The one millionth tire pressure thread

[mattd1205]

Hey guys,


I’ve been researching tire pressures on more modern tires for these bikes and it seems the consensus is that the pressure for tires should be set higher than was prescribed in the owners manual. I stopped by the old school motorcycle tire shop by my place and one of the guys who has worked there since these bikes were new suggested 34 psi up front and 40 psi for the back on my 1978 GS750. Dunlop’s fitment guide suggests 32 psi up front and 36 for the back on my specific tires and bike. I’m a little hesitant to deviate from the owners manual even though I feel like handling could be improved a bit by inflating them a bit more. At this point I’m wondering what exactly has changed since the late 70’s-early 80’s to warrant the change in tire pressure since my tires are still bias plys and have similar load/speed ratings as the stock tires that came with the bike originally. Do these recommended tire pressures sound right?


Thanks
Matt

[Nessism]

I always follow the tire manufacturers spec. You are not using the original tires so the Suzuki's recommendation isn't applicable.

If you don't like this advice do some research on how to check for pressure rise while riding. I think that's the purest method to determine proper inflation since it takes into account the load placed on the tires.

[mattd1205]

I always follow the tire manufacturers spec. You are not using the original tires so the Suzuki's recommendation isn't applicable.

If you don't like this advice do some research on how to check for pressure rise while riding. I think that's the purest method to determine proper inflation since it takes into account the load placed on the tires.

That sounds reasonable to me. The tire manufacturer puts a lot of engineering behind their tires so they’d know under which conditions their tires perform best. I’m ok with that. It’s just kind of a new concept to me as I’ve always just gone by the vehicle manufacturers recommendations and never really gave it much thought after the compressor gets put away.
I think I’ll try it out and see how it responds. Thank you Ed!

[Grimly]

The OEM recommended pressures for mine are 28F and 32R and for years I ran them at that, quite happily. Those years were spent doing long journeys at motorway speeds and the running pressures were correspondingly higher due to the tyres warming up.
Only later did I bother changing my normal pressures and now I run them at 32F and 36R, as these settings are better for the backroad scratching I do now.
These two sets of pressures are irrespective of the makes of tyres I used.

[bwringer]

Google "motorcycle tire pressure 10% rule" for the more scientific approach.

Basically, adjust cold pressure until hot pressure is 10% more than cold. Takes a few days of experimentation.

Yes, modern tires generally want a bit more pressure. Ignore the sticker on the frame that was intended for the disco era tires. The rubber we have today is light-years beyond that stuff.

[Gorminrider]

But air has not changed. And it's air that holds the bike and rider up.

Air pressure in tires is per the load it supports, the comfort the air cushion provides, and the factors that heat and expand the air, all while keeping the best traction that the rubber offers....so any overall recommendation is a generalised guide.

[mattd1205]

I bumped it up from 25/28 to 32/36 as per Dunlop’s fitment guide and wow it feels so much more nimble. The handling was on my bike always kinda meh even though I’ve got all the suspension goodies. Now, I’m just itching to take it through the nearby switchbacks and see how it does. I think I’ll also do the 10% rule just to make sure I am where I need to be pressure wise. Thanks for your help fellas!

Matt

[Admiral Beez]

I’m running 28 front, 32 rear in my Bridgestone Battlax on my 1082 GS650G. This is what the manual calls for. I’m just back from a 1000 km club ride and it seems to give a good ride. Thoughts?

[KiwiAlfa156]

This from guru Dave Moss is the most scientific tyre pressure advice I've seen

https://www.youtube.com/watch?v=XaIuWKfnGEI

The best tyre pressure is the one that works/feels the best for you. It depends on what you ride, how you ride, what you want from your bike and the roads/conditions you ride; and the tyre make and model. Saying one set of pressures is ideal is like saying cook all food for 15 minutes regardless of what it is.

There is a range of pressures that 'work'. Pressure affects profile deformation and contact patch size. More air = less carcass flex/ small contact patch = less heat, less bump absorption, more responsive steering, less wear and less grip. Lower pressure = more carcass flex/larger contact patch = more heat, more bump absorption, more steering effort, quicker wear and more grip. The right tyre pressure is the one that puts them in that sweet spot.

Air might have not changed, but tyres certainly have since GS were new. Silica compounds actually work in the wet and give vastly superior grip with acceptable wear at lower pressures. I currently run 29 psi front and 31 psi rear in the Shinko 712 sport touring tyres on my 650G daily rider. Suits my riding style and suits that bike on those tyres. I get light steering, reasonable wear, and what is most important to me excellent feedback and confidence-inspiring grip.

When I was a kid in the 70/80s the accepted wisdom was that factory recommendations were way too low for aftermarket 'premium rubber' like Pirelli, Michelins or Metzelers. So the dealers would send you out on new hoops running 36 psi front and 42 rear... which looking back might explain some of the more 'inexplicable' slide outs and resulting road rash given the expensive sticky tyres fitted.

