Jump to content

Tire pressures


Paul4450

Recommended Posts

Paul4450

In the gas mileage thread, the conversation took a slight turn toward tire pressures. There was talk about how higher pressures would yield better gas mileage and tire wear. I was also told to try lower pressures front and rear, by Ted Porter’s BMW shop in Scotts Valley when I bought the TracTive dampers. It was suggested to use 32 psi front and 36 psi rear. 
 

Since the tires are radials with belts under the tread I wouldn’t think lowering the pressures would affect wear or mileage. Regarding wear, I would think that lower pressure would allow the contact patch to enlarge slightly but at less pounds per square inch. It does give a more comfortable ride - I don’t feel every little imperfection, especially in the rear. I run Dunlop RS3’s. I did not see any change in gas mileage. It hasn’t caused any handling issues.  I feel just as confident leaning over, and there has been no weaving or wobbles.  I haven’t seen any increase in pressure buildup from cold to “running temp” (ie, after riding for an hour or more).  
 

I’m just wondering if others have tried lower pressures. I’m happy and satisfied with the lower pressures.  I’ll try the RS4’s when it comes time to replace these. 

Link to comment

I wouldn't go any lower than 32 in the front due to the weight of the bike and the generally crappy state of our roads. Any lower and you risk bending a wheel in a pothole. Early police bike RT's would bend a front wheel on driveway lips if there wasn't at least 35 in them!! BMW changed the alloy (IIRC) to fix that issue. I don't know why some want the tires up so high, I only use the 37/42 pressures when fully loaded touring. The tires run cooler under heavy load with higher pressure.

I have found 35 front and 38 rear work well for me for solo sport/street riding. (Track pressures are a completely different story) 

 

Link to comment
realshelby

Since pressure "tests" are ONLY valid on the same bike, rider, and tires...

 

I use the Bridgestone T-31 GT tires on this RT. Ran 36F 42R per BMW suggestions. Front will cup/feather a bit at 36 psi. 38 psi seems to be the sweet spot for ride, handling, and less uneven wear. But I still back it down to 35 when in the twisties for a couple days. Rear...handles fine at 42. But..I have since dropped rear pressure to 39 when riding solo. Over 4 rear tires of the same exact brand/model...I will tell you the 39 psi has significantly better wear in the center area. I am looking at around 1500 miles better wear easily. Never noticed any fuel mileage change. 

Link to comment
dirtrider
10 hours ago, Paul4450 said:

In the gas mileage thread, the conversation took a slight turn toward tire pressures. There was talk about how higher pressures would yield better gas mileage and tire wear. I was also told to try lower pressures front and rear, by Ted Porter’s BMW shop in Scotts Valley when I bought the TracTive dampers. It was suggested to use 32 psi front and 36 psi rear. 
 

Since the tires are radials with belts under the tread I wouldn’t think lowering the pressures would affect wear or mileage. Regarding wear, I would think that lower pressure would allow the contact patch to enlarge slightly but at less pounds per square inch. It does give a more comfortable ride - I don’t feel every little imperfection, especially in the rear. I run Dunlop RS3’s. I did not see any change in gas mileage. It hasn’t caused any handling issues.  I feel just as confident leaning over, and there has been no weaving or wobbles.  I haven’t seen any increase in pressure buildup from cold to “running temp” (ie, after riding for an hour or more).  
 

I’m just wondering if others have tried lower pressures. I’m happy and satisfied with the lower pressures.  I’ll try the RS4’s when it comes time to replace these. 

Morning Paul

 

Tires & riders are different but my personal preference is 32 front & 36 rear based on  68°f at measurement time (you see about 1psi difference per 10° of temperature change). This is for no passenger & not a lot of luggage weight.

 

It is a balancing act as most motorcycle tires have a rounded profile so the centers wear out first, so the last thing I want to do is inflate higher, puff out the tire center even more, therefore make the tire center wear even faster. 

 

But this needs to be balanced against too low on tire pressure makes it easier to dent a wheel, & in some cases lower tire pressures can allow more edge cupping or feathering.  

 

The factory recommended pressures are based on a lot of things-- First is safety, they pretty well assume most riders do not adjust their tire pressure when adding a passenger or more luggage load so err on the high side.

 

They also don't know the riders weight (it could easily be over 300+ pounds) & they don't want to offend customers by stating in the manual that over-weight riders need to  add more air pressure.

 

The factory tire pressures also take into account that the tire pressures won't be checked daily so they recommend  a pressure high enough that won't be too low as ambient temperature change, or don't get regularly checked over a reasonable amount of riding time.  (if they say 42 psi they don't expect the rider to check & add air every day if it is only 41psi at ride off).

