Does anybody remember who first posted this about a year ago? It put me on the dark side.
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I guess I'll my "buck-two-ninety-eight" to the discussion.
So far I haven't found a single post ANYWHERE on the WWW that speaks negatively of using a car tire on the rear of a motorcycle...other than the standard warnings from tire manufacturers that specify "correct" style tires for everything that rolls, and we all know where from those litigation fearing statements come.
"Modern" motorcycle tires--as found on the Rocket and other high-performance bikes, are mostly about "style"--meaning race-track APPEARANCE "technology" being applied to street tires because they have "the look", albeit street tires are compounded completely differently and so cannot possibly offer the same level of frictional interface (grip) as a racing tire on dry pavement...and this little factoid SERIOUSLY calls into question the rationale for using such completely rounded profile tires on the street.
What "round" tires bring to the table for street use--other than "the look", is to make the bike more responsive with less rider input. On a new set of Rocket tires the bike feels as if it's rolling on ball-bearings, and lean response is instant with little rider effort. However, a bit of applied logic also reveals another fact about round tires...being so responsive to lean input, they also tend to deliver LESS feeling of stability at speed, especially in high-wind conditions. And, as we all know, a "proper" rounded street tire becomes seriously DANGEROUS the instant pavement ends due to not having tread-blocks and SIDEWALLS with tread-blocks that bite into soft stuff rather than slide around.
When I was a kid growing up living on a dirt road, riding my Honda CB-175 to school everyday--complete with "semi-rounded" tire profile on the rear, and significantly more "blocky" tread, I never "feared" riding a dirt or gravel road, but with the factory tires found today I avoid dirt and gravel roads like the plague. Get an 800 pound monster in sandy gravel--with the stock tire, and it can easily slide out from under...and in such a situation, the low CG plus mass of the Rocket is seriously working AGAINST the rider--as opposed to a 295pound dirt bike with knobbies, where the rider's weight composes a significant portion of the total--and the bike's CG is located higher relative to the rider's, giving the rider more control (tires notwithstanding).
As best I can tell, a car rear tire offers something "more" for normal riding NEEDS, mainly in being "square" in profile, as it is forced into a leaned condition, and the inside sidewall compresses (with the tread section staying MOSTLY on the ground), the resultant shape delivers superior frictional interface because it behaves like a "wedge", with a portion of the contact patch located well INSIDE the wheel rim. This is completely opposite to what happens with a now "conventional" motorcycle tire. First, the round tire doesn't HAVE to be "forced" into a leaned state because it's profile is...well, ROUND, hence the ball-bearing-like responsiveness we've all experienced. Additionally, even under the added G-force compression of a lean, lacking a sidewall (of significance), the tire does not form a "wedge" shaped contact patch inside the wheel rim, instead, the tire relies completely on frictional interface, enhanced by increased contact patch due to turn-induced G-loading.
Here is the best analogy I can come up with to create a visual impression: Take a rubber ball and place it against a surface--like pavement. Now, push the ball forward a note how it "grips" before your pushing force overcomes the frictional interface causing it to "slide" Notice with light pressure it slides easily, but if we apply additional force it "digs in" and becomes harder to slide...same thing as with a round motorcycle tire in a lean under loading. If we went on to add instrumentation to a purpose-built test-rig, using an actual mounted tire, we'd be able to determine the tire's "static" frictional interface", as well as the amount of lateral G-loading at which it would lose grip and slide merrily along....which for street compounded tires is around 1.2 give or take. In a motorcycle traveling at a steady speed in a 45 degree lean, the tire will be under 1 G. Obviously, since the R3 can't lean to that angle, under normal conditions, a street compound tire has ample safety margin on the order of about .4-.5 G for most people. To belabor a point, racing compounds are FAR more "sticky" and thus allow racing bikes on excellent pavement to achieve lean angles sometimes exceeding 45 degrees, relying on frictional interface and the hand of a pro-level rider to keep the tires from sliding out, but this is also WHY racing tires are rounded, and even somewhat "peaked" at the center, because with such grip available from the rubber compound itself, PROFILE is determined by the desire for exceptional responsiveness--NOT frictional interface (grip).
Now...back to our little home-made test rig. Mount a square profile tire in the rig and apply the same initial downward force--then try sliding it sideways...it will take OODLES more force (that's a technical term) to force a square tire sideways than a round one, but of course you say, that's because they are MADE to resist lateral force--and you're right, but here's the difference, At some specific frictional "number" the round tire will slide laterally EQUALLY whether it is oriented vertically or 45 degrees off vertical. Now, place the square tire at a 45 degree lean angle being sure to place it under the same "initial loading" (like an 800 pound monster), which, with the proper inflation pressure, will result in a tire with most of the tread area "flat" to the surface, and the "inside" sidewall appropriately flexed, or compressed into a "bubble shape"...viewed from the "end" the contact portion of the tire will have assumed at sort of "wedge" shape. Now start applying pushing force to move the tire laterally to the surface and note how much is required, THEN add-on the force of 1G to mimic a 45 degree lean force. You will find--assuming EQUAL rubber compounds, the square profile tire will require substantially more lateral "sliding force" to move it as compared with the round tire. The relatively much larger contact patch of the square tire under leaned compression will have more resistance to sliding--a fact reflected in the comments of EVERY person who's ridden one on a motorcycle. Whereas one must be conscious not to use too much throttle in or coming out of a lean on a round tire, the square profile can handle significantly more without slipping, NOT just due to a larger contact area, but also that little "wedge effect" that is working to prevent the tire from rolling further, creating what is best described as a mechanical interference fit between the tire and road surface.
