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Motorhead Memo: A loaded question

By Kip Woodring

Far be it from me to let the wind out of your sails (or the air out of your tires), but has anyone really thought about what’s behind what’s up front on trendy custom baggers? I’m talking about the fact that for some unknown reason it has become very fashionable to ditch the OE 16″ front wheels and tires and replace them with an 18″ or even 21″ combo just for looks.

The trouble is nobody’s paying attention to the ramifications of such a change—except the tire manufacturers, and they are decidedly displeased with the 21″ notion (see: attached “dealer” form letter from one of their distributors)—although Metzeler (and Avon) make 18″ front tires that skirt the issue nicely. This whole deal is far more complex than it might appear at first blush. I’m not a tire expert, but it seems to me their point is well taken, yet not completely logical.

Real deal or CYA?
If it helps to set the stage at all, the best example to use to begin our enlightenment process, might well be from a pickup truck! Some years back Ford decided to hotrod one of their workhorses into a racehorse—the so-called Lightning. Truck freaks will know a lot more about the details than I do but the big change, most germane to our discussion, was the switch from “regular” pickup wheels and tires to “dubs” about three or four inches taller and seriously low-profile sporty-type tires to go with. Well, that change alone meant if one had the standard truck upon which the Lightning was based, one could tow a buttload (more than 5,000 pounds anyway) and bed payload was, well, about the half ton you’d expect from a half-ton pickup. On the other hand, thumbing the pages of the owner’s manual of a Lightning would yield a shocker in that area of the specifications. Want to guess what the tow capacity of a Ford Lightning is? Zero! Nah-dah! Zip! No can do! Same flippin’ chassis, body, (basic) suspension and more than “plenty” of horsepower, but useless at a truck’s main function. Hauling (other than your derriere) is simply not on the Lightning’s job description for Pete’s sake—because of those bling wheels and rubber band tires!

OK, if a simple swap of wheels and tires on a truck can have such a profound functional effect, what do you suppose it does to a dresser?

I’d venture to say none of us are chassis engineers, let alone gurus of payload distribution and vehicle dynamics on our single-track machines, but anyone who’s ever ridden can verify the fact that the addition and location of mere pounds can affect stability and safety as much or more than adding tons to a truck ever could! More importantly, I believe there’s more at stake here than carrying capacity alone.

Rubber side down—which side do we take?
Mind you, none of the tire manufacturers are warning us adamantly about some of the silly things we do with rubber in the rear. It’s those front tires, notably the 120/70-21 size, that they’re freakin’ about. Among the questions you have to ask yourself are: “Why that size? Why the front? Why the hell not?”

Why that size? This sorta falls under the same category as the Lightning truck deal. Mainly because less sidewall means less ability to safely sustain a payload (or sudden shift of same) in emergency situations under all conditions. You simply use up all the “cush” quicker, since there isn’t as much of it (vertically) should a crushing shot come from some stutter-bump or pothole. That wall of rubber acts as a spring as well, and the volume of air, as well as the volume of rubber involved, affects things hugely.

Why the front? This seems to me both the simplest and most complex aspect of this issue. First off, the front wheel/tire is the one where most of the action is and it can be pretty radical action to boot. Think about wheelies. When one pulls daylight under a front tire, then slams it back on deck, you have loads involved—to say the least—that might meet or exceed the limits of even the stoutest tire. We’ve even seen cast wheels crack from wheelies or high-speed whacks of a dozen varieties. Frankly, rear wheels don’t have it half so hard. Add the fact that in the rear the trend is bigger whilst up front tall and tiny is the happening thing. Smaller, obviously, means even more stress and operating closer to the limit all the time. This leaves damn little margin in a crisis, which is the crux of the issue. That’s the simple part. The complex part emerges from pondering in depth the logical probabilities, emanating from the four basic functions of a tire.

  1. Tires must support the vehicle load.
  2. Tires must transmit traction and braking forces to the road surfaces.
  3. Tires must absorb shocks from the road surfaces.
  4. Tires must change and maintain the direction of travel.

Vehicle load indeed!
There’s a load-rating chart somewhere close to this body of blather—look at it! If you’ve already sneaked a glance at the “letter” referred to earlier, you can see that by switching to a fat 21″ tire on a bagger you just gave up roughly 200 pounds of load-carrying capability. An awful lot! Certainly enough to make the tire outfits sweat, and their concerns are legitimate in absolute terms. Bikes with fairings are right on the ragged edge, without doubt, since they have as much as 40-odd extra pounds or so hangin’ off the front end, that Road Kings (typically) do not. So forcing a tire intended for lighter loads to lug an extra burden, is asking for it… isn’t it? Well, if it is, it isn’t like there’s no precedent.

On runs or events, am I the only one who’s seen a huge bedroll tied to the forks of a Softail… or Dyna Wide Glide… or Springer… or Chopper… or whatever? S’pose those riders realize they’re pushing the load limits of their front tires? Never mind that strapping that stuff to the handlebars has a fairly disastrous effect on steering and therefore is a danger in and of itself—we’re talkin’ about tires! Yet, I’ve never seen or heard of a crash or hassle directly attributable to packin’ too much up front for the tires to handle—have you?

For the record, the load capacity on stock, factory 19″ tires is between 441 and 507 pounds. OEM 21″ tires “weigh” in at 467 pounds of payload capability. Also, it should be noted that even though those factory fairings are on the forks or the frame up near them, there’s very little difference in weight distribution. For instance, a Softail carries approximately 46 percent of its poundage on its front skin, whereas an Ultra Classic (full fairing and all) mounts about 43 percent of its total tonnage on that fat 16. So what’s really up with 783 pounds of payload “reserve” on a bagger or FL Softail? Well, there’s only 27 pounds between the FLSTC and the slightly more corpulent FLHR, but there’s a whopping 169 pounds separating the lightest Softail from the heaviest bagger. It would therefore seem there’s a tire-to-weight ratio of sorts at work, with the end game a practically fixed safety margin of some 300–400 pounds of load per tire for rider, passenger and gear. That margin can become nearly nonexistent with a “21 swap” on a (loaded) bagger.

