Load rating help (dilemma)

[mrainv23]

The pirellis I purchased for my 21s are load rated 93y in the front 255/30zr 21 and 96y in the rear 295/25zr 21. Bmw m6 gran coupe calls for 99 upfront and 101 in the back. Is this a serious issue should I be concerned?

[M6-Coupe]

As long as you don't over load your car, should be fine.
I think the maximum allowable load for M6 GC is ~900 lb and your 96 rating tires up front can handle the portion of this load is transferred to the front...

[mrainv23]

Technically following these guidelines the only 21" tire set up that is deemed appropriate would be Michelin pss 305/25/21s and 265/30/21s. Which are rated 98 in the rear and 99 in the front. Back is off by 3 clicks in rear from Oem guidelines but at 40psi is reasonable.

Of Course these tires are also on national back order.

[mrainv23]

Worried because my kids ride in the car with me and tire shop told me I am crazy and taking a big risk with my current set up.

[mrainv23]

Let me know if my math/ thought process is correct. GVWR for M6 is 5315lbs. Divided by 4 is 1328lb per tire. 93xl tire is rated for 1433lbs and 96xl is 1565xl in rear. This gives me approx 105lbs to 235lbs of wiggle room.

[M6-Coupe]

Originally Posted by mrainv23

Let me know if my math/ thought process is correct. GVWR for M6 is 5315lbs. Divided by 4 is 1328lb per tire. 93xl tire is rated for 1433lbs and 96xl is 1565xl up front. This gives me approx 105lbs to 235lbs of wiggle room.

Oh I read the numbers incorrectly... My bad... For some reason I thought your front was 96 and didn't notice the rear...
Yeah I agree that 93 front and 96 rear is risky. Your calculation is a little off too.
Your M6 GC has 52.3/47.7 weight distribution so with no load at all, Curb weight ~4450 lb you have ~2330 lb on front or 1165lb on each tire and 2120 lb on rear or 1060 lb on each tire. The majority of back seat passenger's weight is applied to the rear axle and for driver and passenger (front seats) almost evenly to both front and rear axles so if for example you have 2 persons each 200 lb front and 2 persons each 200 lb on back and 50lb weight in trunk, then you are putting ~1300 lb on front and ~1350 lb on rear tire. It might look ok with your current setup BUT there is one important thing you have ignored and that is "hard braking" and "hard acceleration" subsequence. When you apply hard braking/hard acceleration there is weight transfer to front/rear end of your car and I'm sure if your car is loaded then your tires will be under big stress which is dangerous and that's why manufacturer has considered 200-250 lb more rating (99 front, 101 rear); so if I were you, I would definitely either replace the tires or drive like grandma when I had passengers...

[mrainv23]

Very helpful. Thank you!

[mrainv23]

So what I learned today is that anyone that drives an M6 that wants to upgrade to 21" wheels is out of luck because tires are not in stock that have the correct load ratings ie pilot sports.

[M6-Coupe]

Yes you are right. Another thing you and others must learn is that "this state of art car" is designed and manufactured by one of the best automakers and engineering teams so what they have suggest and installed on the car is the most reliable and optimum part/component for the safest, most pleasant, and highest accurate driving experience if you want to get the best performance of your car .... cosmetic part (spoilers, diffuser, ...) are exceptions.

[chask]

M6-Coupe is right on target. I would add that adjusting tire pressure might help but it is not a panacea for all ills in this case and you have to be careful when increasing as there are other factors involved too. Would you please post exactly what you are running - brand, model and size? There are some specs online that might help.

However, I would disagree somewhat about cosmetic things, at least in the aero area. If you drive fast (even just at highway speed limits or below) they can produce extra load on the car via increased down force - i.e. greater load on the tires without actually adding any weight to the car. It will be much greater if you are driving into a good head wind. You would be surprised how much force just a single psi (pound per square inch) can add in force when applied over a fairly large surface. Measure a door in inches and calculate the area, then multiply by half a psi to get an idea of the ponds force applied by some good winds. You could also stick your hand out the window and into the airstream at normal highway speeds. There's a big difference between holding your palm parallel to the ground and perpendicular to the ground. Think about that force applied over the entire area of your rear spoiler (if you have one) or the front spoiler built into the car. The numbers (pounds of force) can be surprisingly big.

[M6-Coupe]

You are right about those factors, but actually the M performance CF (I'm just talking about OEM BMW part) diffuser/spoiler have better down force performance than the plastic one is installed on stock car and the reason they are not standard on stock car is the pricy CF.

[chask]

I just meant if the OP has some aero on his car, including the stock aero to some extent, it will also add to the effective net weight at speed. That is where one needs to be concerned, and especially if he lives in an area where they have some good winds and/or he drives fast - factors that would affect the aero. Those conditions can push the effective load closer to the load limit of the tires. Some of the aero packages I have seen for sale look like they could have a significant affect on down force. The tires have no clue what is causing the extra weight. All they know is that it is there.

I'd look at the load limit of the tires. Compare it to the load limit of the tires that came off your car. Also compare this to the load limit of the other tires BMW sells the car with - they may be more or less than the ones that were on your car.

