Why 20" front and 21" rear?
07-31-2019, 09:14 PM
#16
Larger outer diameter results in a longer contact patch.
Increased grip: More rubber surface area in contact with the road, less pressure variation across the contact patch and rubber in contact with the road for a duration all increase grip (the longer time means the visco-elastic rubber has more time to “flow” into better contact with the pavement, especially important at higher speeds).
Reduced rear slip angle: the longer contact patch also decreases the relative angle of the rear tire’s axis in relation to the road under cornering load. This helps the heavy rear more faithfully follow the front. This improves precision, in particular reducing the time needed transition from corner to corner.
Larger rim diameter reduces sidewall height which also reduces rear slip angle improving precision and transition time (less sidewall flex).
Larger rim diameter increases weight and gyroscopic effect, so it’s less desirable on the more lightly loaded front. Hence the desire to stagger on a 911, especially one that is rear wheel drive.
Plus it looks better.
I run a couple different tire diameters on on my early 911 (17s, 18s and 19s). The staggered 18s and 19s do handle noticeably better. That’s on a light car with stiff sidewalls.
Increased grip: More rubber surface area in contact with the road, less pressure variation across the contact patch and rubber in contact with the road for a duration all increase grip (the longer time means the visco-elastic rubber has more time to “flow” into better contact with the pavement, especially important at higher speeds).
Reduced rear slip angle: the longer contact patch also decreases the relative angle of the rear tire’s axis in relation to the road under cornering load. This helps the heavy rear more faithfully follow the front. This improves precision, in particular reducing the time needed transition from corner to corner.
Larger rim diameter reduces sidewall height which also reduces rear slip angle improving precision and transition time (less sidewall flex).
Larger rim diameter increases weight and gyroscopic effect, so it’s less desirable on the more lightly loaded front. Hence the desire to stagger on a 911, especially one that is rear wheel drive.
Plus it looks better.
I run a couple different tire diameters on on my early 911 (17s, 18s and 19s). The staggered 18s and 19s do handle noticeably better. That’s on a light car with stiff sidewalls.
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