E36 tyre pressure - Upgrade

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Hi,
I've recently changed from 15" wheels to 17" wheels on my 318ti compact. Can anyone help me with a method to calculate the correct tyre pressure to run
my new wheels at?
The new tyres are 225 45 R17 W.
Cheers.
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tires should have a rating on them. The pressures shouldnt have anything to do with the car, unless the tires are not load rated for your vehicle.
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tire and don't have anything to do with your tire/wheel/car combination. Use them only if you prefer a truck-like ride and questionable handling.
If the manual or placard doesn't specify pressures for your size tire, start with the OEM recommended pressures (probably around 30 pounds front and 33 rear) and then you might experiment with increases of around 5~20%.
Tom K.
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dependant, hence the pressure for a given wheel and tyre will usually be different on different makes or models of car. As Tom says, the rating on your tyre is the max pressure.
Tom, as I'm going from 205 15 inch to 225 17 inch and hence a wider surface area, shouldn't I decrease the pressure from OEM?
Thanks again.
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zerouali wrote:

You need to determine the design size of the tire footprint and inflate appropriately. Here's how.
Take max load and pressure listed on the sidewall. Divide max load by max pressure. Now you have the design footprint size in sq in. Next you have to figure the load on the tires. Notice that front passenger weight is close to evenly distributed between front and rear, because the front seats are approximately in the middle of the car. Weight of passengers and luggage in the rear, however, goes predominately on the rear tires alone.
So, for example, with my car, max load per tire is 1300 lbs at 44 psi. Dividing, we get a footprint of 29.545 sq in.
The car weighs 3120 lbs at 50/50 weight distribution front/rear. Add two front seat passengers at 150 lbs each, plus one rear seat passenger at 150 lbs plus 100 lbs of luggage in the trunk.....
Front Tire Pressure = (780 + 75) / 29.545 = 29 psi front pressure Rear Tire Pressure = (780 + 75 +125) / 29.545 = 33 psi rear pressure
These are the numbers on the driver's door placard for tire pressure at medium load. There is also a listing for higher pressures at max load for the car.
This method works just as well for my 4x4 truck with floatation tires and a 900 lb load - gives the 26 psi specified on the glove box placard. (Aren't those engineer guys clever?)
I use these numbers as minimum, and add maybe 1 psi or so based upon handling. Of course, if I have a heavier load, I recalculate and use the resulting higher pressures
Hope this helps
Frank
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Absolutely brilliant Frank, thanks very much :-)
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More clear than pure water ! Thanks

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Wow! While I alwasy knew there was a formula, I never actually have seen it. Having said that, 30 psi in all four tires works pretty well for the vast majority of passenger car applications.
I am not arguing the point, indeed I think it is great that you would know something like this. I'm such a simpleton, I have gotten by for 35+ driving years putting 30 psi in my tires when I bother to fill them at all.
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Nedavno Raybender pise:
| Take max load and pressure listed on the sidewall. Divide max load | by max pressure. Now you have the design footprint size in sq in. | Next you have to figure the load on the tires. Notice that front | passenger weight is close to evenly distributed between front and | rear, because the front seats are approximately in the middle of the | car. Weight of passengers and luggage in the rear, however, goes | predominately on the rear tires alone. | | So, for example, with my car, max load per tire is 1300 lbs at 44 psi. | Dividing, we get a footprint of 29.545 sq in. | | The car weighs 3120 lbs at 50/50 weight distribution front/rear. Add | two front seat passengers at 150 lbs each, plus one rear seat | passenger at 150 lbs plus 100 lbs of luggage in the trunk..... | | Front Tire Pressure = (780 + 75) / 29.545 = 29 psi front pressure | Rear Tire Pressure = (780 + 75 +125) / 29.545 = 33 psi rear pressure
That does not work for my car. It weight is 2150 lbs and max load per tire is 1170 lbs at 44 psi. So I get a footprint of 26.59 sq in.
FTP = (537.5 + 75) / 26.59 = 23 psi front pressure RTP = (537.5 + 75 + 125) / 26.59 = 28 psi rear pressure
But from my manual - tire pressure at partial load:
FTP - 29 psi ; RTP - 30 psi
and at max load:
FTP - 32 psi ; RTP - 38 psi
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Yvan wrote:

