Tire size to use in a loop frame
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Width: Most modern tires are now specified in metric widths. However, the Guzzi documentation specifies 4 inch as the appropriate tire width. A little math will tell you that 4 inch is equal to 101.6 mm. So, a 100 tire is the closest you can get to the factory specification. It is possible to fit a 110 or a 120 on a loop and still clear the fenders.
Height: Most modern tires specify the height as a percentage of the width. As near as I can tell, back in the day when Guzzi specified a 4 inch wide tire, the tire was also 4 inch high. That is, the height was 100% of the width. Finding a tire that is the same in both height and width is not easy with many modern tires. Instead, most tires applicable to loop frames will have a height that is 90% of the width.
My first set of tires were 120/90-18 front and rear. I thought they were fine until I put on my second set of tires, 110/90-18 in the rear and a 100/90-18 up front. Wow, what a difference! My skill level in the twisties went way up immediately. The bike turns much more quickly and I love it. No more 120/90-18s for me.
That being said, some very well respected owners like to run the wider 120s.
- One reason to run the wider tires is that it fills the fenders more and looks better. If you are really interested in running a 130 or 140 wide tire in the rear, contact Mark Etheridge of Moto Guzzi Classics as he can provide you with the modified components to make the switch easy.
- Another reason is to gain a little extra cornering clearance. A 100/90-18 would give a height of 90 mm. A 110/90-18 would give a height of 99 mm. A 120/90-18 would give a height of 108 mm. So, to compare these figures, subtract the height difference between the tire sizes. Using a 100/90-18 as a reference point, a 110/90-18 would be 9 mm taller and a 120/90-18 would be 18 mm taller. These calculations are simplistic and don't take into consideration all factors...but the main point is that the height differences are real.