There are a lot of good technical tire articles on the web, but being too technical is not the point of this. I want to concentrate on the things that matter to safety and to our upcoming trip, and maybe blend in a little technical and cost information that you may not have heard before. So let’s start by examining the two most popular choices of tires for my bike, the BMW R1200GS Adventure, and then you can take the knowledge on to almost any tire decision for almost any vehicle.
The two most common tire types for my bike are the Metzeler Tourance and the Continental TKC 80.
The TKC 80 is a classic “knobbie” tread design that is approved for both on road and off road use. Many riders swear by TKC’s preferring to have them on their bike at all times. Claims are made that they are good on the road and great off road:
The Metzeler Tourance tire is definitely more of a street tire, but is also considered suitable for most non-hardcore off road riding:
Of course the ideal solution would be to have a street tire on while on the street, and an off road tire on while off road, but that’s just not practical or inexpensive. Some people solve that issue by having two sets of wheels, each mounted with a different set of tires. That way when they are commuting or traveling on road they use a street tire, and when off road they use the knobbie. With cross-spoke wheels costing around $1,000 each, not including duplicate brake rotors and ABS ring, this is simply too expensive for most.
Why does BMW use a cross-spoke wheel? Well, a solid aluminum wheel does not flex well when struck with a large force, like hitting a large rock or a curb, they tend to break. That is not very fun if you have a big heavy bike and are off road!
If you examine the picture below (taken with a wide angle lens that makes things on the edges appear bigger), you will see that the spokes, unlike most bikes, are STRAIGHT. That makes them stronger. They also attach to the rim of the bike on the outside of the wheel versus the inside. This allows a tubeless tire to be used (versus one with spokes in the middle), AND it allows the wheel to FLEX when striking objects without bending the spokes too severely. I can testify that I have hit some very large objects and have been amazed that wheels are still round!
This brings me to the discussion about tire pressure. I’ll come back to the Tourances and TKCs after awhile and offer my opinion, but let’s talk about pressure…
On my bike for two riders (or heavy –which is always for me), the manual recommends running 41 psi in the rear and about 37 in the front. This is a good street pressure, but I think most people find that running a TKC off road at those pressures makes them feel hard and doesn’t allow the sidewalls to flex as much, thus it doesn’t allow the tire footprint to spread out.
OFF ROAD:
While riding off road with EITHER tire, but especially the Tourances, my technique is to deflate the front tire into the 22 to 25 psi range and to deflate the rear into the 25 to 27 psi range. Be cautious and aware of a couple of things if you do this… under-inflating the tires leaves the wheels more susceptible to damage from striking objects, that is why I do not deflate less than 20 psi under almost all circumstances.
Also, the TKCs ride okay, but not as well, if you ride on road with low pressure, but the Tourances with low pressure definitely degrades on-road handling characteristics. Thus, do not be lazy – if you deflate, re-inflate! There are several more good reasons to re-inflate when back on pavement besides handling, one being fuel economy, another being tire wear, and yet another big one is SAFETY that has to do with hydroplaning and I’ll cover that one in a minute.
Deflating the tires DEFINITELY makes the Tourance handle like a completely different tire off road than on the street. In fact, when on gravel or any HARD off road surface, I find that I don’t even notice the difference between the two tire types. However, get the Tourance in mud or in soft sand and then it definitely does not have as much “grip.” That is where having the TKCs really pays off, but everything’s a compromise isn’t it? We’ll get back to that thought.
To summarize so far, I would say that the Tourance is the equal of the TKC off road in most dry conditions as long as it’s deflated. In wet or sandy conditions, however, the TKC will vastly outperform the Tourance. That’s off road, how about on road?
ON ROAD:
Here there is even more to consider in my opinion. The Tourances are without question smoother, quieter, have a lower propensity to wobble, and wear at least twice as long as the TKC. Why?
Tread design is just one reason why, the compound of the rubber is another.
The tread design obviously affects the way water is routed out of the tire footprint. It also affects the noise as the edges of the tire strike the pavement. The shape also affects how the bike handles as the tire is leaned onto its side. For example, a new TKC rides mostly on the center row of knobs when upright when the tire is new, and as it wears will flatten out and actually increase the size of the footprint. That happens a little on the Tourance but not as much.