Very interesting. Apparently the tires are oversize for the car. A couple of questions...
1. Cars as small as yours are usually front wheel drive with more like 60/40 front/rear weight distribution, so you need to change the calculation to reflect that. Still doesn't come up with the correct answer, though, and it is interesting that your recommended pressures are higher in the rear - usually the opposite for front wheel drive.
2. Are these the original OEM tire size? What is the actual tire spec? (185/65 etc?)
3. Is your car front wheel drive, and do you know the weight distribution?
4. Does your manual give the rated max load for your car?
Additional info on my car.
For the max load, the placard pressures on the door jam are 2 psi higher than calculated by this method - interesting - see below for my guess as to why
My placard also has pressures for optional, oversize tires. Standard tire is 205/60 -15. Optional tire is 225/45-17 Front, and 245/40-17 Rear. Looking up the specs of these tires on the tirerack website, the footprint for the front tire is actually smaller than the standard. For the rear tire size, some brands have a smaller footprint, some have a larger - very interesting. The handling characteristics of the car would change quite a bit with different tire brands, because of the slip angle differences the different footprint sizes would produce.
Anyway, at the partial load on the oversize performance tires I get...
Front Tires: Calculated pressure = Placard pressure (different than pressure for standard tire) Rear Tires: Calculated pressure is higher or lower than Placard pressure depending on tire brand. Didn't find a brand that matched the placard. Car is 9 years old, so I guess ratings for this size tire have changed during these years.
With oversize performance tires, the calculated pressures at max load are less than the recommended placard pressure. Again, we have a safety margin added at max load.
So with my car, the calculated partial load pressures match quite well with the placard pressures for either the standard tires, or the optional, oversize tires. The max load case includes carrying stuff on the roof, raising the center of gravity, so the recommended pressures are apparently set higher to reduce the tire footprint and encourage sliding instead of tipping if you "lose it".
Of course you should use the placard pressures with your car, but I'm curious for more info to see why the calculated pressures are so far off.
Frank
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Nedavno Raybender pise:
| Very interesting. Apparently the tires are oversize for the car. A | couple of questions... | | 1. Cars as small as yours are usually front wheel drive with more | like 60/40 front/rear weight distribution, so you need to change the | calculation to reflect that. Still doesn't come up with the correct | answer, though, and it is interesting that your recommended pressures | are higher in the rear - usually the opposite for front wheel drive.
It's '87 BMW E30 316 rear wheel drive. And I had whiechle weight wrong. It's 2300 not 2150 lbs. It does not make some difference, but still not as in the manual (FTP-29psi, RTP-30psi):
FTP = (578.5 + 75) / 26.59 = ~25 psi front pressure RTP = (578.5 + 75 + 125) / 26.59 = ~29 psi rear pressure
| 2. Are these the original OEM tire size? What is the actual tire | spec? (185/65 etc?)
When it was new it had standard 175/70 R14 84 T on it. Now its 185/65 R14 86 H. I looked up in the manual and that dimension is not listed. It seems that I have following options for my rim size:
Tire size partial load max load F R F R
175/70 R14 84 T 29 31 32 38 175/70 R14 84 H 29 31 32 38 195/60 R14 85 H 29 31 32 38
I do not knew what are last two digits and letter. So maybe the problem is that I have wrong tire size, but it looks strange to me that I can put 175/70 and 195/60 but not 185/65.
| 3. Is your car front wheel drive, and do you know the weight | distribution?
Rear wheel drive, I do not knew weight distribution, but someone here might (as I wrote it's '87 BMW E30 316 1.6 litre not 1.8)
| 4. Does your manual give the rated max load for your car?
Yes, 2300 lbs at partial load, and 3500 lbs at max load.
| Of course you should use the placard pressures with your car, but I'm | curious for more info to see why the calculated pressures are so far | off.
I am inflating mu tires to pressure indicated in my manual, but I just wanted to test how does your calculation results match values in manual.
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Yvan,
Some interesting stuff - see imbedded comments.
Yvan wrote:

Are you sure of this weight? When I look up E30 sites I found weight listed as 2600 - 2700 lbs. Heavy one was an M3, though, but wouldn't think an M3 would be much heavier than the standard car. At 2600 lbs for car we'd have
FTP = (650 + 75) / 26.59 = 27 RTP = (650 + 75 + 125) / 26.59 = 32 psi
So it looks like front-rear weight distribution is not 50/50? Anyway, we're much closer. Also, see comment below about change in tire spec load rating.

The last two digits represent load rating (84 = 1102 lbs or 500 kg) and the letter represents the speed rating. T = 190 km/hr H = 210 km/hr
I agree, strange that your 185 tires are not listed. What is also strange to me is when I look up specs for these tire sizes (on tirerack.com) there is quite a bit of variation. Some tires have max pressure of only 35 psi, some have max of 44 psi for the same size and load rating, for example. Another guess would be that tires have changed significantly since your car was built and that placard stuck on the door - maybe the 185/65 size was not available in 1987? Also, load ratings for the same tire sizes apparently have changed - Your placard most likely assumes a load rating of 8402 lbs, but your tires actually have a rating of 1170. Makes a difference. If we use the original spec (back in 1987?) for a load range 84 tire of 1102 lbs, the footprint drops to
1102/44 = 25 square inches - 1.5 inches smaller, so calculated pressure goes up.

