Motorcycle tire size

[VanTheMan]

So I took the front wheel off my 94 Honda Magna and took it to a cycle shop to have a new tire put on. I called ahead to see if they had an OEM replacement tire and he said they didn't, but he had one that would work fine. I drove the 30 minutes to the shop and find out that the tire he had was a 120/70/17 instead of the 120/80/17 that is OEM for my bike. I asked if the size difference would matter and a couple of them told me it wouldn't, so I went ahead and had them mount it.

It wasn't until after I got home that I happened to think about the size change screwing up my speedometer. Since it stands off the wheel 10 fewer millimeters than the stock tire, it will have a smaller circumference and will turn more times at speed than the OEM tire. I started looking around on some motorcycle tire websites and most of them will NOT recommend a size other than OEM for a replacement tire. I haven't ridden on the tire yet, so it's still brand new. Does anybody have experience with this or know if any other problems might arise from changing the tire size? Should I have them order an OEM replacement and switch it?

Seven-year-old thread got necro-bumped. Locking.

Zenmervolt - AnandTech Moderator

 

[MiataNC]

See if you can find a forum/usergroup online for Magna's. I am sure they will have tire information and options.

 

[KMurphy]

It would be 12 fewer mm, but that still shouldn't effect your speedometer reading. I would imagine that speed is based off the drive wheel (rear wheel) relationship to the drive train, not how fast the front wheel is turning; so as long as the rear wheel outer diameter is stock, there will be no difference in reported speed.

120mm * 80% - 120mm * 70% = 12mm sidewall height reduction

The biggest differences to the ride is your handlebars will be ~12mm closer to the ground and a little more road texture feedback. The front wheel will be 24mm shorter measured from the road to the top of the tire and there will 12mm more clearance between tire and fender.

 

[JulesMaximus]

Originally posted by: KMurphy
It would be 12 fewer mm, but that still shouldn't effect your speedometer reading. I would imagine that speed is based off the drive wheel (rear wheel) relationship to the drive train, not how fast the front wheel is turning; so as long as the rear wheel outer diameter is stock, there will be no difference in reported speed.

120mm * 80% - 120mm * 70% = 12mm sidewall height reduction

The biggest differences to the ride is your handlebars will be ~12mm closer to the ground and a little more road texture feedback. The front wheel will be 24mm shorter measured from the road to the top of the tire and there will 12mm more clearance between tire and fender.

No, it isn't actually, at least not on old motorcycles with cable driven speedos. The shortest distance is from the gauge to the front wheel so that's where they mount the gear that turns the speedo cable on old bikes.

 

[mooseracing]

damn your on here to eh?

I responded on greatlakes.

Like I said there though, I wouldn't worry about it.

 

[jagec]

Originally posted by: JulesMaximus

Originally posted by: KMurphy
It would be 12 fewer mm, but that still shouldn't effect your speedometer reading. I would imagine that speed is based off the drive wheel (rear wheel) relationship to the drive train, not how fast the front wheel is turning; so as long as the rear wheel outer diameter is stock, there will be no difference in reported speed.

120mm * 80% - 120mm * 70% = 12mm sidewall height reduction

The biggest differences to the ride is your handlebars will be ~12mm closer to the ground and a little more road texture feedback. The front wheel will be 24mm shorter measured from the road to the top of the tire and there will 12mm more clearance between tire and fender.

No, it isn't actually, at least not on old motorcycles with cable driven speedos. The shortest distance is from the gauge to the front wheel so that's where they mount the gear that turns the speedo cable on old bikes.

Not just old bikes...even on new bikes with electronic wheel speed sensors, they often measure from the front wheel. Tradition I guess...

 

[JulesMaximus]

Originally posted by: jagec

Originally posted by: JulesMaximus

Originally posted by: KMurphy
It would be 12 fewer mm, but that still shouldn't effect your speedometer reading. I would imagine that speed is based off the drive wheel (rear wheel) relationship to the drive train, not how fast the front wheel is turning; so as long as the rear wheel outer diameter is stock, there will be no difference in reported speed.

120mm * 80% - 120mm * 70% = 12mm sidewall height reduction

The biggest differences to the ride is your handlebars will be ~12mm closer to the ground and a little more road texture feedback. The front wheel will be 24mm shorter measured from the road to the top of the tire and there will 12mm more clearance between tire and fender.

