Optimum air pressure in 'over-sized' tires?

[papa al]

Many riders seem to adopt the somewhat controversial practice of fitting larger than factory supplied tires to their bikes.

If this is done, should the tires still be inflated to (1) pressures iterated in the owners manual,

or should a (2) little less pressure be used,

or (3) more pressure?

Pundits...?

[AllanB]

I under-inflate the slightly oversized tyres on our Wave by about 5psi, purely for comfort and since we both ride quite slowly, don't think there is any significant risk. The tyres don't get hot, even this weather and we both corner quite conservatively too, just in case we meet something.

The ride on Isaan's bumpy roads has improved a lot since we fitted them, especially two up and that matters on what is an overloaded bike. Over-inflating them would surely defeat the object of the exercise.

On the basis that the "balloon" tyres fitted to ATV's run at very low pressures, i assume you lower the pressures anyway.

Riding off-road I would lower pressures too, but for reasons of grip, as well as comfort.

Edited March 29, 2014 by AllanB

[Shurup]

I think the manufacturer's calculate their recommended pressure based on the vehicle weight + whatever it's designed to carry. I remember reading somewhere that for a pick up, if you're planning to load it up, you should bump up the pressure as well. I would think the same goes to MC.

Based on this, IMHO, the oversized tire should be inflated to the same pressure as recommended by the manufacturer.

[Spoonman]

30psi is the same in any tyre no matter it's dimensions.

[andreandre]

30psi is the same in any tyre no matter it's dimensions.

Exactly right its all about pressure, not volume ... bigger tyre on same bike should have same pressure as smaller tyre.

[papa al]

Consider:

In bicycling (push-bikes) rim/tires used for racing are narrow and high pressure 100 up to 120 psi.

Fat tires like on mountain bikes and casual cruisers use low pressures 20-40 psi.

Intermediate size general usage tires usually call for ~40-70 psi.

Total weight of bike and rider is pretty similar in all three cases.

I'm leading toward the bigger tire--> less pressure argument, not because of volumetric considerations, rather because of contact area considerations.

A wider tire will present a larger patch to the pavement.

The larger patch will support the same weight with less pressure per area (another way of saying less PSI) at the rubber/road interface.

The PSI at the interface is essentially equal to the PSI within the tire, consistent with equal and opposites forces required by classical physics.

Maybe I'm thinking too much, or

maybe I'm ignoring crucial considerations.

Any analytically minded engineering types, please critique my analysis.

[Spoonman]

What size are the original tyres and what size are the fat replacements.

[andreandre]

Consider:

In bicycling (push-bikes) rim/tires used for racing are narrow and high pressure 100 up to 120 psi.

Fat tires like on mountain bikes and casual cruisers use low pressures 20-40 psi.

Intermediate size general usage tires usually call for ~40-70 psi.

Total weight of bike and rider is pretty similar in all three cases.

I'm leading toward the bigger tire--> less pressure argument, not because of volumetric considerations, rather because of contact area considerations.

A wider tire will present a larger patch to the pavement.

The larger patch will support the same weight with less pressure per area (another way of saying less PSI) at the rubber/road interface.

The PSI at the interface is essentially equal to the PSI within the tire, consistent with equal and opposites forces required by classical physics.

Maybe I'm thinking too much, or

maybe I'm ignoring crucial considerations.

Any analytically minded engineering types, please critique my analysis.

Yes you are thinking too much, no problem with that in general..but the bikes you use as examples have far too many variables that are not relevant to this simple issue OP

This is about putting a larger tyre on an original rim and you are quite limited by rim width, just how much bigger you can go anyway.

A wider tire will present a larger patch to the pavement.

Often the opposite happens because the wider tyre is fitted to the narrower rim, it changes the profile and the contact patch can actually decrease.

[Shurup]

@papa al

Each of described bicycles have different application and requirements so their weight isn't really considered in tire pressure calculations, sports bicycle uses very skinny tire to minimize the contact surface with the ground, higher pressure keeps that surface area small. Less the contact area - less the force of friction and less of the cyclist's power goes to overcome that force. Offroad bike (or any other vehicle) uses low pressure fat tire for a number of reasons - to maximize the traction through the larger contact area, to make the ride less bumpy, and to protect the terrain from destruction. General use bike isn't really good for racing nor offroading, it's somewhere in between the 2 and so it's tire pressure.

Changing the tire pressure from skinny to fat, will already change the contact area due to the fat tire having larger curvature, lowering the pressure will increase the contact area even more. the bike's dynamics in turns will be greatly affected and I'd think affected in a bad way.