[phydeauxmutt]

But air has not changed. And it's air that holds the bike and rider up.

Very true, air has not changed.

However, the container that holds the air (the tire) HAS changed. The tire itself does have some support for the bike. If the tire has stiffer sidewalls, it might not need as much air pressure to hold the weight. Softer sidewalls will require more pressure to hold the weight and stabilize the tire.

[KiwiAlfa156]

Very true, air has not changed.

However, the container that holds the air (the tire) HAS changed. The tire itself does have some support for the bike. If the tire has stiffer sidewalls, it might not need as much air pressure to hold the weight. Softer sidewalls will require more pressure to hold the weight and stabilize the tire.

Yep, which is why radials require higher pressures than bias ply tires.

[Gorminrider]

Very true, air has not changed.

However, the container that holds the air (the tire) HAS changed. The tire itself does have some support for the bike. If the tire has stiffer sidewalls, it might not need as much air pressure to hold the weight. Softer sidewalls will require more pressure to hold the weight and stabilize the tire.

If you're saying where the 1981 owner's manual says it wants 30 psi it should now have less? I think not. Most people put too much air in their motorcycle tires and not enough in their cars....it's all about the rubber on the road and what the wear on the tire tells about this. Radials are no doubt "different" but cars, which have had radials for years, seem to want about the same pressures as they did with the bias tires. And with tubes too. I think it'd be a mistake to equate 1980's bikes with their old fashioned frame and suspension to today's modern racebikes that need to have tire-warmers, fashions in profiles and all that. You'd have to wonder whether more "sidewall stiffness" is necessarily a good thing too, quite apart from the difficulty of mounting them. per grip and handling, solid rubber tires are fine for lawnmowers but not be great on a motorcycle-not yet, anyways. And it's not as though they couldn't make a stiff tire in the 80's, some years after the USA landed on the moon.

[Gorminrider]

There's a lot of youtube videos on all this..(a couple below that are interesting) One of them, the guy goes into all the interesting and sure-valid-reasons why not to trust the owners manual and then, in the end changes it a # or so... The second one is more technical but you still have to read between the lines.

https://www.youtube.com/watch?v=vl9gJ1aQg1E

https://www.youtube.com/watch?v=XaIuWKfnGEI

[salty_monk]

32-36 for me... sometimes 38 if I have a passenger.

[KiwiAlfa156]

If you're saying where the 1981 owner's manual says it wants 30 psi it should now have less? I think not. Most people put too much air in their motorcycle tires and not enough in their cars....it's all about the rubber on the road and what the wear on the tire tells about this. Radials are no doubt "different" but cars, which have had radials for years, seem to want about the same pressures as they did with the bias tires. And with tubes too. I think it'd be a mistake to equate 1980's bikes with their old fashioned frame and suspension to today's modern racebikes that need to have tire-warmers, fashions in profiles and all that. You'd have to wonder whether more "sidewall stiffness" is necessarily a good thing too, quite apart from the difficulty of mounting them. per grip and handling, solid rubber tires are fine for lawnmowers but not be great on a motorcycle-not yet, anyways. And it's not as though they couldn't make a stiff tire in the 80's, some years after the USA landed on the moon.

Modern bias ply tires like those run on our GSs have improved in compounds but haven't changed in terms of construction, so air pressures stated by the manufacturers are a good starting point. And I agree that there seems to be a tendency to run bias ply motorcycle tires at more pressure than is optimal. I think a lot of that comes from the 'increased tire pressure for high speed riding' advice. So more pressure became better. Everyone I know used to run their tires at or close to maximum pressure back in the day, because the factory recommended pressures where a joke. Apparently. I do think radial motorcycle tires on modern bikes do run higher pressures and with good reason.

Both air pressure and sidewall stiffness combined is the 'spring' that presses the contact patch against the road. Tires are a big part of the suspension system. Sidewall stiffness in motorcycle bias ply tires compared to motorcycle radials is because of the ply angle, and ply angle doesn't just affect the sidewall, its also means a stiffer and less easily deformed tread. Radial treads deform more easily and evenly under load which is why they can deliver greater grip and lower internally generated heat. Motorcycle radials also have shorter sidewalls (lower profile) than bias plys to improve sidewall rigidity. One way of looking at it is that motorcycle bias plys require less air pressure to overcome their inherent stiffness of their construction compared to motorcycle radials.

I don't think comparison of motorcycle radials with radial car tires holds. Cornering loads in car tires are forces that are 90° to the rim and direction of travel, so the sidewalls are deformed sideways. Motorcycle cornering is essentially increased load at close to 0° to the rim (varied by tire width and rider body position). Add to that slip angle deformation differences...