The compounds of the tire are very important for both longevity and grip. The only reason the TKC can be certified for use on the road is because the compound is soft enough that it will grip more on a hard surface than a hard tire compound would on a hard surface. That COMPROMISE means that the tire wears much faster. Still, in some instances that softness is not enough to ensure a good grip, and that’s important enough to spend some time talking about.
Both these tires are RADIAL tire designs. That means that they hold the road better than a non-radial tire, but instead of losing their grip progressively, they tend to lose it all at once. I had that happen not too long ago when, on wet roads and in heavy traffic on a two lane road, I was following behind a minivan who stopped suddenly as the car in front of them stopped suddenly to make a left hand turn across oncoming traffic. I was following too close and got on the brakes hard enough that the anti-skid (ABS) began to cycle. It cycled all the way to a stop only two feet behind the rear bumper! That was with the TKC tires, and I KNOW that it would not have been even close with the Tourances. The Tourances are a WAY, WAY better tire on wet asphalt, especially for stopping distance with a heavy bike.
To summarize, when on road, the Tourances are much quieter, much smoother, have better braking performance on dry asphalt and MUCH, MUCH better braking performance on wet asphalt.
When on road, I try to run the Tourances at 37ish front, and 41ish rear. I run the TKCs on asphalt at about 35ish front and about 37ish rear, but again will deflate BOTH when leaving the hard road surface.
HYDROPLANING:
One reason to run the tire pressures at a higher pressure on road is to delay the onset of hydroplaning . Hydroplaning is a condition where the surface tension of the water is preventing the tire from making direct contact with the road surface. There are actually several types of hydroplaning, our discussion will center on viscous hydroplaning which is the most common and again is simply due to a film of water between your tire and the road.
Imagine, if you will, a level and flat slab of concrete with a quarter inch of standing water on it. If you take a vehicle and park it on the concrete, the weight of the vehicle is going to push the tire surface against the road surface and break the tension of the water. A much different dynamic occurs with speed. As the vehicle speeds up, the water may not have enough time to “get out of the way” and the surface tension of the water “floats” the tire in the same way that a water skier stays on top of a lake at speed and sinks when stopped.
LOL, now that’s funny. Look, even a KTM can do it…
No, I won’t be trying that on my Adventure any time soon!
The speed at which the vehicle tire stays on top of the water is the hydroplaning speed.
Motorcycles, in general, are less susceptible to hydroplaning than cars by virtue of their thin tire design. The variables to hydroplaning speed are (not necessarily in order of effect):
1. TIRE PRESSURE – This is the Number ONE determiner of the speed that a tire will hydroplane (of the variables that you can control). That is not intuitive to most people, so let’s examine it a little closer… if the slab of concrete had grooves cut in it and was sloped, then the water would begin to run off and the peaks in-between the grooves would be above the water and could meet your tire, thus breaking the surface tension of the water. Without grooves in the concrete, then it’s up to the tire to have grooves that allow the rubber in-between to pierce the surface tension and make contact with the road. Imagine a large flat inflated plastic pool lounge sitting on a road with standing water… even though the water is not deep, it will still float an empty inflatable. Now imagine a knife hitting the wet road on edge… the knife will definitely break the surface tension. While this may sound very simple, experiments with tire inflation and hydroplaning prove the dynamic.
In fact there is a simple formula you can use to calculate the speed at which the average tire will hydroplane... it is roughly 9 times the square root of your tire pressure!
Thus, if your tire is inflated to 36psi, the square root is 6, and you would multiply that times 9 to come up with 54 mph. At 41psi, the calculation would produce a 58 mph viscous hydroplaning onset speed.
Actually, the multiplier does vary with the other conditions that act upon hydroplaning speed and it can vary from as low as 8x with poor conditions to as high as 10.5x with optimum conditions. So, this formula is an approximation only, don’t bet your life on a mph or two! It will put you in the ball park, however, and shows that if you only inflate a tire to 20psi then your hydroplaning onset speed can be reduced to as little as 40 mph or less (keep your wife’s car tires inflated please…)!
2. Tire width – Obviously, a wide racing slick will hydroplane sooner than a thin motorcycle tire. Once your tire is installed, however, you lose control of this variable as you do most of the others except for weight.
