Performance of smaller diameter wheels vs larger on the bends

[Agusts]

Maybe you guys can settle this hypothetical discussion we had with friends the other day, I know larger diameter wheels do better on pot holes but what about on the bend, my feeling is that a smaller diameter wheel keeps me from skidding on the corners better than bigger wheels...!? It has lower center of gravity and more traction with the road, what you think...? 

 

Let's make it clear,  take exact same bike, put on 14" wheels and say 18" wheels (assume possible), put on same tyoe and width tires exactly. Now take exact same corner on same road. Would you be able to take that corner with more speed and more lean with smaller wheel than larger before going down ...!? (I guess you take slightly different line, but assume it's a normal road, so can't change the line of approach and curve too much, it's hypothetical question...!)

 

 

[VocalNeal]

Most modern sportbikes use 17 inch wheels? MotoGP use 16.5 in wheels. Discuss?

[Agusts]

4 hours ago, VocalNeal said:

Most modern sportbikes use 17 inch wheels? MotoGP use 16.5 in wheels. Discuss?

I am pretty sure overall, considering speed, acceleration and stoppage power etc. etc. the 17" wheels are probably the optimum solution for racing.

 

But we are talking about a very specific hypothetical scenario going round the same bend with same bike ... , I just feel being lower to the ground immediately gives some advantage, but I'm no expert...,  maybe unless this is tested for real, it's hard to be sure - well, maybe expert racers know ....! 

[VocalNeal]

There is a whole science about motorcycle cornering and lean angles and centre of gravity. How far a rider should hang off the bike and all the knee down theories. Ie. 

Lots of stuff on the internet. But I cannot settle your hypothetical theory for you. 

[jackdd]

I guess that if the wheel base and the weight of the bike would also stay identical you could take a curve at the same speed. But the lower center of gravity would make the handling easier.

17 inch is probably the sweet spot (on road) for all the factors which are affected by the tire size, so this is what you find on most bikes.

[Kwasaki]

8 hours ago, Agusts said:

I guess you take slightly different line, but assume it's a normal road, so can't change the line of approach and curve too much, it's hypothetical question...!)

You can guess as much as you want, complicated but as "Vocal-Neal" has said MotoGP use 16.5 what does that tell you.

My bike has 17 rear and 16 front it turns quicker than the lastest bike which runs 17's.

[OneMoreFarang]

I also don't know the answer. But what I know is a lower center of gravity means more lean angle for the same speed.

In general I know bikes are designed for a specific wheel size. So I am pretty sure if the bike was designed for small wheels then using bigger wheels won't make it perform better. And the other way around.

[lemonjelly]

The bigger the radius of the wheel, the more it wants to keep a straight line when revolving. If your holding a spinning wheel in your hands by the spindle and try to lean it over, it takes effort. The larger the radius, the more effort is needed. It’s how spinning tops stay upright. A larger wheel is going to be hard work, but a really small wheel is going to be too twitchy, plus it’s going to overheat much quicker. 

[Agusts]

Very good points mentioned, I was also trying to imagine the amount of surface that a small wheel has with the road as oppose to a larger diameter one, assuming same width tires. I think the amount of traction you get is relative to that surface area...

 

But I'm not sure which one is actually gives more surface area... !? 

[eisfeld]

16 hours ago, Agusts said:

Very good points mentioned, I was also trying to imagine the amount of surface that a small wheel has with the road as oppose to a larger diameter one, assuming same width tires. I think the amount of traction you get is relative to that surface area...

 

But I'm not sure which one is actually gives more surface area... !? 

Surface area has nothing to do with traction. This is a bit counterintuitive but physics doesn't lie. Traction is simply a matter of force pressing two surfaces together, in this case weight/gravity, and the friction coefficient which is determined by the tire compound. Giving the same compound tire but different sizes or widths will make zero difference in terms of traction.

 

There's usually a difference in maximum lean allowed by different tires but see below. Lean does effect the downward force and therefore traction.

 

What wheel diameter will impact is the center of mass which in turn impacts lean angle. Also gyroscopic effects will be different. There's quite a few more forces at play but in the end the rule of thumb is: smaller wheel = less stable, more flickable; larger wheel = more stable, less flickable.

