Why Are Radial Tires Better Than Bias Ply Tires?

In short, they provide a smoother ride with better grip, better handling and better braking. They make a motorcycle feel noticeably more "planted" when riding, especially on turns, even at legal or 10 MPH over speeds. At hyper-legal speeds the difference is night and day.

Smoother ride: Because radials have softer, more flexible sidewalls they do a better job of absorbing road imperfections.

Better grip and handling: The carcass of a bias ply tire has two cable plies oriented at ninety degrees to each other. Because of this construction bias ply tires have the same amount of flex from bead to bead. As you'll see below, the more flexible sidewalls of radial tires give them a better contact patch which results in better grip and handling.

Better braking: Due to the improved grip, radials tires also brake better than bias ply tires.

Safer riding: Since radial tires deal with road imperfections better while improving handling and braking, overall they're safer tires to ride on.

Bias Ply Tires:

The carcass of bias ply tires has two two plies from bead to bead at ninety degrees to each other. Because of this the sidewalls and tread have the same flex.

Radial Tires:

The carcass of a radial tire has one ply that goes from bead to bead radially and then another ply on the crown underneath the tread area. Because of this construction the tire has softer, more flexible sidewalls with stiffer construction under the tread.

Due to the different flex patterns of bias ply and radial tires, radial tires flex more so that they have a wider, shorter, more consistent contact patch.

The pressure on the contact patch of a radial tire is more evenly distributed which leads to better overall handling, better steering response, better braking and more consistent wear across the tread.

Radials tires also weigh a bit less and dissipate heat better than bias ply tires.

Copyright credit: The images above are screenshots from the following Michelin YouTube video:

 https://www.youtube.com/watch?v=UJJxSlls-tA

 

Why Did K Bikes Come With Bias Ply Tires?

If radial tires are so great then why did the early K bikes come with bias ply tires from the factory? The original K bikes, the K100 and then K75, were designed and built in the early 80s. Although radial tires had been available for cars for a while by then, tire manufacturers didn't start producing radial motorcycle tires for the consumer market until the mid to late 80s. The first production bike to come with radial tires was the 1984 Honda VF1000R.

When BMW came out with the K1 and K100RS4V in 1989 both of them came from the factory with radial tires. The later K1100RS also came with radial tires.

The K1100LT, which came out between the K1/K100RS4V models and K1100RS, still came from the factory with bias ply tires. My guess as to why the K1100LT still came with bias ply tires is that the engineers at BMW thought that a heavy bike like that, fully loaded and being ridden 2-up, probably needed the stiffer sidewalls of bias ply tires. (You can now get GT versions of many radials which have stiffer sidewalls for heavier bikes - like the K1600.)

I've run regular, non-GT, radials on the K1100LTs that I've owned and riding 1-up, fully loaded they ride just fine. If I planned to ride 2-up, fully loaded then I might consider using GT radials.

What Size Radial Tires?

Due to how radial tires are constructed, they usually have a lower (shorter) profile than bias ply tires so you won't be able to find a radial tire in the same exact size as the bias ply tires that BMW recommends in the rider manuals. The term for this is running "off-sized" tires.

The exception to this is the K1100LT 110/80-18 front tire which comes in both bias ply and radial.

I, and many other K riders, have safely ridden oodles of miles on the "off-sized" tire sizes recommended below. They're what I've been using for over a decade and, since they're better handling tires, I consider them to be safer than the BMW recommended bias ply tire sizes.

Note: When buying tires, all radial tires will have the letter R somewhere is the tire size designation even if it isn't explicitly spelled out in the product description.

K75/K100 - Two Wheel Styles

Depending upon the model and year, K75s and K100s came with either "Y-Spoke" or "3-Spoke" wheels. Aside from a different styling appearance, the rear wheels also varied in width. Both the Y-spoke and 3-spoke front wheels are 2.5x18.

Y-Spoke	3-Spoke
Front - 2.5x18	Front - 2.5x18
Rear (disc) - 2.75x17	Rear - 3x17
Rear (drum) - 2.75x18

Radial Tire Sizes

The tables below show recommended radial tire sizes for the various K bike wheels. A couple of things to note when changing from bias ply tires to radials:

Bike will be lowered: Since radial tires have a shorter profile than bias ply tires the front and rear will be slightly lower. (See the difference in radius in the tables below.) This will make it a little harder to get the bike on the center stand but not obnoxiously so.