3. Vehicle weight – the heavier the vehicle, the HIGHER the hydroplaning speed, but the effect is not nearly as great as increasing tire pressure.
4. Tread design and tire wear – At high speed, grooves do not “carry the water away” as most people believe. The grooves simply provide ridges so that that portion of the tire is not as wide and can more easily break the surface tension of the water. Think of the pool raft versus the knife. This sounds like semantics, and it is somewhat, but at high speeds water simply cannot displace at the speed of a fast moving tire no matter the water being thrown up by the tire, that’s just an after effect.
5. Road surface condition – This is why concrete runways and some highways are grooved. Peaks in the road surface penetrate the surface tension. This is also very important in stopping distance where a locked tire on water can quickly create what is known reverted rubber hydroplaning. A locked tire on water quickly heats a microscopic layer of water to the point that it turns to steam and floats the tire! It can even MELT the tire! Not kidding, this is obviously way more likely at high speeds and can and does happen to airplanes not equipped with anti-lock brakes.
There are several other variables, but their effects are not as great. Below is an excellent technical article for those who may want to learn more:
Breaking traction with the road is NOT just a wet road phenomenon…
Riding a motorcycle is, in fact, very similar to flying in that it shares some dynamics, like the dynamics of turning… when an airplane turns, lift is broken into two components, a vertical component and a horizontal component:
In level turning flight, the vertical component of lift must always equal the weight of the aircraft or it will not hold altitude. That means that the wing has to generate more lift than the aircraft weighs to support the turn. A level turn at a 60 degree angle of bank will produce two G’s, or twice the weight of the aircraft must be supported by the lift of the wings.
The same exact correlation is true with a motorcycle.
When you go leaning up in a corner, the tire contact area, instead of the wing, is supporting the combined weight of the bike PLUS the centripetal force of the turn. Street tires are round so that the contact area stays large as you lean the bike, this is another disadvantage of a knobbie tire design and one you should be aware of if you’re leaning hard at 110 mph (I’ve never done that)! The truth is that when you do that, you are “on the edge” in more ways than you may realize, and if that’s your thing, then knobbie tires are not for you (I know there are people who will argue this point and point to their past successes – that’s great until the day you meet the marginal condition and the tire breaks free)!
Motorcycle wheels and tires, when in motion, become great big gyroscopes as most riders realize:
Gyroscopes have more force the faster they turn and the more weight that is distributed towards the OUTSIDE of the gyro. Thus a heavy tire and wheel will have more gyroscopic force than a small one. This is why I love the Adventure when it’s up to speed… at 45 mph on a gravel or dirt road the force is so strong that you can almost literally not fall over if you tried!
But gyroscopes are tricky when you apply a force to one in motion. If you can imagine standing to the side of a spinning wheel and pushing on the top of the tire, you would NOT lean the tire away from you by doing so! No, it would swing inwards to you from the side ahead in the plane of rotation. That’s because all gyroscopic forces act upon a gyro 90 degrees ahead in the plane of rotation.
Why is that important to know? It’s the reason that the handlebar forces reverse at speed, and the reason that many amateurs have accidents when their natural instinct is to steer away from danger… that leans the bike in the opposite direction intended and causes it to go into danger.
Here’s a fun experiment to try on a long and lonely road… at highway speed, balance the bike up and take both hands off the bars. Stick your right hand straight sideways into the wind and what direction do you think you’ll turn? Most people would think the drag on the right will turn them to the right, but in fact you will turn left. That’s because the drag force acts 90 degrees ahead in the plane of rotation upon the gyros that are holding you up and allowing you to take your hands off the bars! The faster you move, the stronger the force.
PICKING THE RIGHT TIRE FOR THE RIGHT JOB:
That may have been pretty basic for the experienced riders, but here’s my take on the two tire choices for my bike and the trip through Central and South America…
The vast majority of the riding will be on road and thus I will be riding on the Tourances. My line of logic goes like this… Safety is the number one priority. Most riding in mud and loose sand is at relatively lower speed than highways. I would rather be stuck in the mud or fall over in sand than lose traction cornering on a wet road and slide into the path of an oncoming truck! Sure, there are risks both ways, but to me the higher risk is on the road and not off road. Yes, I love the TKCs for offroad, no question. But they are inferior on road and on a 20,000 or 30,000 mile trip will be a pain to have to change all the time and we haven’t even addressed cost yet.
