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Old 11-22-2014, 01:13 PM   #1
DiabloADV OP
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Tire size Q

Hey ... I'm trying to pick a pair of tires for my modified airhead project. Rims are 17x2.5 front and rear.

I can get Bridgestone BT45s in various sizes...difference being aspect ratio:

110/70 and 120/80 H-rated
or
110/80 and 120/90 V-rated

What will the difference in the height be...with respect to handling? Will I even notice?

Any other ideas for a pair of tires for this setup? I am space-limited in the rear. A 120/90 Anakee II tire just barely fits. I can modify the swingarm to fit another 1/2cm if I have to.
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Old 11-22-2014, 01:18 PM   #2
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related Q -- does it matter if I buy two "front tires" instead of "front and rear" from a given brand, if the sizes are otherwise right?
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Old 11-22-2014, 11:26 PM   #3
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The difference between a 110/70-17 and a 128/80-17 is pretty large. Were it me, I'd stick with the 110/70 on the front, and a 120/90 for the rear. I'm going to assume that these will be bias-ply and run with tubes. It'd be a tough order to fill to make radial tires fit and work on such narrow wheels. Airheads tend to work best with narrow tires anyhow, as wider rubber can expose the hinges in the frame at inopportune moments. BTDT. The wider tires will also slow down the steering, which will likely make the bike feel truckish at lower speeds. You'll also get longer tire life and more even tread wear by sticking to the narrow tire/narrow rim combo.

Running front tires on the rear is generally frowned upon, but running rears on the front (reversed when mounted) is done once in a while. Motorcycle tires are constructed as either fronts or rears based on the way that the carcasses are laid up in the molds. Fronts are built to take steering and braking forces, while rears are built to handle cornering and the drive forces. Your Airhead is a reasonably heavy machine, and your best bet is to try to stick to what the tires are made to do.
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Old 11-23-2014, 01:03 AM   #4
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These are standard tubeless tires -- wheels are 17x2.5, sealed for tubeless. I think the BT45s are bias-ply.

Anyway, the front choices are 110/70 or 110/80. Rear are 120/80 and 120/90. Trying to understand which aspect ratio to choose, and why.

Few things different about the bike...front end is modern USD Showa forks...so more robust than standard airhead. Back end is standard dual-shock w/beefed up subframe; motor is at the high end of 1000cc airhead HP range.

Primary purpose is day rides in the hills and valleys of Northern California and Sierras.
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1970 R60/5

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Old 11-23-2014, 02:38 AM   #5
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The 110/80's will be 22mm taller than the 110/70's. Cut that in half (11mm) for the ride height difference and for reduced clearance between the tire and the fender.

On the rear, the 120/90's will be 40mm taller. Ride height change and clearance will be 20mm.

About the front/rear question. There are reasons not to. First, even in the exact same size, the profile of a rear tire will be flatter and a front tire rounder. Also, front tires need to deal with aquaplaning more than rears, which is one reason you sometimes see different tread patterns between front and rear. The other reason that tread patterns are different is that a rear tire needs grip while accelerating and a front tire just the opposite. If you put a rear tire on the front where the tread pattern is specifically designed for the rear, mount it backwards so that the tread pattern is optimized for braking.

Hope that makes sense.

Any reason you are stuck on the BT45's? Why not a 110/80-17 in an Avon AM26 that uses the same tread design front and rear?
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Old 11-23-2014, 04:16 AM   #6
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I have gone form 70s to 80s and back.
Yah you notice it, but it goes away after the first ride or so.
It,s not a huge difference.
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Old 11-23-2014, 10:19 AM   #7
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Quote:
Originally Posted by DRONE View Post

Any reason you are stuck on the BT45's? Why not a 110/80-17 in an Avon AM26 that uses the same tread design front and rear?
Didn't see the AM26 comes in the pairing I need. Not easy to find.

The AM26 would work, too. Any reason to choose that over the BT45?
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1970 R60/5

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Old 11-23-2014, 12:52 PM   #8
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Given sizes like 120/90-18 are so-called "nominal" sizes and rarely does the actual tire have exactly those dimensions. If you are lucky the manufacturer can give you the exact dimensions and Bridgestone does give these on their website. I favored Bridgestones on my airhead and tried both BT45s and Spitfires. The latter last forever but 45s handle better.

Rear tires have 7 to 9/32 thread depth when new. Fronts are almost all about 5/32" deep. Fronts have to handle steering inputs in the yaw axis and deeper thread means taller blocks and these aren't as stable and tend to wiggle or bend over when stressed. Front tires handle better on the front than a rear will as a general rule. Tires didn't evolve to have deeper thread on the front. Would you notice the handling difference in riding? Maybe not - darksiders run rears on the front all the time but then they have a car tire on the back.
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Old 11-23-2014, 01:27 PM   #9
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The Anakee II have have on the rear now is 120/90, and is 118mm wide as-mounted. I could have about 10mm more total width and still have swingarm clearance.
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1970 R60/5

'91 Bill Holland Steel w/Dura Ace
'01 Z3 3.0 Coupe. The Clownshoe.
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Old 11-23-2014, 10:03 PM   #10
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Quote:
Originally Posted by DiabloADV View Post
Didn't see the AM26 comes in the pairing I need. Not easy to find.

The AM26 would work, too. Any reason to choose that over the BT45?
No, the BT45's are fine. I just kinda like the idea of an olde English branded tire on an old airhead Beemer better than a Japanese tire. Quality-wise I think they are in the same class.

You can find the Avons at Revzilla, Bike Bandit, Motorcycle Superstore, Dennis Kirk, AMT, and anywhere else Avon tires are sold.
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Old 11-24-2014, 09:56 AM   #11
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Quote:
Originally Posted by DiabloADV View Post
The Anakee II have have on the rear now is 120/90, and is 118mm wide as-mounted. I could have about 10mm more total width and still have swingarm clearance.
If width is what you want, OK, but don't go wider than 120/90 on a 2-1/2" rim--you risk the bead breaking under load. I don't think I'd even try a 120/80 if the 90's fit OK. The shorter sidewall on an 80 would put the bead at a steeper angle and again you risk having the bead let go.

On the other hand, the contact patch on a 110 is almost the same as a 120, and it's the right size for a 2-1/2" rim so ya got no worries about bead failure.
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Old 11-24-2014, 10:02 AM   #12
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Thanks for the tips. Much appreciated...
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1970 R60/5

'91 Bill Holland Steel w/Dura Ace
'01 Z3 3.0 Coupe. The Clownshoe.
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