No, it isn't actually, at least not on old motorcycles with cable driven speedos. The shortest distance is from the gauge to the front wheel so that's where they mount the gear that turns the speedo cable on old bikes.

Not just old bikes...even on new bikes with electronic wheel speed sensors, they often measure from the front wheel. Tradition I guess...

Yeah, I was going to say most bikes but I haven't taken apart any new ones to see how the gauges work. :laugh:

 

[VanTheMan]

haha, yeah I'm on here too Moose. KMurphy, aren't the numbers a measurement of the tire? The 120 is 120mm wide, the 80 is 80mm measured from the edge of the rim to the crown of the tire and the 17 is a 17 inch wheel. Since my stock tire is 120/80/17 and the new tire is 120/70/17, that just means that the distance from the edge of the rim to the crown of the tire is 10mm less on the new tire. Miata, I tried Magna forums from work but they were all blocked by the freakin iPrism thing we have at the hospital. Thanks for the replies guys.

 

[KMurphy]

Got the sensor part wrong, but the number after section width is the aspect ratio in percent. This is the sidewall height in relation to the tread width.

See sidewall aspect ratio.

 

[GuitarDaddy]

Originally posted by: VanTheMan
haha, yeah I'm on here too Moose. KMurphy, aren't the numbers a measurement of the tire? The 120 is 120mm wide, the 80 is 80mm measured from the edge of the rim to the crown of the tire and the 17 is a 17 inch wheel. Since my stock tire is 120/80/17 and the new tire is 120/70/17, that just means that the distance from the edge of the rim to the crown of the tire is 10mm less on the new tire. Miata, I tried Magna forums from work but they were all blocked by the freakin iPrism thing we have at the hospital. Thanks for the replies guys.

No, KMurphy is correct the second measurement is the Percentage of height to width.

On a 120/80/17 the sidewall is 120*.8= 96mm
On a 120/70/17 the sidewall is 120*.7= 84mm (12mm shorter)

 

[VanTheMan]

Ah, thanks. Good to know. Well I guess it's 2mm more difference than I thought it was. I guess I'll just live with it and go back to 120/80/17 on my next change.

 

[Zenmervolt]

Originally posted by: VanTheMan
Ah, thanks. Good to know. Well I guess it's 2mm more difference than I thought it was. I guess I'll just live with it and go back to 120/80/17 on my next change.

Total difference though is only a 4% reduction in overall diameter (assuming that my quick math is correct). When the speedometer reads 50 mph you'll actually be going 48 mph. When it reads 100 mph, you'll actually be going 96 mph.

Not enough to worry about in the real world.

ZV

 

[The Stigenator]

first off.. most motorcycle speedos (heck even some car speedos) are setup to read a bit over what it actually reads. You have a taller tire, hence as ZV states, you are actually goign slower than what you are to be going.

As for the rideability, you actually turn in faster due when you lean in, some people like it, i know i do. I still run 110/70/17 and 140/70/17 (my stock is 130/70/17) and the rears i got them bigger however i find that because of the wider tire it is not as nimble but it sure is a lot more stable in the corners, but i got to be careful not to chicken strip not riding to the edge.

 

[VanTheMan]

Thanks for working that out Zenmervolt and thanks for the tips GG. I don't do much sharp cornering or anything on my Magna like it sounds like you do on your GS500, but it's good to have an idea of how my bike might handle differently.

 

[JulesMaximus]

Originally posted by: TheGoodGuy
first off.. most motorcycle speedos (heck even some car speedos) are setup to read a bit over what it actually reads. You have a taller tire, hence as ZV states, you are actually goign slower than what you are to be going.

As for the rideability, you actually turn in faster due when you lean in, some people like it, i know i do. I still run 110/70/17 and 140/70/17 (my stock is 130/70/17) and the rears i got them bigger however i find that because of the wider tire it is not as nimble but it sure is a lot more stable in the corners, but i got to be careful not to chicken strip not riding to the edge.

Haha, I have no choice on my bike. If I lean it over too far I scrape the pegs (I've done this a few times) so chicken strips are something I guess I will always have on my current bike.