[papa al]

Andre blogs:

"This is about putting a larger tyre on an original rim and you are quite limited by rim width, just how much bigger you can go anyway."

This is an excellent point.

Seems like on these small bikes the manufacturer decides what tire is appropriate, then puts the smallest (hence cheapest) rim with it that is feasible.

Installing a wider tire on the stock rim exceeds industry standards.

On the second-hand Nouvo I acquired, the previous owner replaced the 80mm rear (on 1.6" rim, the limit, by the charts.) with 90mm.

Out of spec but seems to perform swell.

(Anyone else find this to be true?)

The stock Yamaha Spark 135 rear is 80mm tire on 1.6" rim also.

I know a guy who runs a 110mm on the stock rim, reporting no probs.

Crazy. Really, how much bigger can you go anyway?

What are the dangers?

And :

"Often the opposite happens because the wider tyre is fitted to the narrower rim, it changes the profile and the contact patch can actually decrease."

Possibly.

But here we are fitting the wider tire to the same sized rim, not a narrower rim. (Narrower than 'recommended', of course.)

The profile on each would be essentially circular, with the wider tire presenting with a greater diameter.

So I'm visualizing a larger contact area.

Edited March 30, 2014 by papa al

[andreandre]

Andre blogs:

"This is about putting a larger tyre on an original rim and you are quite limited by rim width, just how much bigger you can go anyway."

This is an excellent point.

Seems like on these small bikes the manufacturer decides what tire is appropriate, then puts the smallest (hence cheapest) rim with it that is feasible.

Installing a wider tire on the stock rim exceeds industry standards.

On the second-hand Nouvo I acquired, the previous owner replaced the 80mm rear (on 1.6" rim, the limit, by the charts.) with 90mm.

Out of spec but seems to perform swell.

(Anyone else find this to be true?)

The stock Yamaha Spark 135 rear is 80mm tire on 1.6" rim also.

I know a guy who runs a 110mm on the stock rim, reporting no probs.

Crazy. Really, how much bigger can you go anyway?

What are the dangers?

Tire:Wheel Chart.png

And :

"Often the opposite happens because the wider tyre is fitted to the narrower rim, it changes the profile and the contact patch can actually decrease."

Possibly.

But here we are fitting the wider tire to the same sized rim, not a narrower rim. (Narrower than 'recommended', of course.)

The profile on each would be essentially circular, with the wider tire presenting with a greater diameter.

So I'm visualizing a larger contact area.

"Often the opposite happens because the wider tyre is fitted to the narrower rim,

My bad explanation...i did mean fitting a wider tyre to the original rim ie, as you say narrower than recomended...

[papa al]

Shurup:

"Changing the tire pressure from skinny to fat, will already change the contact area due to the fat tire having larger curvature,..."

I'm guessing you meant to say size, not pressure.

And "change the contact area..." means increasing it?

Right?

(Not trying to put words in you mouth Shurup, just trying to clarify,)

[andreandre]

From Dunlop bike tyres info site..

TOP

Can I put wider tires on my motorcycle?

This is possible in some cases, but again, it should only be done with the approval of the motorcycle or tire manufacturer. When considering wider tires, you must factor in clearance for width and diameter, the effect on stability and handling, along with whether your rim is wide enough. If wider tires are approved for a motorcycle, it is usually permissible to increase by only one size designation. When fitting a larger tire, always allow for some tire growth from the new to used situation. All tires increase in size after they have been inflated and are run for a few hundred miles. In some cases, putting a wider tire onto the same wheel will actually give you less “rubber on the road”, by changing the profile and reducing the contact patch.

[Shurup]

Shurup:

"Changing the tire pressure from skinny to fat, will already change the contact area due to the fat tire having larger curvature,..."

I'm guessing you meant to say size, not pressure.

And "change the contact area..." means increasing it?

Right?

(Not trying to put words in you mouth Shurup, just trying to clarify,)

Yes, that's what I wanted to say. My fingers type faster than I think.

Sent from a Nuclear Submarine.

[papa al]

Andre: Intereting! Thanks for that.

I'll try to find the full article.

It seems unlikely that Yamaha and Michelin (or any manufacturer) would write letters of approval for the mods we have been discussing, as the unnamed author suggests.

Actually, I would be surprised if anyone has ever gotten such an official approval.

We all know people who have done these mods.

Anyone know of negative outcomes?

I wonder under which circumstances a wider tire would put down a smaller patch.

A clear explanation for that would be well received by me.

Edited March 30, 2014 by papa al

[seedy]

Imagine a tire designed for a 2 inch rim. Picture in cross section the curve from bead, around sidewall, across tread, and back to the bead.

