Community Forums

Sky_Boss · 01-22-2013, 08:32 AM

Does anyone know why Newmar derated the Mountain Master IFS axle from 14,600# to 14,200#? When ever I talk with Spartan they always say that it is rated for 14,600#.

The closest theory I have is that they opted for Michelin 305/70Rs instead of going bigger. I also think that gives buffer for an imbalanced set up where one tire weight might require the full 120 PSI or more. That would be a HUGE imbalance to make that happen. Even at 115 PSI (7570# max wheel weight) this would be a set up for excessive L - R balance problems.

bonzocfi · 01-22-2013, 10:00 AM

You might want to ask Newmar that question. On my 05, Newmar derated the steering axle from 14,600 to 13,200 because they had Spartan put 275/70 tires on. Two years ago, after talking with Spartan, I put the 305/70's on and picked up that extra 1400 lbs. As a side note, since the 06 model year, Newmar has put the 305/70 on the Dutch Star.

Route 66 · 01-22-2013, 10:14 AM

Newmar used to reduce ratings for model separation. Higher priced models had more "capacity".

However, as noted above, contact Newmar.

SteveLevin · 01-22-2013, 01:23 PM

Brings up a good point in all this... an axle limit is the maximum of the tires fitted to it, or the actual axle rating, whichever is LESS. It can be easy to forget that a beefy axle doesn't magically make your tires carry more load -- or vice versa.

Steve

Sky_Boss · 01-22-2013, 01:34 PM

Quote:

Originally Posted by SteveLevin

Brings up a good point in all this... an axle limit is the maximum of the tires fitted to it, or the actual axle rating, whichever is LESS. It can be easy to forget that a beefy axle doesn't magically make your tires carry more load -- or vice versa.

Steve

True...so true. LOL I once checked out a 2002 MADP 4302. The Camping World had gotten it in recently with relatively new tires...WRONG SIZE! Instead of the 275/80R LRH it had 275/80R LRGs. With the tile floor and a previous weight slip, I figured that with full fuel and water, driver & passenger we were only 100# below the max tire rating.

No way those tires would stand the designed GAWR.

Sky_Boss · 01-24-2013, 05:02 AM

Newmar Answer:

The reason that the front end would be rated at the 14,200 lb. rating instead of a 14,600 lb. rating would have been because of the tires. We build a tolerance into the front end to give some extra clearance in the tire ratings for safety reasons. The tolerance would also be built in because the front weight side to side can be different and is usually not the same side to side on any coach. If we can be of further assistance, please let us know.

Brigadoon · 01-24-2013, 07:03 AM

Exactly the reason to know the weights on each tire and to know the capacity of the tires in use as well as their proper inflation.

Sky_Boss · 01-24-2013, 08:39 AM

Quote:

Originally Posted by Brigadoon

Exactly the reason to know the weights on each tire and to know the capacity of the tires in use as well as their proper inflation.

I think this may have been a extreme example of CYA on Newmar's part. The 305/70R tires are very capable. At 115 PSI 7570# and 120 PSI 7830# each. Even at 115 PSI (total 15140#) it would take a significant imbalance to put the full 7570# on a single tire. Given the typical practice of adding 5 PSI more safety I can't see this being more than a buffer for the lawyers. Don't get me wrong, I understand any company trying to avoid product liability claims from folks that just don't use the grey matter between the ears. I just don't like