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Discussion Starter · #1 ·
Howdy,
My 1979 Yamaha XS750 Special has a user manual with it, and the recommended tire pressure is in the mid 30's PSI.
My after market tires have 40 PSI stamped on them.

I pumped them to 40 and have ridden them for about a week like this. They feel fantastic and the bike handles much much easier. Turning, leaning, simple low speed maneuvering, is all easier. This would make sense since the bike itself is 507 lbs.

Is it dangerous to keep the tires at their spec'd max PSI? My only really concern is popping. I don't ride aggressively and am not taking chance with swift moves or anything. Just very casual riding. And the ease and enjoyment factor is improved greatly by the 40 PSI.
 

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Your manual was for different tyres than the ones we use now. Most of the tyres on my bikes are 36 to 40 pounds.
After many miles, check to see if the center is wearing too soon. If so drop the pressure a littel, and or lean me going around the corners.

UK
 

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I pumped them to 40 and have ridden them for about a week like this. They feel fantastic and the bike handles much much easier. Turning, leaning, simple low speed maneuvering, is all easier. This would make sense since the bike itself is 507 lbs.

Is it dangerous to keep the tires at their spec'd max PSI?
I'd suggest keeping them a couple of pounds lower as the pressure does increase as they heat up...
 
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I'd suggest keeping them a couple of pounds lower as the pressure does increase as they heat up...
It might say something in the fine print about the suggested pressure when cold. As in they have factored the increase when hot.
On the track bikes we measured when cold and hot.

For the dirt bikes it depended on the terrain. Mud versus rock for instance.

UK
 

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My calls for 36 but 40 works so much better. Enough so that I could not care less if I get slightly less tire wear. Handling is more important to me on this 900+ pound beast. And I have TPMS sensors and can watch if I want to see how much it affects heat and I found they heat up less with the higher pressure. Before they would go from 36 to 60 on the interstate. Now they go from 40 to 58. That's quite the drop. And yes, the ride is not as plush. So what!
 

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My calls for 36 but 40 works so much better. Enough so that I could not care less if I get slightly less tire wear. Handling is more important to me on this 900+ pound beast. And I have TPMS sensors and can watch if I want to see how much it affects heat and I found they heat up less with the higher pressure. Before they would go from 36 to 60 on the interstate. Now they go from 40 to 58. That's quite the drop. And yes, the ride is not as plush. So what!
That is interesting! And yeah, I'll take better handling over softer ride 98.2% of the time! :smile_big:
 

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I wouldn't worry, I usually run my tires at 41F, 42R cold, they are rated at 42, the bike says 36F 40R .
Has worked fine for the last 100,000 miles :)
 

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I run all my tyres on all my vehicles just a few pounds under max. Never have had a blowout, wear is even (unless the tyre is ****), and handling is amazing. You have my vote!
 

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I run 36 front 41 back on both my bikes
 

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The more air in the tire, the smaller the contact patch on the ground.

Cycles have tiny contact patches to begin with, so making it even smaller isn't the best idea. There may be 100 ways that max air "improves" this and that...but I know for sure it makes STOPPING more difficult. Also, at max pressure, your risk of a blowout increases substantially; there's just no room in the tire to absorb the impact of a big pothole (edit)...think of a balloon...fill it with air until it's ready to pop...then squeeze it..>BOOM!..same deal with inflating tires to their max pressure...they're just expensive balloons.

This paper is very technical, but here's the gist of it:

> Over-inflating the tires by 20% increased the distance needed to stop by ~17%.

>"The vehicle had the shortest braking distance when the tires have been inflated 20% less than the prescribed pressure."

Tire Inflation Pressure Influence on a Vehicle Stopping Distances

Say a bike needs 250 feet to go from 70mph to 0mph....at 20% over-inflated, that bike now needs 300 feet to stop...that 50 feet is literally life or death...the difference between not hitting the object or hitting it at ~25mph...

There's a sweet spot when it comes to inflation pressure...20% over/under the recommended are definitely the outer bounds..I wouldn't stray too far from what the people that made the bike recommend.
 

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The more air in the tire, the smaller the contact patch on the ground.

Cycles have tiny contact patches to begin with, so making it even smaller isn't the best idea. There may be 100 ways that max air "improves" this and that...but I know for sure it makes STOPPING more difficult. Also, at max pressure, your risk of a blowout increases substantially; there's just no room in the tire to absorb the impact of a big pothole (edit)...think of a balloon...fill it with air until it's ready to pop...then squeeze it..>BOOM!..same deal with inflating tires to their max pressure...they're just expensive balloons.

This paper is very technical, but here's the gist of it:

> Over-inflating the tires by 20% increased the distance needed to stop by ~17%.

>"The vehicle had the shortest braking distance when the tires have been inflated 20% less than the prescribed pressure."

Tire Inflation Pressure Influence on a Vehicle Stopping Distances

Say a bike needs 250 feet to go from 70mph to 0mph....at 20% over-inflated, that bike now needs 300 feet to stop...that 50 feet is literally life or death...the difference between not hitting the object or hitting it at ~25mph...

There's a sweet spot when it comes to inflation pressure...20% over/under the recommended are definitely the outer bounds..I wouldn't stray too far from what the people that made the bike recommend.
Good points! I did up my front pressure this morning before riding (Max is 40, I put in 38) and it definitely made "turn in" much sharper. Of course, it had been a bit low at 32 so it was a significant increase. After reading a bit more, I'll probably keep it around 36 which is what is recommended on the bike...
 