 

Now you might think "hey, why do race cars and race bikes have wider tires?" The answer to that is because that way they can use a softer compound which increases traction. They need wider tires for two main reasons: more total useable rubber to consume and a more stable tire (not wheel) itself when leaned as the tire deforms during a turn, especially with a really soft compound.

[papa al]

traction ≠ friction.

[Agusts]

Yeah, as someone mentioned it's too complex, let's leave it at that.

 

Let's not waste time on hypothetical question. Now for practical one I hear people complain about unstable cornering for scooters as oppose to proper bikes, I understand single one shock on the side ones like Click could wobble, but the other reason maybe due to engine mount, smaller tires etc. ! - but generally, should they ...?

 

Now on my old model Forza, with two shocks, and a fairly fat 13" back wheel, while cornering and specially coming out of a bend, when I give it some throttle, it wobbles like hell, far more than smaller 125 scooters I have tried. I adjusted tire pressure, even tried a bit more than recommended like 35psi, but no difference.

 

My next move is to adjust the shocks to stiffer settings and see if it makes any difference. But it maybe just the way it is, larger wheelbase, heavy back-end, and good torque when reved, equals wobbly scooter... !!!? 

 

 

[VocalNeal]

10 hours ago, Agusts said:

I hear people complain about unstable cornering for scooters as oppose to proper bikes,

My old Lambretta had scratch marks on the bottom of the exhaust. Have come home with a mate and he had grass on his helmet. 

 

If the Forza has a wobble give the engine mounts a good shake to see how they are. Also if the rear tire is much wider than 140 that will have an effect.  

[Agusts]

1 hour ago, VocalNeal said:

My old Lambretta had scratch marks on the bottom of the exhaust. Have come home with a mate and he had grass on his helmet. 

 

If the Forza has a wobble give the engine mounts a good shake to see how they are. Also if the rear tire is much wider than 140 that will have an effect.  

I still have both factory tires on it, which is 140/70 13" at the back.

 

I forgot to be more specific, the wobble is clearly at the back wheel, at first I thought it was just a bad patch on the road, but noticed it happens anywhere, mostly if revving out of a bend, not sure if it is dangerous, but it just feels weird and unsettling.... when not reving I don't really notice it, so I relate it to that...

[eisfeld]

12 hours ago, Agusts said:

 I understand single one shock on the side ones like Click could wobble

One or two shocks makes zero difference to the stability of the bike. The force are just spread onto two shocks instead of one but the same net result is there. Most bigger bikes have one shock and are more stable than scooters.

 

Stability has way more to do with wheel base (the larger the more stable), wheel diameter (as mentioned before), tire pressure, suspension, center of mass and the general geometry of the bike.

 

The Forzas I tried felt <deleted>ty in terms of stability. Very un-neutral. I call it a boat on two wheels. I doubt the suspension is adjustable on these bikes so all you can do is make sure the tire pressure is right and nothing is loose.

 

If your back feels unstable then I would try to check tire pressure there and maybe shock preload. How about the tires themselves, how many km do you have on them and how old are they?

[canthai55]

YSS make replacement shocks and fork springs which are a noticeable improvement on the stock components

[Agusts]

Old model Forza has 5 step preload adjustable shocks at the back, mine has both oem tires and have 7700km now...

I report back with stiffer preload adjustment, I will go to Koh Samui for a few days from Phuket,  I see how it feels on that trip ... now that's a trip you enjoy on a Forza...????

 

I'm sure YSS makes an improvement,  specially for two up and heavy load, but I wish I could test it before I buy, see if it really worth it..., need to swap bike with someone and try....

[Agusts]

I have set my Forza 2016 to the most stiff setting of 1, that makes the spring length the shortest as it pushes it up.

 

I can say round the bend it has made some difference for better, and I feel less wobbling and instability on the back wheel, of course it makes the ride a bit more stiff, but not by much,  I will leave it on there for now...

 

By the way, the guy had to loosen up the exhaust to get the wrench working on that side, wouldn't move otherwise, he tried and lift up the shocks by hand while I was twisting it by hand,  but wouldn't go, not a big deal though...

 

I think a little bit of this wobble is also due to hard acceleration/skidding and fat small tire, so only so much the shocks can help in this respect, maybe that is the curse with heavy back-end and small tires, and what they say about scooters round the bends in general... I bet a Ninja or CBR is far better...