While you’re pondering that, on to number two for tires—those pes-ky traction and braking forces—and how they are transmitted to the road. All Harleys, except baggers, have pretty “lazy” rake and trail numbers. Baggers have relatively “quick” rake and trail specs for that marvelous balance of stability and agility we all love about them. Then, there’s those unique “backwards” triple-trees on all touring models (a sort of pendulum effect that makes for quick, effortless steering) to consider. Switching to a 21″ doesn’t undo all that. If anything it puts some of the dynamics of a system set up for a 16″ into hypersensitive mode. Though the actual rolling diameter of either size tire doesn’t change much, the contact patch does—not so much statically as dynamically. (Think about why those guys in the olden days began swapping to 21″ tires on old Pans and Shovels… quicker steering!) You may already know—oh, say—one square inch of rubber on the road is all you get with either a skinny tire or a fat one, but the fact is the shape of that contact patch is way different. Going around corners with extra weight on a long narrow patch (such as a 21″) might affect handling and will almost certainly generate more heat in the tire’s carcass. Too much of that could mean heat-soaked rubber coming off in chunks or squirming like a greased weasel—at the worst possible time. In short, a skinny tire simply does not work the same way with a bagger’s forks and steering geometry as it does with other hogs’.

You’ve read in this column (at length) about the fact that tires are also what really stop you. No Harley equipped with dual front-disc brakes comes with a 21″ tire. Figure that dual discs generate more actual power than your engine (they do) and that if they gang up on that long, skinny contact patch too much (or too easily) you could find a whole new dimension to the expression “skidding to a halt.” Just be sure to bring clean underwear, in that case.

Moving on to number three—absorbing shocks. Come on! Has everyone forgotten why we got “balloon” tires on Harleys in the first place? The fatter the tire, the more the “cush”—period! True then on rigid frame stuff—true now on air-suspended, hydraulically damped baggers. Baggers get a more comfortable, capable ride from factory 16″ tires than they ever could from any 21″.

Finally—number four—“change and maintain direction of travel.” No argument (for some reasons already stated) that a 21″ does these things as quick or quicker than a 16″—up to a point. Or should I say a razor’s edge? Push a 21″ hard on a winding road, or try some downhill switchbacks, and you just might learn what the road racer types mean when they say a tire “goes off”… simply ceases to stick. In the end, this might be the real case the manufacturers are making—that you can completely overwhelm a smaller section tire with an inferior load rating—aboard a heavy, high-powered machine ridden like it was meant to be.

If you’re out there doin’ burnouts, wheelies and other motorin’ mayhem with a 130 hp bagger – literally riding the tires off it, might be just what happens – if there’s a 21″ up front! The info on this chart and/or the writing on the (side)wall are not to be taken “lightly” when it comes to tire selection for heavy motorcycles. One of the most underappreciated, ignored and abused ratings there is might be the most critical of all! If you literally ride to live and want to live to ride, these are the guidelines that will help it happen. They pay damn good engineers a lot of money to figure this stuff out for your safety, so don’t think you can easily outguess them. When it comes to a choice between what looks good and what works good, you don’t have to guess which line I’m in. How about you?
I’d bet this was mandated, if not actually written, by the liability lawyers who work for this distributor—or Metzeler—or both. There’s not a word of it that’s not absolute fact, and Avon or Dunlop (or whomever) certainly concur, with or without a formal letter to the effect. No 21″ tire from anyone has a sufficient load rating for bagger duty—yet. But there’s room to read between the lines a bit as well. Road Kings have the advantage of less weight on the forks than fairing-equipped baggers, but all the same: If you never get close to max payload on your bagger, or never, ever ride hard enough to crowd the tires (whatever size), and your real riding happens between coming off the trailer and picking up the trophy (or trophy girl)… ’tain’t that big a deal in unreal life
The highest load rating for a factory Dunlop 19″ tire (as you see here) is 61, which equals 567 pounds. The 120/70-21 tires are all rated at 62, or 584 pounds – essentially equivalent. What’s been bugging me about all this, and what I know for a fact, is that for over a decade Harley-Davidson built a police bike (the FXRP) with as bulbous and heavy a fairing as ever graced any bagger. These bikes were used for high-speed pursuits, with all that weight up on spindly 35mm forks, using what are now apparently considered “underrated” 19″ tires! No problems! Ask yourself how we got from that point to this—and see if any rational answer doesn’t seem like litigation rather than engineering. Like I said, I’m no tire expert, but I do recall a little “living” history, after all.

Deflated dreams or inflated expectations?
On the other hand tires, regardless of size or brand, are a personal choice. Like most things that make this sport a sport, not a mere pastime, there can be more than a hint of danger in the results of those choices. What the hell—the truth is most of what we’ve discussed would qualify as extreme conditions issues anyway and lots of folks are getting away with it (and worse) just fine in spite of warnings of dire consequences. So, now that you are aware of the downsides (and the possible high-sides) of fooling around with 21″ front tires on your bagger, if you want to do it anyway, it’s a free country after all. Let’s just say, in the words of Dirty Harry, “A man’s got to know his limitations.” So, if you choose to gamble on this game of 21″, don’t come crying to me, or go gunning for the tire company, if it don’t work out. Because you can’t say you weren’t warned!

LeMansLetter

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