Load Rating? How critical is maintaining load rating near OEM Specs?

[rsiwicki]

I want to change tire sizes on my Q and have a question about going down in load rating from a 95 rating to say a 93 rating. In otherwords, I want to switch from a 215/65/15 to a 245/40/18, but the load rating goes from 95 to a 93. How critical is this on tire performance and safety issues? I will be driving my Q hard and auto crossing every now so if load is an issue I need to make sure that I get it right now versus finding out later on the auto cross track.

[Exar-Kun]

meaintining the lod rating on a heavy vehicle is very important, if you lower the load index signifigantly, you ususally ahve to incresse the air pressure to meet the load bearing requirements of the MFR, ie a 32 psi on a 95 load index tire amy mean if you use a 93 you'll need 35-38psi, etc....

I would keep it as close tos tock as possible, if not greater.-chet

[rsiwicki]

thanks chet for your response...now that makes some sense about increasing air pressure to compensate for lower load rating. I am still on the yeah neah side of things and don't know which way to go yet. I will talk with a local tire shop tomorrow to see what they say.

[AZhitman]

Rob - May want to consider a 255/40/18 for our application (as we were discussing). Conti makes a nice set in this size...

[Q45tech]

Load rating has a secret benefit [besides defining the failure point].

When you go above the factory minimum, the slip angles decrease for a given load! The tire has the ability to handle better besides the fact it last longer. About 1/3-1/2 of the extra load rating translates to better handling and braking distance....3-4% improvement as the tires don't reach a higher internal temperature for the same load.

A smart owner would always run tires which are 3 points [roughly 120 pounds] stronger than oem in the same make model tread design.

Example on old Q45 215/65/15 95V [1521 pounds] for years I have been using 235/60/15 98H [1640 pounds]. Many have great luck with 255/50/16 [98-99 rating].

Besides the safety and legal implications of running an overloaded tire........the wear rates are costly.

Every Nissan I've checked overloads the oem tires in maximal braking........luckily the tires can stand it for 3-4 seconds but it takes its toll especially in summer heat.

The factory rating is the minimum for AVERAGE drivers, they assume enthusiast would understand STRONGER is safer and BETTER.

[Q45tech]

Don't assume that all brands are the same: Many low cost tires just meet the load test failing 1 minute after the proscribed time, some like Michelin [due to MUCH stronger more expensive materials] can go almost indefinitely [at least an hour] at the speed and full load rating.

When I look at cheapo tires I always BUY UP in load rating, hopeing that gets me a little margin.

Tire load rating is tested without any CAMBER in lab on a cooled steel drum at speed. If you exceed 100F ambient [or 100F tire temperature] or have any camber from lowering or suspension misadjustment/or just agressive suspension.........the load rating can be easily exceeded.........why tires wear on the inside edge.....the forces are concentrated there and the EXTRA heat wears the tread.....even in winter.

[Exar-Kun]

"Don't assume that all brands are the same: Many low cost tires just meet the load test failing 1 minute after the proscribed time, some like Michelin [due to MUCH stronger more expensive materials] can go almost indefinitely [at least an hour] at the speed and full load rating."

excellent point.-chet

[rsiwicki]

thanks again Q45tech for the insight into load rating. Looks like I will be changing the sizes of my tires.....I thought it was interesting that almost all 245/40/18 tires are a 93 rating while I can get a 275/35/18 that is a 95 rating. I would have thought the 35 series would not be higher than the 40 series.

[Q45tech]

My point is that a 95 rating only applied to oem size 215/65/15 with oem wheels

A higher than oem rating would be a good thing when going to different tire brands and sizes.

Q eat tires so a 97- 98-100 rating would be desirable.An 18" wheel and tire places more stress on tire: more weight and possibily more agressive driving.

[Grant@tirerack]

[Dori Dori]

And why on earth everyone seems to use 215/40/17's is beyond me. I think some of you guys are too concerned w/ the JDM stretched look.