M6-coupe is absolutely right though about weight distribution. I am not a tire specialist (in other words - all of this may be wrong) but I would think you would need to balance the vehicle weight first by axle (front and rear) using the weight distribution spec provided by BMW would be the place I would start. Then look at where the extra weight you put in the car is located. Though some of the weight put in the trunk will be added up front, it would seem logical that most of it will be on the rear wheels - as it would seem logical that the rear passengers weight would primarily be there also (at least most of it). The drier weight will most likely be as M6-coupe suggested - at least that is what I would think. But what do I know about this? As I said - next to nothing. Talk to an expert.

I'd also be careful about "experts". I remember one place that would inflate the tires to the maximum pressure on the side of the tire - in the case I am thinking of that was 50 psi. That is the most pressure that should be put in the tire, not what you should run them at during normal routine driving. That said, I am pretty sure that maximum load for a tire is proportional to the inflation pressure.

[chask]

Originally Posted by M6-Coupe

You are right about those factors, but actually the M performance CF (I'm just talking about OEM BMW part) diffuser/spoiler have better down force performance than the plastic one is installed on stock car and the reason they are not standard on stock car is the pricy CF.

My M6 came with a CF diffuser. Is that the "better" part you are talking about or a more cosmetic rather than functional piece? It definitely has a different appearance than most of the stuff I see for sale online or that you guys are buying. I can't bend over to really take a good look at what is going on down there but I could see how a good piece down there might help a lot. With all the wind we get around me I'd really like to have some good aero on the car - better than stock if possible.

[M6-Coupe]

Originally Posted by chask

My M6 came with a CF diffuser. Is that the "better" part you are talking about or a more cosmetic rather than functional piece? It definitely has a different appearance than most of the stuff I see for sale online or that you guys are buying. I can't bend over to really take a good look at what is going on down there but I could see how a good piece down there might help a lot. With all the wind we get around me I'd really like to have some good aero on the car - better than stock if possible.

Like I said the spoiler and diffuser are cosmetic; however in case of diffuser, the CF BMW M Performance even delivers slightly better aerodynamic rear end character and reduces the turbulence for those want to have better lap time ie on track or 1/4 mile (IMO). On the other hands, most of the aftermarket rear spoilers in market are almost the same size and shape of stock one but CF. I highly believe modifying this kind of high performance cars needs engineering design and knowledge and as long as I'm not convinced the aftermarket parts or tuning are designed and manufactured based on truth technical specs of this car and will not hurt the whole functionality and life time of other components/parts (like extra torque and power in case of engine tuning that I believe will reduce the life time of DCT due to more input torque), I would never install any aftermarket parts on my car and that's why my M6 is bone stock except the 305 rear tires due to traction issue because I love launching my M6

[chask]

Originally Posted by mrainv23

The pirellis I purchased for my 21s are load rated 93xl in the front and 96xl in the rear. Bmw m6 gran coupe calls for 99 upfront and 101 in the back. Is this a serious issue should I be concerned?

I checked the Tire Rack website for specs on Pirellis - all of the data below is from their website. I have no idea if it is accurate.

The only P Zeros that fit with your load ratings were the 255/30ZR21 (93Y - 1433 lbs max load rating) and a 245/35ZR21 (96Y - 1565 lbs max load rating). These are way smaller than the tires that came on the car - 265/35 up front and 295/30 in the back.

Have you checked the rim width for your tires and made sure they fit the rims? The 255 is usable on a rim width range of 8.5-9.5". The 245 is okay for rim widths ranging from 8-9.5".

In addition, if I am reading your posts correctly it sounds like you have a 255/30 in the front and a 245/35 in the rear. That is not a good thing on a car that is supposed to have a 295 in back and a 265 up front. I would not drive like that at all. Especially not in a M6. Please tell me I am mistaken about all of this.

My car came with 20" Pirellis - 265/35 up front (99Y - max load 1709 pounds) and 295/30 in the rear (101Y - max load 1819 lbs).

It looks like the 285/30ZR21's would work up front (100Y - max load 1764) and the 325/25ZR21's in the rear (102Y - max load 1874). However, the 285's are about 4/10 of an inch taller than the stock 265/35ZR20 tire I have (Michelins are supposed to be the same). Not sure if they would fit but I think so from what I have read posted around here. Twitch Massacre has the 325/25's on his car. I don't recall what he is running up front. There is a pretty long thread he started about his process of getting wheels and the tires on his car.

Also I believe that the load rating is proportional to inflation pressure and that the maximum load rating is achieved at the maximum inflation pressure. I think the door panel sticker on my car shows an inflation pressure of around 33-34 psi. If you use a different size you will have to adjust the pressure to whatever tire you are using. You might want to check that out as you set your tire pressure. I think there are several sites that have information about inflation pressure and load capacity posted on them.