 

On your tire pressures not changing cold to hot-- They MUST change from cold to hot, or after riding for a while. If you are basing your (no pressure difference) on the dash readout then you won't see a lot of change as the factory tire pressure monitoring system is temperature compensated based on a 68° nominal. So at 78° tire air temperature  the tire pressure would go up 1psi  but the dash should still show no increase.

   

 

 

 

 

 

Link to comment
56 minutes ago, narcosis said:

Seems I recall that lower pressure in the front tire exacerbates cupping.

Depends on the tire but..... generally yes, that has been my experience.

On the ST I use 32F/34R.... But that bike gets ridden a lot harder.....

Link to comment
Paul De

One of the things I found with radial tires on motorcycles is that lower pressures may give a smoother ride but as the sidewalls loose stiffness i eventually find a squirmy feeling that really kills my confidence in corners when riding frisky.   Aside from a greater tendency to cup, there would be concern for excessive internal heating of the tire if you set the pressure too low.  As a side note I found with the early Metzler radials (MEZ series), I needed to increase pressure from the recommended setting to get rid of that squirmy feel. Lower pressure is not always better for overall ride quality

Link to comment
Paul4450

Thank all of you for your comments. While 32/36 was recommended, I actually went down in increments and ended up at 34/38. No squirming in corners, still very secure feeling. No cupping of the front yet either. I still get the most noticeable wear at “10-11 and 1-2 o’clock” on my front. I’m very close to the edge on my back tire (<1/4”), and the kind of riding I do wears my rear tire pretty evenly from edge to edge.  My cornering technique is to trail brake into the first 1/4-1/3 of the turn to get my desired lean, then crack the throttle open through the middle/apex to “balance the weight distribution F/R”, then roll on throttle exiting the turn. I tend to find a pace between turns in the twisties that doesn’t require hard braking at the end of a straight, but does require moderate braking. That’s my comfort zone and a pace I can keep a long time. 
 

DR is correct about not seeing much if any rise using the TPMS (vs stopping and using a gauge, which as he said would indicate a rise in pressures).
 

Everyone will end up using what they think is best for their situation, skill, load, and personal preference. But as DR said, BMW’s liability lawyers decided what was the “correct” pressures.  

Link to comment
On 3/23/2021 at 8:39 AM, narcosis said:

Seems I recall that lower pressure in the front tire exacerbates cupping.  True?  False?

Possibly, keep in mind that "cupping" is the result of slip. Rear tires cup because of heavy throttle, and front because of heavy front brake usage. Lower pressure would allow more deformation leading to more cupping. The real way to reduce cupping is to reduce slip, which means less aggressive braking and throttle.

Link to comment
Hosstage
23 minutes ago, WBinDE said:

 The real way to reduce cupping is to reduce slip, which means less aggressive braking and throttle.

Hence, my tires cup!

  • Like 1
Link to comment
dirtrider
11 hours ago, WBinDE said:

Possibly, keep in mind that "cupping" is the result of slip. Rear tires cup because of heavy throttle, and front because of heavy front brake usage. Lower pressure would allow more deformation leading to more cupping. The real way to reduce cupping is to reduce slip, which means less aggressive braking and throttle.

Morning  WBinDE

 

Tire slipping, or better put, tire scrubbing is a major cause but not so much due to heavy braking or heavy acceleration as most think. It needs to be kept in mind that a tire will not cup or scrub if the part of the tire that is cupping is not touching the pavement.

 

If the cupping is in the normal riding contact patch then it could be from braking or acceleration but most cupping occurs out more towards the tires edge  (not a normal area that is touching the pavement during acceleration or hard braking). 

 

One of the big causes of cupping is scrubbing due to different parts of the tire having different diameters, therefore different circumferences coupled with softer rubber out toward the outer edges.

 

If you look at a  conventional motorcycle tire it is  not flat across the tire contact area like an automobile tire is, it is a more rounded or oval in profile.

 

That means that at the exact same tire rotational speed, or RPM, each part of that rotating tire is "trying" to travel a different distance for each revolution. This causes a scrub or slip as a tire is leaned into a curve, or even worse in a higher lean angle turn. The more rounded the tire profile & the farther it is leaned onto the smaller outer diameters the more it scrubs in trying to equalize the larger diameter center travel to the smaller diameter outer area travel (therefore scrubbing & cupping, or sometimes even feathering). 

 

Some tires are better than others at handling the scrubbing due to tire construction, tire profile, tire rubber compounds/rubber hardness across the contact area of the tire, even tread design can have an effect. 