Now use a 1.5 inch rim. Picture what happens.

The two beads are squeezed closer together, which decreases the curve of the tread, effectively reducing the amount of tread on the road by lifting up the sides of the tread as the beads are squeezed closer together.

[Shurup]

Imagine a tire designed for a 2 inch rim. Picture in cross section the curve from bead, around sidewall, across tread, and back to the bead.

Now use a 1.5 inch rim. Picture what happens.

The two beads are squeezed closer together, which decreases the curve of the tread, effectively reducing the amount of tread on the road by lifting up the sides of the tread as the beads are squeezed closer together.

True, but oversized tire would have a larger curve to begin with. Squeezing it slightly will reduce the curvature but at the end it probably still going to be larger than the original skinny tire. I think there are few variables so it's not all that simple, that's why the quote by @andreandre says "in some cases".

[Shurup]

^^^ Variables: tire height, width, tread pattern, how much it's squeezed and the tire pressure will all play part in curvature and contact area.

Tire is a lot thicker at the tread than at the sidewall, when squeezing, the side wall will bend more than the tread, which will tend to keep it's curve at least until the tire start wearing out.

[seedy]

double post

Edited March 31, 2014 by seedy

[seedy]

I wonder under which circumstances a wider tire would put down a smaller patch.

A clear explanation for that would be well received by me.

This is what I was replying to.

[andreandre]

Imagine a tire designed for a 2 inch rim. Picture in cross section the curve from bead, around sidewall, across tread, and back to the bead.

Now use a 1.5 inch rim. Picture what happens.

The two beads are squeezed closer together, which decreases the curve of the tread, effectively reducing the amount of tread on the road by lifting up the sides of the tread as the beads are squeezed closer together.

Thats it exactly...

[andreandre]

Andre: Intereting! Thanks for that.

I'll try to find the full article.

It seems unlikely that Yamaha and Michelin (or any manufacturer) would write letters of approval for the mods we have been discussing, as the unnamed author suggests.

Actually, I would be surprised if anyone has ever gotten such an official approval.

We all know people who have done these mods.

Anyone know of negative outcomes?

I wonder under which circumstances a wider tire would put down a smaller patch.

A clear explanation for that would be well received by me.

Here's the Dunlop link...interesting all round,..tyres and tyre pressures are a lot more involved than many people realise [moreso for bigger bikes but applicable to all bikes to some degree IMO]

http://www.dunlopmotorcycle.com/info-center/faq/

EDIT;

This one sentence covers it all really from Dunlop site; it is usually permissible to increase by only one size designation.

If you adhere to this you are talking of maybe 10 mm bigger so the issues raised are probably irrelevant .

A larger tyre without wider rims is just for looks..there really is no real advantage that i can think of...handling performance wise which is what its all about.

A lot of riders seem to be into the ''look at my bike " thingo..more show than go.....thats up to them but not for me

Edited March 31, 2014 by andreandre

[papa al]

Cosmetic considerations is definitely a plus of larger tires for many people.

I doubt whether better cornering would accrue, as a smaller tire runs hotter and hence would be 'stickier.'

A more massive tire resists acceleration & braking more, so poorer performance there too.

Smoother ride (especially with a bit lower pressure)...a plus.

Increased resistance to tire/rim damage or loss of control when striking road irregularity (especially at high speed & at heavier loads (big or multiple riders)), a plus.

I'm wondering if the author of the referenced Dunlop FAQ piece was an engineer type or a lawyer type.

[andreandre]

Cosmetic considerations is definitely a plus of larger tires for many people.

I doubt whether better cornering would accrue, as a smaller tire runs hotter and hence would be 'stickier.'

A more massive tire resists acceleration & braking more, so poorer performance there too.

Smoother ride (especially with a bit lower pressure)...a plus.

Increased resistance to tire/rim damage or loss of control when striking road irregularity (especially at high speed & at heavier loads (big or multiple riders)), a plus.

I'm wondering if the author of the referenced Dunlop FAQ piece was an engineer type or a lawyer type.

I'm wondering if the author of the referenced Dunlop FAQ piece was an engineer type or a lawyer type.

I think a bit of each...so below an unbiased view IMO from another site, but confirms more the engineer type ..

What’s the widest tire I can fit on my bike?

