850T Tire Sizes

[swooshdave]

I know the recommendation from Guzzi is 100/90-18 front and 110/90-18 rear but does anyone run the 100/90-18 on both ends? Does it make any difference? The bike currently has the same size tires on the front and back and I guess it handled fine. It has the same size rims so the slightly wider rear would spread the rear tire out a little?

[mar54]

Front and rear have different load code

[swooshdave]

I went with the different sizes from Avon. Maybe when I wear out these tires I'll try the same size and see if there's a difference.

[rodekyll]

Dave,  I'll answer the OP backwards --

You can fit some 130s back there (depending on brand and model tire), so a 120 isn't going to rub on anything.  If you get a 130 that rubs on the swing arm, it's easy and functionally unnoticeable to add a 5mm shim on the axle to space the tire away.  So can you go bigger?  yes.

Would I recommend tires larger than oem spec?  Not any more.  I used to run the fattest rubber I could spoon on the rims because I thought fat tires looked cool, gave more contact patch, and helped stability in the curves.  I was wrong on all counts and the tradeoff was slower transitions from bank to bank going through the twisties, a feeling of 'oversteer' as the front tire began turning before the back tire was locked and loaded, and more of a 'snatchy' feeling rolling the throttle on and off.  Altogether a poor tradeoff.

Can you go skinnier?  On my short wheelbase Tonti I'm running 100/90 and 100/90. I'll do a 110 on the rear if I have to.  I run really heavy when I'm x-country travelling and have had no issues with the load range of "skinny" tires.  In fact, on my trike (1550# loaded) I'm running 4.00" (~100) x18 on the front and 4.25" (~110) x19 on the back.  These take huge sideloads as well as the straight weight of the machine without any adverse behavior, and 6k into ownership they look like they'll be good for at least 10k-miles.

So in my opinion, all else being equal, skinnier is better, and all else being equal, there is no functional advantage to different size tires front and rear.

[swooshdave]

So in my opinion, all else being equal, skinnier is better, and all else being equal, there is no functional advantage to different size tires front and rear.

There can be a difference in handling by using a narrower tire up front, but it can depend on the bike too.

[rodekyll]

There can be a difference in handling by using a narrower tire up front, but it can depend on the bike too.

Yes, there can be a difference.  The question is -- is it a good difference or not?  Some tires with weak sidewalls can flex if the profile ratio is not right (too tall) for the bike or riding style.  Some can be too stiff.  Some, with the properly-proportioned wider rear rim and smaller diameter can work with the larger diameter, skinny front tire to give a positive effect.  And if you go around a circle track on the rub bars all day, there is a branch of science dedicated to sorting out the answers.  But my opinion is based on your 850T and "all else being equal." We need to change the bike being discussed and the "all else" to make a context for those sorts of cases.

[Moto]

I've gone back to 90/90 front, 90/110 rear this Spring, Avon RoadRiders. The narrow front really quickens the steering, and I was wobbling all over the street at first. But I like it again now that I've gotten used to it. I ran 100/90 F, 120/90 R, a while back, and it made my T3 feel like a barge.

The 90/90 is closer to the inch specification in the manual than the 100/90, even though Guzzi specifies 100/90 metric.

Rodekyll's comments agree with my experience.

Moto

[Tom]

Run higher cold air pressure in the tires.  Sounds like the sidewalls were flexing too much.

[Moto]

Run higher cold air pressure in the tires.  Sounds like the sidewalls were flexing too much.

No, not that kind of wobble. More of a quick response because of less contact patch on the pavement, or maybe less trail because of the smaller diameter that I think is implied by 90/90.

[Tom]

Okay, thought I'd throw that in because a lot of people run with low air pressure in their tires.

[Drew Vickery]

FYI bigger rear tyre for lower RPM. 

In the Bridgestone bt46 range the 4.00-18 64H W tyre fits my bike with 4mm clearance to my 850t3 swingarm (see photo). Tread measures 116mm wide, which is same as what Bridgestone specs say https://www.bridgestone.com/products/motorcycle_tires/products/detail/pr176

A benefit of the 4.00-18 64H W is more diameter than 110/90. This results in slightly taller gearing. My T3 does about 3800rpm at 100kmh where it used to do about 4000rpm for 100kmh on a 110/90 BT46.

[John Croucher]

Dave,  I'll answer the OP backwards --

You can fit some 130s back there (depending on brand and model tire), so a 120 isn't going to rub on anything.  If you get a 130 that rubs on the swing arm, it's easy and functionally unnoticeable to add a 5mm shim on the axle to space the tire away.  So can you go bigger?  yes.

Would I recommend tires larger than oem spec?  Not any more.  I used to run the fattest rubber I could spoon on the rims because I thought fat tires looked cool, gave more contact patch, and helped stability in the curves.  I was wrong on all counts and the tradeoff was slower transitions from bank to bank going through the twisties, a feeling of 'oversteer' as the front tire began turning before the back tire was locked and loaded, and more of a 'snatchy' feeling rolling the throttle on and off.  Altogether a poor tradeoff.

Can you go skinnier?  On my short wheelbase Tonti I'm running 100/90 and 100/90. I'll do a 110 on the rear if I have to.  I run really heavy when I'm x-country travelling and have had no issues with the load range of "skinny" tires.  In fact, on my trike (1550# loaded) I'm running 4.00" (~100) x18 on the front and 4.25" (~110) x19 on the back.  These take huge sideloads as well as the straight weight of the machine without any adverse behavior, and 6k into ownership they look like they'll be good for at least 10k-miles.

So in my opinion, all else being equal, skinnier is better, and all else being equal, there is no functional advantage to different size tires front and rear.
[/quote

The tire could rub inside the rear fender, hit tail light wire, or mountin screws at 130. 

[n3303j]

Can't imagine a reputable manufacturer would not specify tire sizes that best represented their machine's capabilities.

[Tom]

Throwing this into the mix.  Consider the bike that you have and the state of motorcycles being made today.  The size of the tires in oem conditions vs the same sizing today.  You would still be better off staying with the oem size but with tires from today.  My 1980 Convert with today's tires would be better in the oem size.  My '93 Daytona 1000 is better with oem size tires from today.  Definitely my 1973 V7 Sport is better with oem size with today's tires.  etc.  1000S.  I could on.  🤦‍♂️

[wirespokes]

Lino Tonti supposedly said that the Tonti bikes handle best with the size tires he originally called for. As an example, my 87LM handled like a dump truck when I got it.  130x80 rear, 110x80 front. Switching to 120x90 and 100x90 made all the difference. The wider rear tire on the T3 made for poor handling - changing to a 110x90 was just what it needed.

On another note, the same size from different brands don't measure out the same. Some 100x90s are larger or smaller than others. I know of one rider who needed to go down to a 90x90 front because the 100 rubbed the fender mount/brace on a 90S. But I'm sure a different brand in 100x90 probably would have fit.