WEAR & COST:
My experience with wear for the way I ride (fairly aggressive) is that the Tourance will last 7,000 to 8,000 miles. The TKC will last 3,500 and that’s it. Both tires cost the same to buy and to mount and balance. Right now you can buy either for about $270 to $300 per set online or about $350+ plus another $100 to install at a dealer.
That’s $450 per set. If you divide $450 into 8,000 miles, you will find that the Tourances cost about 5.6 cents per mile. If you divide $450 into 3,500 miles, you will find that the TKCs cost about 12.8 cents per mile, or 2.3 TIMES as much!
Here’s some fun with calculators that will probably get your attention, it got mine… my bike gets 40 miles to the gallon under almost all riding conditions (except speeds above 80, not that I’d know, lol). If I pay $2.70 per gallon for gas, then my cost per mile just for gas is about 6.75 cents, only slightly more than the cost of Tourance tires. But here’s the rub, I bet you didn’t realize that the cost of riding on TKCs, at 12.8 cents per mile, is TWICE the cost of FUEL! So, complaining about $3 gas is really not cool when you’re paying so much for knobbie tires! Oh yeah, they do look cool!
I know there are a lot of people who swear by their TKCs for everything. That’s also cool as long as they can spell out their arguments in favor and are riding within the safety margins provided by the tire design. I love off road riding as much as anyone, but my experience is that a partially deflated Tourance is hardly distinguishable off road from the TKC. The same cannot be said on road.
Of course there are other tires that are suitable for my type of bike, they all have their plusses and minuses, as the design of a tire is like anything else - it’s a compromise, a trade off of one quality for another.
As I ride 20,000 miles PLUS over the next four months, I’m planning on burning up 3 sets of Tourances. It would require at least six sets of TKCs to do the same thing. Of course your mileage may vary and I’ve heard stories of people getting thousands of miles out of a TKC… my question would be what fun is that?! Hey, you may be right, I may be crazy!
Did you know Billy Joel is a bike collector/designer?
Your tire debtate is very interesting. Bicyclists (us no-engine guys) who are out for "speed" (as in 19mph average or better) all use slicks. Their ratio of weight to contact area is too great to ever hyddroplane at speeds they are capable of going. Much better traction can be had with slicks than with any tread pattern. Of course, off road is a completely different matter.
ReplyDeleteSo, with human powered bikes, it's more of a no-brainer. Road=slicks. Off road=knobbies.
"The vast majority of the riding will be on road and thus I will be riding on the Tourances. My line of logic goes like this… Safety is the number one priority. Most riding in mud and loose sand is at relatively lower speed than highways. I would rather be stuck in the mud or fall over in sand than lose traction cornering on a wet road and slide into the path of an oncoming truck! Sure, there are risks both ways, but to me the higher risk is on the road and not off road. Yes, I love the TKCs for offroad, no question."
ReplyDeleteYou say that now. Let's see how you feel if it's at all muddy in SA... Personally, I find knobbies (Husky) to be adequate on road. The front knobby will slide on pavement during assertive stops vs the Pirelli DS tire I normally run, which is good enough to do stoppies with. Traction is very different.
However offroad, the knobbies're the way to go hands down.
"But they are inferior on road and on a 20,000 or 30,000 mile trip will be a pain to have to change all the time and we haven’t even addressed cost yet."
Let's be honest - it's about the $$ and the convenience factor. :) And how many dirt roads are you going to run, anyways? Too bad they don't make a Dunlop 908RR in your size.
Oh, you're right about that, Eric! It is all about convenience and $, but it's also about having a better tire where it's most important, and that's on the road. I agree that there will be times I'm wishing like hell that I had the TKCs, unfortunately I can't have everything I want all time, wish I could!
ReplyDeleteInteresting observation about bicycle tires, Thomas. There's the knife on concrete scenario in regards to hydroplaning for sure.
ReplyDeleteBTW, if it were a short trip I'd probably take the TKCs... and depending on conditions, I may actually run the TKCs for awhile if Arno and I plan on any serious legs off road.
ReplyDelete