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Discussion Starter · #14 ·
Upon looking closer, turns out my tires don't actually indicate a max psi.
Instead, they give a max load for "40 PSI Cold"

Front tire... "Max load 507 Lbs at 40 PSI Cold"
Rear tire.. "Max load 639 Lbs at 40 PSI Cold"

So.. any thoughts as to what that might mean regarding what the tire's max psi is? Again, this is an aftermarket tire, new probably two years ago.

Clearly the tire manufacturer thinks its normal and acceptable for them to be pumped to 40 PSI. I mean, I think?
 

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Keep in mind, 20% of 40 PSI is 8 pounds.
If it says 40 when cold, you know you are safe at 40, and the smart guys have figured you will be safe when they get hot.
You can do the math on the weight of your bike and what the tyre says.

My 412 pound sport bike says 36 front and back. I often start with 38 knowing they will drop a little bit over the near future.
I may favor 38 to 40 on the heavier bikes. 475 and 575 pounds.

On my belch mobile truck, I run 35 when empty, and 50 to 75 loaded.
For trucks and trailers, it is the tyres that determine maximum load. Hopefully the axles and springs can match.

Just had another thought. Braking distances have always been given as stopping distance from 60 mph. Breaking from tradition causes me to be suspicious.

UK
 

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Just had another thought. Braking distances have always been given as stopping distance from 60 mph. Breaking from tradition causes me to be suspicious.

UK
The 60mph "standard" is a relic of the US era of the Federally mandated 55mph limit.

Federal speed limits were repealed in 1995 giving the power to set speeds back to the States where it rightfully belonged. ~70mph is a relatively common highway speed across the USA, so the 70 to zero measurements are actually a more useful piece of information than 60 to zero.
 

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I agree 70 to zero may be a more useful standard for some. But if we are to compare new to old, we need the 60 to zero numbers.
100 to zero would also be helpful.

UK
The "to zero" measurements are used to compare between different tires that you're thinking of installing on your machine...not between a 2019 tire and a 1995 tire...I, and I would bet most of the drivers in the USA, regularly brake from 70+ to zero on a routine basis...100 to zero would be interesting as at that speed most tires are at their design limits...but few stop from a 100 to zero...I know it's hard to let go of the past, but ya gotta try.
 

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Upon looking closer, turns out my tires don't actually indicate a max psi.
Instead, they give a max load for "40 PSI Cold"

Front tire... "Max load 507 Lbs at 40 PSI Cold"
Rear tire.. "Max load 639 Lbs at 40 PSI Cold"

So.. any thoughts as to what that might mean regarding what the tire's max psi is? Again, this is an aftermarket tire, new probably two years ago.

Clearly the tire manufacturer thinks its normal and acceptable for them to be pumped to 40 PSI. I mean, I think?
I always thought the max load listed on a tire, at a given pressure, would also be an indication of the maximum recommended air pressure for said tire.
One thing you can be sure of is that if you run the tire at the max load pressure you are safe from any type of tire failure due to inflation pressure.
Tires ain't that delicate and the tire maker ain't about to put an inflation number on the sidewall that could be dangerous under any circumstance. Their lawyers check for that stuff...

Generally the motorcycle manufacturer knows what tire air pressure their bike should run as long as you're using the same type of tire the bike was intended to use.
On some bikes a guy might want to run a little higher pressure than recommended for better HIGH speed stability.
Sometimes a little higher pressure in the front tire can help avoid tire 'cupping' on the heavier machines or bikes subjected to HARD braking.
On the other hand... backing off the pressure some, from max, will increase available traction but if you back off too much stability will suffer. You gotta find the right compromise for your bike, tires and riding conditions.
 

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The "to zero" measurements are used to compare between different tires that you're thinking of installing on your machine...not between a 2019 tire and a 1995 tire...I, and I would bet most of the drivers in the USA, regularly brake from 70+ to zero on a routine basis...100 to zero would be interesting as at that speed most tires are at their design limits...but few stop from a 100 to zero...I know it's hard to let go of the past, but ya gotta try.
The relic of 60 to zero is still used by the major US bike magazines. The also provide stopping distances from other speeds. The UK does the same. Comparing new stuff to old stuff is a common practice.
I also find specific bike info from the US and UK, is better than an obscure test done in Slovakia with and old Citroen.

The TT100 tyres I mentioned further up the page, were named as TT100, because they lapped the IOM at 100 mph. With a lap average of 100, I would suggest there were many times they exceeded 100. The bikes they used back then were Triumph triples, and 750 Nortons. They were supposedly stock, but I will suggest they were far from it, and quite capable of 120 mph.
I could start another thread on how they fudged things.

The reason I like the higher speeds for stopping, is it will often show some brake fade. Many good sport bikes do not have this problem, but heavy cruisers could easily suffer. Riding over the Cascade Mountains, Hwy 20, in WA at reasonable speeds would also show up fading brakes.

Speaking of the past: Back in about 1974 I rode a bike that pushed 6 pounds for every horsepower. Only one of my more modern bikes beats that, at 4.9 pounds per hp. Is that giving up the past.

Put a smiley to the above. I thought you raised a bunch of good points. And I am leaving out a dig at the US, for being the only non metric country.

UK
 
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