Speedometer reading will be higher for a given actual speed: Not only do radial tires have shorter profiles than bias ply tires but they also have smaller circumferences (see tables below.)  This means the for a given actual speed, say 60 MPH, a radial rear tire will need to rotate slightly faster than a bias ply tire going 60 MPH, thus sending more pulses to the speedometer via the speedometer sensor.

However, since K bike speedometers usually read about 10% high from the factory I don't think this is a big deal, just something to be aware of. (And when switching to radials it is a good time to calibrate your speedometer - click here.)

On the bright side, you get a slight increase in torque from a tire with a smaller radius and circumference.

 

 

 

Radial Tires Are More Expensive Than Bias Ply Tires

Because they cost more to manufacture. However, if you do the math, they really aren't all that expensive. For example, if you change tires every 10,000 miles and radial tires cost $100 more per pair then it's only costing you an extra penny per mile. To me that's worth it for a better, safer and more enjoyable ride.

Brand Preference

Y-Spoke Wheels: The only tire I've found that comes in a 130/80-17 rear radial is the Continental Road Attack 3 so that's the tire to get if you have Y-spoke wheels. On top of being the only radial tire available in a good size for the Y-spoke rear wheel, they're an excellent sport touring with many rave reviews.

2024 Update: The Road Attack 3 tires are now out of production. However, Continental makes Trail Attack 3 dual sport tires in 110/80-18 and 130/80-17. These are an 80/20 street/off-road tire that is suitable for street bike usage and what I would use on Y-spoke wheels.

3-Spoke Wheels: Both Michelin and Bridgestone make great sport touring radials. My personal preference is Bridgestone T32 tires since they seem to perform just as well as the Michelins and usually cost a bit less. (I also run the GT version of T32s on my K1600GT.)

Bridgestone often has spring/summer/fall seasonal rebates where you get a $50 rebate via a pre-paid Visa card for each pair of tires purchased.

Check for current Bridgestone US promotions at this link:

https://www.bridgestonemotorcycletires.com/en-us/promotions

(Note: The link above is a generic link and I do not get any affiliate kickback if you click on it. I'm just posting it here to help fellow K riders save money.)

Where To Buy Tires (US)

I usually find the best prices at either Chaparral Motorsports or Rocky Mountain ATV/MC. Please note that those are generic links and I don't get any affiliate kickback if you click on them. Those vendors are just where I've usually found the best prices. If you can find a better price from another vendor then buy from them.

How I Got Hooked On Radials

In my early days of K bike riding I always used the factory recommended bias ply tires and, for the most part, they worked just fine. However, I found that on my K75RT and later a K1100LT that when doing sweeper turns at 90ish (out in the country with nobody else around) that the rear end would kind of wander around. It wasn't extreme and didn't feel particularly dangerous but it was not, for lack of a better term, confidence inspiring.

On the K11OG forum I'd read several positive reviews of switching the K1100LT over to radial tires so decided to give them a try. The next time I was out touring and needed to get new tires I got radial tires. The next morning, once I had them scrubbed in, I found that at moderate speeds the bike handled fine but also rode a bit smoother and felt more "planted" in the turns than it had on bias ply tires the day before.

Then came the acid test, how will it ride now on high speed sweepers? One word: amazing. The rear end no longer wandered around and felt very planted in high speed sweepers. Unlike the bias ply tires, it was confidence inspiring. I was then sold on radials and regretted that I hadn't switched sooner.

After that I migrated all of my K bikes to radials whenever they needed new tires. (And sometimes a little sooner.) I like riding smooth turns on a smooth running K75 and radial tires make it that much smoother. I'll never run bias ply tires on a K bike ever again.

My Thoughts On Running Bias Ply Tires On A K

Is there anything wrong with riding on bias ply tires? No. Radials just tend to perform better. And if you choose to ride on bias ply tires to save a few bucks then that's fine with me. It's your K bike.

However, having ridden on different bias ply tires in the past I do have one recommendation: Don't buy bias ply tires with a straight center groove in the front tire. In my experience, when riding on the grooved pavement that some roads have to reduce hydroplaning, these tires tend to "track" so that the steering jumps around a little. It's not particularly dangerous or anything but it can be annoying if you have to ride over long stretches of grooved pavement.

Example:

 

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