Bias front, radial rear: does it matter on a motorcycle?

Here's how it came up. When I went double dark on my Gold Wing, I put a Bridgestone BT46 on the front. It's a rear tire, which is the whole point of double dark, and I run it the way the rotation arrow points. What I didn't stop to think about was construction. The BT46 is a bias-ply tire. The car tire on the back is a radial, like pretty much every car tire made. So now I'm running bias up front and radial in the back.

If you search this, you'll find plenty of people saying the "never mix bias and radial" rule is a car rule. Four wheels, two axles, the car pulls toward whichever corner grips differently. On a bike there's one tire up front and one in the back, so who cares? I'd read that too. It sounded right to me.

It isn't where the warning comes from, though.

The warning is about bikes too

The motorcycle side of the tire world says it about motorcycles. RideApart's piece on radial versus bias quotes the plain version: the two constructions behave very differently, so don't mix them on one bike unless the bike or tire maker approves it. Witchdoctors, a cruiser parts shop, goes further and tells riders to run two radials or two bias tires unless the owner's manual says otherwise.

The reason has nothing to do with how many wheels you've got. A front tire and a rear tire do different jobs. The front steers and does most of the braking. The rear drives. A bias tire and a radial flex differently, soak up bumps differently and give back feel differently, so pairing them changes how the bike steers and how it talks to you through the bars. Witchdoctors also points out bias tires tend to run hotter than radials. On a two-wheeler, the front and rear have to work together. That's the whole bike.

But here's the other half

"Don't mix" isn't the same as "never mix." According to RideApart, a bias front with a radial rear comes from the factory on the Harley-Davidson Breakout and CVO Breakout and on several Triumphs, and Triumph says that combination gives those bikes the best stability. A reader commenting on the Witchdoctors article says the factory specs for his 2017 Indian Roadmaster Classic and his 2023 Indian Pursuit call for the same thing. That's one guy's comment, but it lines up.

So bias front, radial rear is a combination engineers have actually signed off on. On specific bikes, after testing. I haven't found anything that says the same about a radial front with a bias rear. That's the one I'd stay away from.

Where that leaves a darksider

The car tire on your rear is a radial, period. So the question is your front. If your bike came from the factory with a bias front and a radial rear, a double dark setup keeps that same pattern and you're closer to stock than you might think. If your bike came radial front and rear, like the GL1800, a bias front is one more change stacked on top of the car tire. That's my situation.

It doesn't mean it can't work. Plenty of double darksiders run exactly this. It does mean you're the test rider, and nobody tested it before you. Here's what I'd do:

• Read your owner's manual and see what your bike shipped with, front and rear.
• Pay attention at highway speed for any weave or wobble, especially with a load or a passenger.
• Check the front for uneven or cupped wear. One GL1500 rider reported a reversed rear on the front cupped badly by about 6,000 km.
• Keep pressures honest and check them cold.
• If something feels off, believe it. Going back to a stock front is cheaper than finding out the hard way.

I'll update this once I've got real miles on my own setup. If you run a bias front with a car tire on the back, I'd really like to hear how it's going. Send me your setup.

This is one rider's research and opinion, not professional advice. Read the full disclaimer.