
nice

great replies

yes, contact patch is something I'm trying to 'get a grip' on in my mind, hahaha,

I sketched up a simple graphic, with some theoretical lines passing thru the axles, along with a steering axis and perpendicular line as 'guides' but not to represent any weight or frame necessarily.

All other things being equal:

Thinking about how the V-strom [layout (B)] has a "falling" or "receding" axle-to-axle line, and how that would affect how far the bike needs to lean to turn (tire width being equal.

While the theoretical [layout (C)] bike (rising axle-to-axle line) would not need to "roll" or "lean" as much as layout B.

Would you 'Stromers agree? beat the dead horse..

k
 

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Picture a right cone where the apex of the cone is the radius of the turn

Then the wheels are points on the base of the cone corresponding to a plane where the distance of the wheels intersect with the ground

Its following the natural rotation of this rolling cone that turns the bike by leaning.

Now picture a wind up monkey crashing cymbals together as a result of engaging the improbability drive.
 

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Most dirt bikes are 21" f 19"r
super moto 16.5"f 17"r
Sport bikes 17 17
My guess is that our differing tire sizes are one of the few things that actually qualify the Stroms as a "dual sport" instead of a street bike :jawdrop:
 

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big wheels roll easy over rocks. [period]

Its as simple as the bigger the wheel, the easier it will roll over obstacles. Nothing more. If it had the same size front and rear it would not be considered an adventure bike. Guys that really get serious with their Stroms put 21" spokes on the front....not necessary...but it will work much better off road.
 

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Actually

The other thing is that a bigger contact patch doesn't, in and off itself, give you any more traction.
Not quite true. Rubber exhibits adhesion, which is somewhat different than friction. That is to say, the amount of traction is not a direct function of force pressing down the tire. So a larger contact patch will prove for more traction, hence the use of extremely low-pressure, extremely flexible slick on top fuel dragsters.
 

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Not quite true. Rubber exhibits adhesion, which is somewhat different than friction. That is to say, the amount of traction is not a direct function of force pressing down the tire. So a larger contact patch will prove for more traction, hence the use of extremely low-pressure, extremely flexible slick on top fuel dragsters.
Normal tire rubber does NOT have adhesion, which you can verify just by pushing a bike around. (There's obviously no extra rolling resistance from the back edge of the tire "peeling up" like the sticky from a post-it note.) This is true even of ultra soft roadracing tires. Drag racing is different, not from any fundamental difference in tire rubber, but because of the traction compound that they apply to the track, that does act like glue.
 

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Tony Foale has done a lot of work studying overall motorcycle dynamics ~ might be worth a look there [if you haven't already].
His experiments on the effect of rake have led to some interesting & counter-conventional results.
But I don't think he looked much at tyre width effects.

OTOH, the BMW engineers did some experimentation in the 1980's, which IIRC concluded that a skinny front & wider rear wheel gave best overall stability.
Though I suppose we might have to make some allowance for the torsional wobbliness of 1980's forks, frames & swing arms ~ maybe things could be different with today's suspension components.

The conventional wisdom is that tall skinny wheels are best for off-road [for various reasons].
The almost universal use of the small front wheel (120/70-17] on road bikes is a little more difficult to explain.
Is it just the advantage of a lighter front wheel, on a road surface that has [usually] very little in the way of significant bumps & valleys (potholes) ?
.
 

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Sorry Klimber, I'm not following what your diagrams in post #13 can achieve [by looking at side view of the bike].

Looking at the bike from the front (or back) will show how the centre of the contact patch (of each wheel) moves to the side as the bike banks over, and the contact patch forms the flattened part of the "cone" that Richw mentions in #14. And of course the front patch will move a little to the outer side, owing to the castor effect.

Ideally, from the handling point of view, the motorcycle will have wheels as skinny as a Tour-de-France racing bicycle's.
Realistically, more width is needed for better grip & durability ~ especially rear durability (hence the particularly wide rear wheel of the average motorbike . . . . which the marketers assure us has nothing to do with race/dragster suggestions of immense power & fashionable street-cred.).

In itself, the height of either (or both!) wheels is unimportant to the matter of handling . . . until you get off the bitumen, when compromises of stability and floatation etcetera start to become important.
.
 
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