As I mentioned above, I found numbers for weight, but not front/rear distribution

VERY IMPORTANT. I don't quite understand here. 2300 lbs includes weight of car + partial load, and then 1200 MORE lbs can be added for max load? I really don't think this is correct. My E36 BMW has a max load of only 970 lbs. If weight of your car is 3500 lbs at max load, then subtracting a more reasonable 900 lbs of load we end up with car weight of 2600 lbs - matching the number I found for E30 weight.
So, it looks like we are now really pretty close. Isn't this fun? I suppose some people would say we should get a real life, but then what do they know?
Frank
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Nedavno Raybender pise:
| Are you sure of this weight? When I look up E30 sites I found weight | listed as 2600 - 2700 lbs.
It's in whiechle registration card.
| The last two digits represent load rating (84 = 1102 lbs or 500 kg) | and the letter represents the speed rating. T = 190 km/hr H = 210 | km/hr
OK, thanks.
| I agree, strange that your 185 tires are not listed. What is also | strange to me is when I look up specs for these tire sizes (on | tirerack.com) there is quite a bit of variation. Some tires have max | pressure of only 35 psi, some have max of 44 psi for the same size | and load rating, for example. Another guess would be that tires have | changed significantly since your car was built and that placard stuck | on the door - maybe the 185/65 size was not available in 1987?
I too was thinking that that may be the reason.
| Also, | load ratings for the same tire sizes apparently have changed | - Your placard most likely assumes a load rating of 8402 lbs, but | your tires actually have a rating of 1170. Makes a difference. If | we use the original spec (back in 1987?) for a load range 84 tire of | 1102 lbs, the footprint drops to | | 1102/44 = 25 square inches - 1.5 inches smaller, so calculated | pressure goes up.
Yes, with load range 84 (1102 lbs) it's:
FTP = (650 + 75) / 25 = 29 RTP = (650 + 75 + 125) / 25 = 34 psi
and that is about right for front tire, and slightly higher for rear. But, as you noted, there might be difference in max allowed load pressure for the tire now and in '87.
| > | 4. Does your manual give the rated max load for your car? | > | > Yes, 2300 lbs at partial load, and 3500 lbs at max load. | | VERY IMPORTANT. I don't quite understand here. 2300 lbs includes | weight of car + partial load, and then 1200 MORE lbs can be added for | max load? I really don't think this is correct.
I made mistake here. 2300 lbs empty, 3500 lbs max load. English is not my native language, so my concentration is not at top all the time, as I am trying to find adequate words to express myself.
| My E36 BMW has a max load of only 970 lbs. If weight of your car | is 3500 lbs at max load, then subtracting a more reasonable 900 lbs | of load we end up with car weight of 2600 lbs - matching the number I | found for E30 weight.
I am from Serbia & Montenegro, and back in '87 customs and taxes combined for import cars up to 1.6 litre 87% (250 above 1.6 litre - hard to believe isn't it), so BMW striped cars to it's bare minimum, and fitted E21 1.6 litre engine in it - rest of the world had 1.8 litre in 316 as a start engine. So data that can be found at Internet may not be correct for my car (I thing BMW had the same engine for Greece too).
| So, it looks like we are now really pretty close.
It's getting there.
| Isn't this fun? I suppose some people would say we should get a | real life, but then what do they know?
:-)
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Nedavno Yvan pise:
| I am from Serbia & Montenegro, and back in '87 customs and taxes | combined for import cars up to 1.6 litre 87% (250 above 1.6 litre - | hard to believe isn't it)
This should be:
Back in '87 customs and taxes combined for import cars up to 1.6 litre were 87% of the sum you had to pay to manufacturer (250% above 1.6 litre - hard to believe isn't it).
My 316 total cost was ~32000 DM (~17000 DM for the car + ~15000 DM customs and taxes :-(
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Yvan,
Your English is very good. Much better than my German, which is the only other language that I know even a little of. I could never have our conversation, here, in German.
I think I get it now, after you have explained that your car was stripped down to avoid taxes. (My sympathies. I think that taxes such as this really hurt many people and keep your economy from growing as fast as it would otherwise)
Anyway, things make sense now. BMW didn't change the placard from the unstripped car. Your allowed 1200 lb max load includes "my" 900 lb "more normal" load plus the 300 lbs that was removed from your car to avoid taxes. Then, apparently, BMW didn't write a new placard for tire pressure to reflect the lighter weight of the car. They just used the "standard" one, because it has the correct numbers for the 3500 lb gross weight with a full load, and it was very important that they recommend the correct pressure with the full load. Having the pressure too high for the partial load just means that you will have a somewhat rough ride over any bad roads.
Does your car give a rough ride with only a light load? Would really think that since the car is actually lighter than the placard partial load pressures assume, you could reduce tire pressure a bit, in this case, and get a better ride. Do the tires tend to wear out in the center faster than on the edges? That would be a sure sign that the pressure is too high for the load you normally carry. Reducing the pressure slightly (stay maybe 1 psi above the calculated pressure for 2300 lb car plus our partial load) would put a larger tire footprint on the road, and give you better handling and braking. Just a thought, but certainly do not reduce the pressure too much, and be SURE that your tire gauge is accurate.
This has been a lot of fun. I've enjoyed the conversation very much.
Frank
Yvan wrote:

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Nedavno Raybender pise:
| Your English is very good. Much better than my German, which is the | only other language that I know even a little of. I could never have | our conversation, here, in German.
Thank you. I am now trying to make sense of Pierburg 1B2 manual I just bought at eBay that is in German (I first bought Haynes carb manual - in English - only to discover later that my carb is 1B2 and not 1B1 or 1B3 that is covered by Haynes manual :-( Just spent a lot of time at google translator translating from German to English. As none of these languages is my native you can imagine how much fun I had. And if you ever tried google or babelfish translating from German to English, you can understand added trouble, as sometimes you probably might not understand anything too :-)
| Anyway, things make sense now. BMW didn't change the placard from the | unstripped car. Your allowed 1200 lb max load includes "my" 900 lb | "more normal" load plus the 300 lbs that was removed from your car to | avoid taxes. Then, apparently, BMW didn't write a new placard for | tire pressure to reflect the lighter weight of the car. They just | used the "standard" one, because it has the correct numbers for the | 3500 lb gross weight with a full load, and it was very important that | they recommend the correct pressure with the full load. Having the | pressure too high for the partial load just means that you will have | a somewhat rough ride over any bad roads.
You are probably correct. BMW did not even bother to print manual for my car. I only received one in German, and it's (poor) translation to my language typed with typewriter. There is only data for 1.8 litre engine, my 1.6 is not mentioned at all.
| Does your car give a rough ride with only a light load? Would really | think that since the car is actually lighter than the placard partial | load pressures assume, you could reduce tire pressure a bit, in this | case, and get a better ride.
I think that I can not detect such a difference.
| Do the tires tend to wear out in the center faster than on the | edges? That would be a sure sign that the pressure is too high for | the load you normally carry.
I knew about that, but I can not tell. This is my fathers car and it was still for some seven years, and even when it was used it was light use (it has only ~30000 miles on the clock). And I still have the original tires somewhere in the shad. As I remember they are worn evenly, not center more than edges.
| Reducing the pressure slightly (stay maybe 1 psi above the calculated | pressure for 2300 lb car plus our partial load) would put a larger | tire footprint on the road, and give you better handling and | braking. Just a thought, but certainly do not reduce the pressure | too much, and be SURE that your tire gauge is accurate.
I will try that when I find spare time. Thanks.
| This has been a lot of fun. I've enjoyed the conversation very much.
Same here. Have a nice day.
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Yvan wrote:

Not a surprise it doesn't work. It's a bogus formula. It assumes the tires have no effect at all on the load-carrying capacity and the tire footprint. They have a very large effect on both.
The tire pressures listed in your manual are those which will allow for uniform wear on the tires at a certain load, plus keep them safe. These have been skewed in recent years since increasing the tire pressure increases the MPG of the vehicle by reducing rolling friction. Since manufacturers like to advertise as high an MPG (and avoid gas-guzzler taxes in the USA) - they tend to set the tire pressure higher than optimal for ride/handling/comfort/wear.
It's a whole bunch more complex than simply dividing PSI by weight.. although I guess that sounds like a nice idea.
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admin wrote:

Well, actually it worked quite well after we found out the true weight of his car vs the weight the placard was written for. His car was modified.
You should read the rest of the thread.
Frank
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Bullshit.
The number printed on the tire is the Max Pressure at the Max Loading. This is NOT the tire pressure to inflate to. The tire pressure to inflate to is printed on a placard on the door pillar. If you don't want to bother reading the placard, put 30 psi in all four tires for all occasions.
If you want the right pressure, read the placard.

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The placard is for the factory fitted tyres on the factory wheels (typically 60% profile on smaller wheels (in this case 15")). IIRC lower profile tyres on larger wheels (17") will require more pressure to remain correctly inflated and in particular keep the rim from damaging the tyre. Why not have a look at the recommended pressures for vehicles that are fitted with these tyres.
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