• Boy, if only it was that easy! There’s a reason that there’s not a common list of stuff that will fit on a given bike – most people don’t sit around their shop with a stack of tires trying to fit different ones onto their wheels. A lot of things need to be taken into account when changing a tire size.
• Tires ‘grow’ at speed – temperature and centrifugal force cause a spinning tire to be measurably larger than one at rest. Tires need clearance in many dimensions. Tires that are too wide can hit swingarms, chains, or driveshafts. Tires that are too large in circumference change gearing ratios and speedometer readouts, and can contact fenders or swingarms. They also may begin shedding water in inconvenient ways –soaking a driver or passenger.
• Wide tires are not necessarily a better thing. They universally ‘turn in’ worse than a skinnier tire of the same make and model, and usually hurt fuel mileage. If a rear tire is made wider and the front not adjusted for, rears have the tendency to ‘push’; effectively making the rider fight to guide the vehicle into a turn.

The designers who engineer bikes usually are highly trained, and they apply that training to find a tire combination that accents a vehicle’s virtues, and minimizes its shortcomings. It’s become very difficult in recent years to beat them at their own game. With all that said, some tire manufacturers have their own ‘plus sizing’ - tire sizes larger than stock that have been a confirmed fit. Keep in mind that a tire that fits is not necessarily a tire that handles or performs better. In fact, it’s likely to be quite the opposite.

[ll2]

never ever put oversized tires on any of my big bikes - apart from my cbr150r with 130 rear tire but it was modded to handle that.

manual for cbr5650f sugests 36-42 for tire pressure but i always set it up at 34 - 40 for normal use and 30 - 32 for track.

[andreandre]

Tyre pressures as listed by bike manufacturers are a calculated guideline sort of an average for all possibilities

There are so many variables depending on the individual rider/types of riding

I mostly ride fast/aggressive and nearly always without a pillion and have used an old rule of thumb that has never let me down and i've always felt confident in my tyres performance..one less thing to be on your mind when riding...

It's simple ..Initially put the recommended pressure in the cold tyres..go out and ride in the manner that you normally do. for say,20- 30 k's to get tyres to optimum operating temp.

Stop and check the hot pressures immediately..if the tyre pressures have increased more than 10% , then your cold pressure was too low

If less than 10% growth,your cold pressure was too high.Adjust to get the 10% difference.

A little trial and error over a couple of days will end up giving you your optimum cold pressure which corresponds to the correct pressure at operating temp Remember it and stick to it.

Carry and use the same gauge every time and check pressures at least every week

Once you have your own personal correct cold pressure reading using the 10% rule, you will be in the right temp range for you and your bike.

Using the same gauge is paramount IMO..it doesn't matter what it reads [ie +or_ as long as it is consistent] doesn't matter if it reads 32psi or 40 psi the cold pressure figure you ended up with is the one for you ..ie where your cold pressure has increased by 10% when the tyre is at its optimum operating temp and thats what its all about.No point in buying the best performing tyres if the pressures are not correct.

Edited March 31, 2014 by andreandre

[seedy]

Honda Gold Wing with car tire on rear.

Over 23,000 miles - not kilometers - on it when changed.

[ll2]

Honda Gold Wing with car tire on rear.

Over 23,000 miles - not kilometers - on it when changed.

Car tire on bike.jpg

yes 23 k miles of straight, boring ride.

what a stupidity!

[andreandre]

Tyre pressures as listed by bike manufacturers are a calculated guideline sort of an average for all possibilities

There are so many variables depending on the individual rider/types of riding

I mostly ride fast/aggressive and nearly always without a pillion and have used an old rule of thumb that has never let me down and i've always felt confident in my tyres performance..one less thing to be on your mind when riding...

It's simple ..Initially put the recommended pressure in the cold tyres..go out and ride in the manner that you normally do. for say,20- 30 k's to get tyres to optimum operating temp.

Stop and check the hot pressures immediately..if the tyre pressures have increased more than 10% , then your cold pressure was too low

If less than 10% growth,your cold pressure was too high.Adjust to get the 10% difference.

A little trial and error over a couple of days will end up giving you your optimum cold pressure which corresponds to the correct pressure at operating temp Remember it and stick to it.

Carry and use the same gauge every time and check pressures at least every week

Once you have your own personal correct cold pressure reading using the 10% rule, you will be in the right temp range for you and your bike.

Using the same gauge is paramount IMO..it doesn't matter what it reads [ie +or_ as long as it is consistent] doesn't matter if it reads 32psi or 40 psi the cold pressure figure you ended up with is the one for you ..ie where your cold pressure has increased by 10% when the tyre is at its optimum operating temp and thats what its all about.No point in buying the best performing tyres if the pressures are not correct.

Getting back to OP....this tried and proven method is the way to answer your query...try it..

[papa al]

Thanks, I will.