I’m asking because, as is often the case, I don’t know enough about the subject. So I ask you guys.
If I understand correctly, a 17” front wheel will produce a quicker steering bike than one equipped with an 18”. This is a pretty prominent message in every article I’ve seen about the Hunter 350, and jives with what I remember from sport bikes back on the 1980s & 90s when I used to ride.
The stock front tire on the Royal Enfield 650s is 100/90-18, many people here replace with a 110/80-18, a wider tire with about the same sidewall height. I understand you are looking for a bigger footprint.
But, and here is my question, why not go for a 100/80-18. I think this would yield a width the about the same as stock, but a shorter sidewall, thus a smaller overall tire. Not a 17” , but more in that direction than the 110/80.
Where am I going wrong (or right) with this thought?
This is a very complicated area but here are a few starting points.
The diameter of the front wheel effects gyroscopic stability so 21" will resist turning more than a 19" and so on as you go down in size. In the 1980s for awhile it was fashionable to have 16" front wheels but we have largely settled on 17" for sporting motorcycles (and even 16.5" until comparatively recently for MotoGP motorcycles). The diameter of the wheel isn't the only thing that matters. Its mass does too so light forged aluminium or carbon wheels are better than heavier ones. The distribution of mass matters too so you want as little mass as possible at the rim compared to closer to the hub. Getting stuff you bolt to the wheels like brakes lighter also helps. So carbon discs both give better braking performance but also better acceleration and turning.
Lightening anything that spins is usually a good idea if you want to improve performance including a crankshaft to get it to pick up revs faster. If you lighten the wheels on your Enfield it will accelerate faster because there is less energy needed to get the wheels to spin. It even matters where the weight is so two wheels that have the same diameter and the same mass will be different if one has more of the weight concentrated in the rim compared to the other.
Okay so lighter, smaller diameter front wheels with their mass concentrated closest to the hub accelerate faster and also are easier to deflect because their moment of inertia is less. You can kind of think of it as a lower moment of inertia is like having less mass so its easier to move or deflect (a gross simplification).
Gyroscopic precession is what happens when you apply a lateral force to the axis of a spinning gyroscope (ie your front wheel). It deflects in the opposite direction. This is what it means to push the handlebars on the left hand side (ie deflect the handlebars to the right and the result is the bike leans to the left. This is what it means to counter steer.
However there is a tyre on the rim too and this has diameter and mass. Wider tyres are heavier than skinnier tyres and higher profile tyres both increase the overall diameter of the wheel/tyre and the proportion of weight that is furtherest from the hub.
So a wider and/or taller front tyre will slow down the steering.
The profile or shape of the tyre is important too. Very sporty tyres will be almost triangular for 2 reasons. One is to help the bike fall over quickly into a turn and the other is to maximise the contact patch when leaned over. All other things being equal more contact patch means more grip and more ultimate speed that can be carried before grip is exhausted.
Independent of the front wheel size is the overall chassis geometry. A steeper rake will turn faster (our Enfields have a pretty sporty 24 degree rake for a retro bike). A shorter wheelbase will turn faster and require less lean angle to hold a a given line than a bike with a longer wheelbase. However with a shorter wheelbase and a steeper rake (and consequent shorter trail) comes less inherent stability. It's all a trade off.
In terms of your question about why not just run a 100/80, the answer is probably because it would change the chassis geometry and reduce ground clearance which is the same thing as what would happen putting on a 17" rim with no other changes. If you think about it the setup on the stock Enfield is kind of giving you an almost 17" front feeling because the rake is so steep. Most people put on wider tyres because of the aesthetics, some because they have concerns when wearing the tyres right to the edge. I have ground the centre stand and the foot peg (with the feelers removed) on the stock tyres so I don't think there is much concern about grip (more on ground clearance) but a little more is probably good insurance if you like to hustle.
Topic: Lets talk tire size theory (Read 1968 times)