Testing Front Tires

TESTING FRONT TIRES

Motorcycle tires aren't all the same. Easy to say, difficult to prove. But it can be proved. CYCLE WORLD published the first comparison of 15 motorcycle tires in August, 1978. The job isn't finished.

The first tire test evaluated tires for cornering on both a wet and dry skid pad and dry pavement braking. All tires were mounted on the rear of a Suzuki GS750 equipped with a set of stabilizing outriggers for skid pad tests and on the rear of a standard GS750 for braking tests and subjective cornering evaluation. The tests answered most of our questions about tires but not all of our questions. Braking forces on a rear tire are tiny when compared to the braking forces on a front tire. How would the tires compare when mounted on the front of the motorcycle?

To find out, a second tire test was started. Letters went out to the tire manufacturers again and tires were gathered. Not all the tires tested in the original tire test are represented in the front tire test because some tires didn’t arrive by our deadline and weren’t available in our area. The Formula and Metzeler tires were left out.

Testing front tires is not the same as testing rear tires. Some of what the rear tire does, the front tire also does. But there are differences in what the two tires do and there are differences in testing front and rear tires.

Take cornering, for instance. Put a motorcycle on a constant radius skid pad and gradually increase speed until the tires break loose and the rear tire will break loose first every time. The rear tire has greater side forces during cornering because the rear tire supports more weight. The rear tire is also transmitting power during constant speed cornering; the front tire is not.

Front tires, however, do slip. They slip over surface irregularities or when turning and braking at the same time. A front tire can also be locked up during heavy braking. One way to compare front tires’ cornering ability would be to install each tire on the CYCLE WORLD outriggerequipped motorcycle used to evaluate rear tires and move the outriggers forward to lighten the front end during cornering. Only it didn’t work.

With the outriggers moved forward enough to lighten the front end the outriggers were no longer passive supports but part of the active support of the motorcycle. Circling the skid pad at the limits with the revised outriggers had the front wheel bouncing wildly up and down, the front handlebars shaking back and forth. There was obviously a conflict between normal steering geometry and that with the outriggers. With the outriggers returned to their original positions only the rear tire would slide. No way to test a front tire.

Front tires can be tested and can be compared. There’s braking ability, for one. Most of the stopping power of a motorcycle comes from the front brake. As an example, the average stopping distance from 60 mph using only the rear brake, for 15 tires previously tested was 267.5 ft. The average stopping distance of 13 front tires, using only the front brake, was 156 ft. Not' only can tires be compared for braking ability but the differences are more important for front tires than braking figures for -rear tires.

To differentiate between different front tires’ cornering limits the tires were mounted on the front of a stock Suzuki GS750* which was run around a test loop at high speed. The track included different surfaces, providing a measure of traction at. the slippery boundary of two surfaces, painted lines and enough room for safe operation at speed. Each tire was run 10 laps. A lap began with a large diameter constant radius left turn exiting into a high speed straightaway followed by a tight right turn, followed by a return straightaway, an 80 mph sweeping righthander and a braking transition back into the large diameter lefthand circle. Sort of an elongated, crooked figure eight. When ridden at, racing speeds by an experienced road racer, differences were apparent. We got the information needed.

Testing was performed on a test track at Long Beach Memorial Stadium. Mounted on the rear of the test bike was a Michelin M45 4.00-18 tire which was felt to provide^ the greatest cornering ability and safety based on our earlier tire test. Because the Michelin is a 95 aspect ratio tire it worked well with tires ranging from the 85 aspect ratio tires up to 100 ratio tires.

Thirteen Street Tires Evaluated for Braking Ability, Handling and Control.

One test considered but not performed was an evaluation of handling on rain grooves. Rain grooves, those tiny cuts made in concrete pavement to prevent automobile aquaplaning, can cause serious handling problems for motorcyclists. Not all motorcycle tires respond equally to •rain grooves, some being better and some worse. The problem is evaluating tires fairly. Because a tire may work well on a Suzuki GS750 doesn’t guarantee that it will work well on a BMW. Then, too, not all rain grooves are the same. California conducted two studies several years ago on how rain grooves affect motorcycles. As a result of the studies, rain grooves in California are all cut at the same spacing and with the same size blades. Not so elsewhere. New York, Oregon and Washington use different size rain grooves.

Tire manufacturers are aware of the problem and have been working on it. Notice that only one of the front tires tested is a straight rib design. Not too many years ago nearly all street motorcycle front tires were ribbed. Turns out nonribbed tires are less affected by rain grooves.

Some motorcyclists have said rear tires are part of the problem with rain groove handling effects. Because testing combinations of front and rear tires over different size rain grooves in different states was unfeasible, the rain groove test was dropped. If rain grooves are a problem, consider installing something other than a straight ribbed front tire.

In the charts which follow is information about each tire tested. Some of the information (model name, size, price, cord material, maximum load and ply rating) was provided by the tire manufacturers. The rest of the information came from CYCLE WORLD measurements.