Originally Posted by M6-Coupe

Oh I read the numbers incorrectly... My bad... For some reason I thought your front was 96 and didn't notice the rear...
Yeah I agree that 93 front and 96 rear is risky. Your calculation is a little off too.
Your M6 GC has 52.3/47.7 weight distribution so with no load at all, Curb weight ~4450 lb you have ~2330 lb on front or 1165lb on each tire and 2120 lb on rear or 1060 lb on each tire. The majority of back seat passenger's weight is applied to the rear axle and for driver and passenger (front seats) almost evenly to both front and rear axles so if for example you have 2 persons each 200 lb front and 2 persons each 200 lb on back and 50lb weight in trunk, then you are putting ~1300 lb on front and ~1350 lb on rear tire. It might look ok with your current setup BUT there is one important thing you have ignored and that is "hard braking" and "hard acceleration" subsequence. When you apply hard braking/hard acceleration there is weight transfer to front/rear end of your car and I'm sure if your car is loaded then your tires will be under big stress which is dangerous and that's why manufacturer has considered 200-250 lb more rating (99 front, 101 rear); so if I were you, I would definitely either replace the tires or drive like grandma when I had passengers...

There are two additional high loads scenarios I could think of. One is hitting a good sized bump or a pothole or any deviation in the road (like a drop in pavement level for construction). Those can definitely be blowout situations. The other is cornering. Even if you don't go pushing it around corners and actually do drive like the little old lady who just puts around you might have to do some serious maneuvering to avoid an accident or other bad circumstances - road hazards, people, bikes, animals, etc.

Originally Posted by mrainv23

So what I learned today is that anyone that drives an M6 that wants to upgrade to 21" wheels is out of luck because tires are not in stock that have the correct load ratings ie pilot sports.

I read somewhere else here (can't remember where though - sorry) that Michelin is discontinuing the Pilot Super Sport (PSS) in the 325/25ZR21 size. I am in the process of trying to decide on tires and want to use PSS tires. I came across some kind of conglomeration website that had tires and specs from not just several makers but several vendors too. I noticed that it did not even list that 325/25ZR21 size in the Pilot Super Sport section. You might want to try a search for the tires you want to run. Somebody probably has them somewhere.


Personally, I would be very leery about driving the car with those tires on it; even if it was just me in the car. The load on any car shifts around as you drive - brake, accelerate, corner and hit bumps. But the M6 has a lot of power and a fabulous suspension. It will shift the load around a lot more than most cars.

Unless you are going to drive it like the proverbial '90 year old lady who only goes to church on Sunday and then the store', I do not think I'd drive with anyone else in the car either, and especially not my wife and kids. I'd never be able to forgive myself if they got hurt. I would go back to a stock setup or get some bigger tires. I am not sure that on there now is very safe. Unless things have changed since I used to do this, generally the reason for going to a larger rim diameter is to run a wider and lower profile tire. I am not sure I understand the rationale behind getting these tires.

The big dilemma for me is getting something wide that will fit under there and not be way off specs. I think if you go to a custom rim and aren't lowering the car you can probably get some pretty wide tires under those fenders without rubbing. I am assuming yours are like mine. Twitch Massacre (TM) has some wide 21" sneakers on his car and they seem to work - check his thread. I will add that I thought they look incredibly good. Tire Rack has the Pirellis in stock (at least they did when I wrote this) and more coming in. TM wrote about having a slight rubbing issue and described the location. But when I got mine up on a lift I could not find any body panels on mine that were anywhere near the tires, and nothing in the area he described where he said he had trouble on his; actually, or anywhere else for that matter. There literally wasn't anything within several inches of the rear tires on my car.

Whatever you decide to do, good luck getting this resolved.

[mrainv23]

The tires a have our Pirelli p zero neros. Front are on a 21x9 wheel 255/30zr 21 93y load rating. Rear are on a 21x10.5 wheel 295/25zr 21 96y load rating. Many ppl are running the Michelin 295/25/21 set up in the rear and apparently that is also not a safe option. You need to run 305s.

[mrainv23]

With a drop finding the right tire without being able to run michelins is now another issue.

[chask]

Originally Posted by mrainv23

The tires a have our Pirelli p zero neros. Front are on a 21x9 wheel 255/30zr 21 93y load rating. Rear are on a 21x10.5 wheel 295/25zr 21 96y load rating. Many ppl are running the Michelin 295/25/21 set up in the rear and apparently that is also not a safe option. You need to run 305s.

Hmm? I missed those tires. That ought to be a reasonable setup, except for the load rating.

I love those rims and love the way they look on your car. I think if I were going to buy rims what you have would be my first choice (21" or 20") and in that same color too. That looks so cool.

Originally Posted by mrainv23

With a drop finding the right tire without being able to run michelins is now another issue.

Check this link -

He had to do some offset adjustment but I believe he was running 325/25 rear.

[mrainv23]

I put the wheels on last week and they fit, ride, and look perfect. I am just sketched out about the load rating.

[mrainv23]

Located some Michelin pss 305/25zr 21s and 265/30zr 21s. Problem solved! If anyone is interested in my pirellis, used for 5 miles, let me know (basically new). I do not recommend for gran coupe due to my previous concerns, but people are using this set up on f12/13 without issues due to lighter curb weight and gvwr.