 

 

 

 

 

    

Link to comment

@dirtrider I don't disagree with you. Slip is unavoidable because - as you point out - diameter decreases as you lean over, and since the contact patch has a nonzero width, some areas (smaller diameter closer to the edge of the tire) will be moving faster than others even without acceleration loads (positive acceleration with the throttle, negative acceleration with brakes). Acceleration magnifies the effect. Even straight-line accelerations can widen the contact patch (squash it out, if you will) so that smaller diameter "edge" areas come in contact with the pavement.

Link to comment
  • 2 months later...
BrianM
On 3/23/2021 at 8:34 AM, dirtrider said:

Morning Paul

 

Tires & riders are different but my personal preference is 32 front & 36 rear based on  68°f at measurement time (you see about 1psi difference per 10° of temperature change). This is for no passenger & not a lot of luggage weight.

 

 Been playing with tire pressures. Have been trying your pressures.

 

Will be going on a longer (2 week) trip. How much increase to compensate for increased luggage load?

2018 GSA. Street tires - Roadtec 01

Link to comment
WBinDE

I don't know whatever possessed me, a completely on-road rider, to put 50/50 tires on my GSA. But I did - Dunlop Trailmax Missions. Not a fan. They grip well and have much better life than I'd expected (front & rear have 2500 miles and they're not even half done!) On the other hand I find they understeer far worse than any road tire I've ever used, and it's hard to corner well with them. I decided to increase pressure on the front from 36 to 40, on the theory that it would deform less and be more predictable. I can't say that it's solved the problem but it doesn't seem as bad as before. Possibly some motivated thinking going on, too.

 

I'm discounting the possibility that riding with full panniers and a drybag on the pillion seat are affecting balance and handling: in previous trips I've had the same PLUS a large drybag with camping gear and a Kermit chair, and not noticed this degree of understeer. So I'm blaming the tires, and looking forward to putting Road 5 GTs back on, 

Link to comment
  • 3 weeks later...
Rob Nowell

BrianM, you never got a response---and I have the same question.  I'm taking off in the morning from L.A. to Dallas.  With the exception of a couple days in and around Taos, NM, most of the riding will be a hot sombitch.  After Texas, I head north to SD, then west to MT (yep, Beartooth).  I have 10 lbs in each side case, and 32 lbs in the top case.  I'm riding a '16 RT solo, I weigh in at 180 lbs, and I'm on two new Michelins. 

 

So, taking into account the weight and, especially the temperature, I'm wondering about tire pressures.

Link to comment

I use 40 psi on the front and 42 psi on the rear of my 2018 R1200RT with Michelin Road 5 GT tires. I have used the same pressures with the Continental RoadAttack 3GT and the Bridgestone T31 GT tires. 

The factory recommends 36 psi for the front and 42 psi for the rear measured cold (20C/68F).

Link to comment
Rob Nowell
28 minutes ago, Bernie said:

I use 40 psi on the front and 42 psi on the rear of my 2018 R1200RT with Michelin Road 5 GT tires. I have used the same pressures with the Continental RoadAttack 3GT and the Bridgestone T31 GT tires. 

The factory recommends 36 psi for the front and 42 psi for the rear measured cold (20C/68F).

You don't think the weather could make that psi rise too much?  I'll be averaging 350 + miles per day.  Garage temp for inflation is about 93 degrees.  

Link to comment

I am not worried about actual tire temperatures, pavement temperatures or other stuff. The designers of the bike and tires decided that the air pressure should be set at a cold temperture to a certain pressure. The max pressure is stamped on the side of your DOT tires at cold temperatures (68F or 20C). Every tire or other sealed container will have an increased pressure when you heat it up.

I ride around Florida year round, even when the temps are above 90F. I don’t alter my tire pressures when I check them.

If you want you can modify the cold pressures when you are measuring at higher or colder temperatures, but be careful.

A under inflated tire will run much higher pressures then a properly inflated tire, at the same temperatures, because it will run hotter.

  • Like 1
Link to comment
alegerlotz
1 hour ago, Rob Nowell said:

You don't think the weather could make that psi rise too much?  I'll be averaging 350 + miles per day.  Garage temp for inflation is about 93 degrees.  

 

For that trip I would use the 93 degrees as your "cold temperature" and set the pressure to 38/42 using a quality gauge (not the bike TPMS sensors that "correct the pressure to what it would be at 68 degrees F").  From there, ambient Temps of 113 degrees would translate to the "cold pressure" having been set about 2 degrees higher in each tire (~40/44).  In the morning when you're starting off, if the temps are in the low 70s, the "cold pressure" would only be down by a couple of psi (36/40).  So your cold pressure range for these high and low temps would be 36/40 to 40/44.

 

What I described above is what I'm going to do for my trip from Los Angeles to Great Falls next week.  I'll set the cold pressure to an average of 38/42 and go with that.  If the weather changes enough I can adjust a couple of PSI, but I'm really not too concerned about it.

  • Like 1
Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...