Tyre size, do they need to be so big?

[gavinnz]

Ok this might end up getting quite technical...

Modern bikes have very wide tyres.
Now why is that?
I can see that a large heavy powerful bike would use a larger area to help dissipate heat and have a larger over all area so wear would distributed more hence slower over all tyre wear. And with heaps of power a larger surface area in contact with the road in a staight line (with a soft compound) would give more grip for acceleration without wheel spin, and more contact patch for braking.

What I am thinking about is.... does a wider tyre really give you that much more contact patch when cornering? I am thinking about the way a tyre rolls into a corner and I can't see how a wider tyre would give much more surface area in contact with the road when lent right over in a corner?

If you are running a large single cylinder engine in a light weight bike where heat build up, wear and wheel spin are not factors then can you fit good narrower tyres without compromising handing and grip to a huge extent?

On my Gilera Saturno they have 110/70/17 and 140/70/17 tyres. I am wondering how much of that tyre choice was really late 1980's fashion and not sound motorcycle engineering. After all not long before that a 110 or 120 tyre was considered fine for the back of a big bore sports bike on the '70's.
The wider the tyre the further away the contact patch is from the centre line of the bike.

Their is also the compound of tyres in relation to their contact patch size that I have been thinking about. A large tyre would need to be softer as the pressure per square inch would be less than a narrower tyre that could be harder compound but have the same real grip as it is being pressed into the road suface harder per square inch.

In the classic racing they get HUGE lean angles on very narrow tyres.
So what gives?

Regards
Gavin

[IdunBrokdItAgin]

Ok this might end up getting quite technical...

Modern bikes have very wide tyres.
Now why is that?
I can see that a large heavy powerful bike would use a larger area to help dissipate heat and have a larger over all area so wear would distributed more hence slower over all tyre wear. And with heaps of power a larger surface area in contact with the road in a staight line (with a soft compound) would give more grip for acceleration without wheel spin, and more contact patch for braking.

What I am thinking about is.... does a wider tyre really give you that much more contact patch when cornering? I am thinking about the way a tyre rolls into a corner and I can't see how a wider tyre would give much more surface area in contact with the road when lent right over in a corner?

If you are running a large single cylinder engine in a light weight bike where heat build up, wear and wheel spin are not factors then can you fit good narrower tyres without compromising handing and grip to a huge extent?

On my Gilera Saturno they have 110/70/17 and 140/70/17 tyres. I am wondering how much of that tyre choice was really late 1980's fashion and not sound motorcycle engineering. After all not long before that a 110 or 120 tyre was considered fine for the back of a big bore sports bike on the '70's.
The wider the tyre the further away the contact patch is from the centre line of the bike.

Their is also the compound of tyres in relation to their contact patch size that I have been thinking about. A large tyre would need to be softer as the pressure per square inch would be less than a narrower tyre that could be harder compound but have the same real grip as it is being pressed into the road suface harder per square inch.

In the classic racing they get HUGE lean angles on very narrow tyres.
So what gives?

Regards
Gavin

I don't know the thin racing tyres you are alluding to, but all I can say is that is why they have started using dual compound tyres. One compound (in the middle of the tyre) for the straights which is focussed upon wear and less grip and the outside compound is focussed more on grip and less on wear (as it will be used less).

Maybe I've oversimplified th eanswer but this is all I can think of.

I have heard that there are three compound tyres out there already (but these are top of the range tyres right now and not street ones).

[LBD]

Steering geometry, lean angles and handling has evolved many times over the years....one of the developments/improvments comes courtesy of wider profile tires that, as you lean, moves the contact patch further from the center line of the bike...

[gavinnz]

Steering geometry, lean angles and handling has evolved many times over the years....one of the developments/improvments comes courtesy of wider profile tires that, as you lean, moves the contact patch further from the center line of the bike...

How does moving the contact patch away from the centre line of the bike help?
Regards
Gavin

[IdunBrokdItAgin]

Only other thing I can think of is that the larger amount of rubber on the road is the larger amount of power put down by the back wheel. Maybe this is why tyres have been progressively getting larger?

When I grow up I want to be a rocket scientist!