Diameter differed by 1.5 in. from the smallest (the Cheng Shin) to the largest (the Bridgestone). Width varied from 4 in. (Yokohama and Bridgestone) to 3.5 in. (Nitto and Nankang). Aspect ratio went from the 85 of the Cheng Shin to 100 on the Carlisle. But the Cheng Shin had the largest contact patch (12 in.) while the Pirelli (7.25 in.) had the smallest. Some figures included in the earlier tire test are not included on the charts this time be cause all the tires were the same. All tires were load range "B" and all started the test laps at the same temperature, 82°. Initial pressure on each tire was 25 psi-as recom mended by Suzuki-and the final pressure on all the tires was 28 psi, the increase caused by heat. Locked up braking dis tances were omitted because locked front wheel braking couldn’t be safely tested.

Durometer readings, a measure of hardness, are included but the significance of the figures, themselves, is questionable. As the charts show, tires with harder rubber (higher numbers) do not necessarily require longer distances to stop or have less traction. Temperature figures are just as curious. Tire engineers have told us temperatures aren’t important unless they exceed 200°. The highest tire temperature recorded was 115°.

Braking distances listed are the best recorded stop, using only the front brake, of three test stops. Baseline stops, repeating the tests on a tire already tested to ensure objective results free from subjective error, yielded the same results. Besides braking distances, control during the stop* is listed. Some of the tires exhibited a tendency to lock up at the end of the 30 mph stop, some tended to lock up earlier.

Avon

$52.50

Bridgestone

$40.30

Carlisle

$23.98

The most controllable tires could be braked hard but kept from locking.

Overall, the tires which stopped from 30 mph in the shortest distance also stopped quickly from 60 mph. The inconsistencies, for instance the Cheng Shin, came from tires which were difficult to control and locked easily. The chart lists tire stopping ¿■distances, in order of combined stopping distance.

SUBJECTIVE EVALUATION

When CYCLE WORLD put rear tires through a subjective track test, the dif ferences between brands were large and obvious. Some tires slipped and slid, send ing the bike sideways at the merest hint of a fast corner~ others stuck well even under power, and when they did slide, did so gradually and predictably. The track test differences between the best rear tires tested and the worst had the potential of being the difference between crashing or not crashing in actual street use.

Cheng Shin

$22.60

Continental

$43.20

Dunlop

$49.79

If subjective testing of front tires proved anything, it proved that the differences between front tires are much more subtle. In most cases, the top-rated rear tire from our previous testing—the Michelin—would slide under hard braking or power before the front tire would slip. But under constant throttle or moderate, post-apex accel-

eration over surface irregularities, subtle performance variances came to light. The difference between the top-rated tires in the front tire test and the very good tires showed up only in slight twitches of the handlebars across pavement changes and painted white lines exiting a constant radius. 45-mph skid-pad turn. Excellent tires didn't slip at all: very good tires twitched the bars with tiny slips. Tires missed the Very GQod rating if the bar twitches described above were more severe or more frequent than the Very Good group. or if the tire slipped perceptiblywhile Excellent and Very Good tires didn't-in the following track sequence: a sweeping. 75-mph right hander: braking from the apex while still leaned over:~, straightening up and braking hard through a rain drainage dip: thrown left back onto the skid pad turn. Tires which earned a Fair rating slipped during the above and at smooth, constant-speed points around the skid pad.

Goodyear

$42.50

IRC

$33.29

Michelin

$44.46

Emphasis throughout the subjective track testing was placed on cornering ability-braking figures are well covered elsewhere. None of the tires tested were ■'dangerous. When we talk of slipping through the 75-mph sweeping turn over parking lot lines, we’re not talking about horrendous front-end washouts that leave black marks on the pavement and bring an expert rider to within an inch of crashing. The slippage was slight in every case, felt more from bar input than actual sideways front end movement. For the average rider at average speeds, all the tires are service able. For sporting street riders, in com bination with a good rear tire, the tires rated Excellent and those rated Very Good will perform well-they're the tires we would put on our own street motorcycles. The differences between the top-rated tires and the next group would probably be apparent only at racing speeds and experience with road test bikes has shown us that at least several of the Very Good tires (Bridgestone, IRC, Yokohama) are racetrack worthy, although for production racing the Excellent-rated tires would be preferred.

Nankang

$24.77

Nitto

$32.50

Pirelli

$48.95

In all cases, however, it is important to note the exact designation of the tires tested. Several manufacturers make other model tires which may not perform as well as the same brand tires tested here. Still other manufacturers build tires not tested here by their choice (the Dunlop K8l, for example) or have designed new tires not in production or not delivered in time for the test.

Yokohama

$35.51

CONCLUSION

Not only are motorcycle tires measurably different, they are different in different ways. Note the three tires judged Excellent in cornering evaluations. They range in rubber hardness (durometer reading) from a very hard to a moderately soft compound. Two are made with rayon cords, one with nylon. Tire contact patch on the three is smaller than average. The tires providing the shortest stopping distances are just as varied.

So who’s the winner? Depends on priorities. The resident road racer at CYCLE WORLD plans on using Michelins on front and rear for his next Box Stock race. It is one of the best handling tires in front or rear, stops well and is very controllable. The Carlisle front tire with the second shortest combined stopping distances and a Very Good rating for cornering has to be the best buy with a list price of $23.98.