So WHY the question begs don't the manufacturer's "know" this...certainly they DO know it, but there are other factors to be considered. Most notably the FACT that a square profile tire mounted at the rear of a two-wheeled, tandem vehicle manifests some undesirable effects...again, reflected in the comments of those who've ridden on them. The rear tire will tend to feel more "squirrelly" over pavement irregularities than would a rounded profile. This is because the contact patch is wider--meaning farther from the virtual centerline, as well as lacking a "gradual" radius. Because of this, whenever the side area tracks over an irregular spot, it will have greater tendency to be "shoved sideways", combined with the point of force being located farther "outboard" of the centerline...acting as a magnifier of such forces. This is most noticeable at slow speed for the same reason as in a car--more contact patch, but as speed rises, tire contact surface is reduced, starting from the sidewalls inboard to the center, thus creating in effect a small radius across the tire. For those with the resources and desire to experiment, it would be useful to try going with a narrower square profile tire to find the optimum marriage of car tire to bike. Without any doubt, going to a narrower tire on the Rocket...say 205-215 section would result in less "squirreling" of the rear at slow speed, however, this would of course tradeoff some of the ultimate cornering grip, not to mention a reduction in the visual "wow factor".
So why don't the manufacturer's make "semi-square" tires for motorcycle rears--they do in smaller sizes and for "period" bikes, using generally less-refined compounds, but there is a reason for what is widely available today. First is what I stated above...round tires have "the look" most people find visually appealing--they LOOK just as aggressive as those seen on superbike racing. Second, modern compounds are capable of delivering adequate STREET traction in a round profile, at the same time exceptional "roll response", AND eliminating the negatives of tracking a large contact patch over irregular road surfaces. Much of what makes a Rocket riding on brand new Metzlers is the sportbike "feel" for such a heavy bike. Also notice how much MORE the superwide rear tire on the R3 is affected by road irregularities than 160 section on a lighter machine--even WITH the round profile the larger contact patch moves the point of contact sufficiently farther to the outside to magnify the effect of tracking influences.
The real reason a wide tire, worn "flat" down the center tends to feel more "squirrelly" is NOT due to absence of tread--look closely and you'll find the tire has NO tread down the center when new, but due to the now relatively wider contact area, with a slight "peak" at it's edge, combined with ever-so-slightly lower weight-to-contact patch area, causing the bike to be more sensitive to road surface irregularities. The reason "modern" motorcycle tires don't last any longer than they did 20 years ago isn't the result of any LACK of engineering, but BECAUSE of technological advances in compounding that allow a more "optimized" tire for most street riders--super-responsive, yet less squirrelly round profile, combined with more "grip"--on pavement, with the sacrifice being tire life.
Handling differences seem modest on the Rocket, yet other types of bikes would be more affected. Sportbikes most of all due to their much shorter trail numbers and lighter weight, which is why there are as many "naysayers" as proponents of going to "the dark side".
If you're a person who demands the optimum in terms of handling, with as few negative effects as possible, factory tires are the way to go, however for those who understand the differences in handling, and tend to drive a lot of miles, wearing out a tire down the center--mostly where "cruisers" ride, then a car tire quite possibly has merit--on the ROCKET, but this does not automatically transfer to all TYPES of bike.
I can see the point on both sides of the issue. I've had my Rocket almost two years and the original rear tire still looks "new" with 4000 miles on it...too many impediments to getting out and riding as much as I want, but even so, based on how my tire looks now, by the time I need a new one the cost won't be a factor. However, if I were able to get out and do some serious coast-to-coast riding, THEN going to something like a Goodyear Eagle F1 would look mighty appealing. My basic position is in the "dark side" camp because people "experimenting" on their own, always outside the bounds of "official sanction" is the mother of innovation. A perfect case in point is; 30 years ago in the aircraft industry, METAL was the preferred, official, and authorized construction material, yet thanks to the "backyard" tinkering of a then unknown engineer (who is now world famous), "composite" materials came FIRST into common usage amongst "homebuilders", then moved UPSTREAM into the commercial and military sectors, yet even with the proof of history bearing down on them, to this day there are still many who cling to the "traditional" construction notion. Another corollary is firearms....the "plastic" M-16 appeared in 1957, was eagerly embraced by USERS, while the "brass" attempted to dismiss it as not being "proper" according to "tradition"....yet here we are 49 years later with the M-16A2 having the distinction of being the "issue" rifle in ALL branches of the military longer than any other type--way, WAY longer than the M-14. That speaks for itself regardless of "naysayers".
So to all the car tire enthusiasts out there I say, DRIVE ON, keep experimenting, push the envelope and don't let detractors who've not been where you are dissuade you. For the rest, enjoy what you feel is "proper", but be mindful of it's origins--intrepid experimenters.