[LBD]

How does moving the contact patch away from the centre line of the bike help?
Regards
Gavin

The exact details are lost from me, but I have read a mag article on the subject not long back on the subject and recall that answer to the same question I had also been wondering about.

And it explains why,way back when...I found that if the front and rear tires were not in line, I cornered one way faster than the other.....

[jonbuoy]

I thought it was because the bigger the cross section of the tyre the bigger the contact patch when tipped over? A small skinny tyre doesn´t have much of a patch between the bottom tread and where the sidewall starts if you see what I mean. A lot of modern tyres don´t look particulary round in cross section more like a curved triangle.

[YellowDog]

I thought it was because the bigger the cross section of the tyre the bigger the contact patch when tipped over? A small skinny tyre doesn´t have much of a patch between the bottom tread and where the sidewall starts if you see what I mean. A lot of modern tyres don´t look particulary round in cross section more like a curved triangle.

Wot e said

[Motu]

The concept of how tyres work has changed - now we have a tyre of selected profile held to that profile by high pressures....and the contact patch isn't that wide if you roll the bike over dusty or wet ground.

I have gone out on a limb with my tyre thinking.I am running Dunlop K70's on my R65 Streettracker - the 4.00x18 tyre at 25psi on the rear puts it's full 4.00 of rubber on the ground....and when it leans over the tyre compresses and molds to the surface,still putting that wide contact patch down.With a low CG the contact patch doesn't move out from the centre line much.The bike is super stable,and the soft rubber grips well in all conditions and surfaces.

I don't like high pressures and contact patches so far out from the centreline - not my cup of tea,but each to their own....

[pritch]

Basically you're stuck with the tyre size the bike was designed with. There are disadvantages to moving either up or down in tyre size. Going up in tyre size can actually reduce the size of the contact patch. It's all about trade-offs, better grip for quicker handling, etc, etc, bearing in mind there's no free lunch.

To make a significant change in tyre size you'd also need to change the rims.

Kevin Cameron covers this in his "Sportsbike Performance Handbook"
Cameron sums it all up like this, "In the end it comes down to crazy, unanswerable questions like is it hotter in the city than it is in the summer?"

It all seems like a lot of hassle for bugger all.

[onearmedbandit]

Look at GP bikes. 125's run thin tyres and due to their light weight and other factors can run higher corner speeds than their larger counterparts. 800cc machines run up to 200 section rear tyres. What is the difference between the two machines? Weight and power. If the GP boys can't run thinner tyres I'd say it's down to those differences, weight and power.

[CookMySock]

Curious isn't it, as the formula for friction force does not include the surface contact area.

http://library.thinkquest.org/10796/ch4/ch4.htm#Sec6

Steve

[george formby]

Going out on a limb I would say the ever increasing size of rear tyres is more to do with looks than practicality. Bike designer Glynn wotsischops has stated this a few times in his articles. I can see the logic in sports bikes, they are getting more powerful, lighter & better handling so consequently need more rubber on the road to make use of these improvements but a lot of less focused bikes are going the same way to keep up with the look. Modern cruisers have huge tyres & relatively low power, the tyre size is just bling. My rear is a 150/70/17 & I never feel that I need more grip or lean angle. I'm happy to be corrected if I'm wrong but watching MotoGP, wets look a lot slimmer than slicks which implies that it is not all about the size of the contact patch, surely a bigger tread area on the track would pump more water or would it cause the bike to aquaplane because the weight & power are spread over a bigger area?
I know for a fact that a bike with skinny tyres is a lot easier to flick around than a bike with big fattys.

[sharky]

'onearmbandit' & 'wild weston' are on the money

[Harvd]

....wets look a lot slimmer than slicks which implies that it is not all about the size of the contact patch, surely a bigger tread area on the track would pump more water or would it cause the bike to aquaplane because the weight & power are spread over a bigger area?
I know for a fact that a bike with skinny tyres is a lot easier to flick around than a bike with big fattys.

I can quite happily use old 125s slicks in the wet racing as they are so skinny there is no real risk of aquaplaning

And yeah its all about aesthetics. My bandit 250 has a 150 rear which for sure is overkill but meh, it looks and feels good. I do laugh at the HDs and similar who do the fat rear end conversions and stick on 280 section rears. they look like big truncated balls of rubber