What advantage/disadvantage to dual rear axles?

[oldryder1]

I am presently researching diesel pushers to replace my Winnie gasser.

I see some of the larger DP's have a pair of rear axles.  I am wondering what the advantage(s) are to that design given the additional weight and suspension complexity.

Is there a length where the extra axle becomes an obvious advantage?

Some other question I should be asking?

thx to anyone taking the time to offer advice.

Mark in MN

 

[sadixon49]

It's purely weight carrying ability. Most duals are rated at 20K #, and fronts can be rated for up to 12K #, so any RV over 32K # requires a second or tag axle. Some wheel axle ratings are lower than those so you can see RVs with tags at 28 to 30K #. RVs with tag axles tend to ride better, but then you get to pay for an extra pair of tires at tire changing time.

 

[Gary RV_Wizard]

As Steve says, it is purely about weight.  For decades motorhome rear axles were legally limited to 20,000 lbs, so bigger/heavier coaches needed an additional axle. Now that limit has been increased to 24,000 lbs, but some state highways and many bridges have lower limits, so most coaches still use 20,000 lbs/axle as a practical limit.

There are fringe effects of having a tag axle, so you are likley to see a lot of pros and cons debated online. However, those items are largely academic. Chassis builders don't employ a tag axle unless there is a need for extra weight-carrying capability.

Ancillary effects of a tag axle design:

1. Pro: the ride is smoother over potholes and RR tracks
2. Con: two more tires to buy and maintain
3. Pro: Better tracking in crosswinds (more tire grip)
4. Con: the tag tires get scrubbed (dragged sideways) in a turn, increasing wear and turning radius
5. Pro: greatly increased GVWR
6: Con: can lose drive axle traction crossing a dip (the tag keeps the drive axle above the road surface)

 

[JD Sharp]

You should consider the added weight of the tag axle and what it is carrying on fuel economy. My brother had a 40' Country Coach with tag axle and I had a 40' Beaver without. Country Coach weighed 42,000lbs and got an average 7.5 MPG and the Beaver was 32,000lbs and got 8.5.

 

[SargeW]

It's purely weight carrying ability. Most duals are rated at 20K #, and fronts can be rated for up to 12K #, so any RV over 32K # requires a second or tag axle.

Not necessarily. My 40' Tiffin Allegro Bus has a 24,000# rear axle and a 16,000# front axle for a GVWR of 40,000#. The tag axle Bus has a GVWR of 51,000#, so you can see that manufactures are finding ways to increase the weight of newer RV's while keeping the footprint the same.  To handle the extra weight chassis comes with 315/80R22.5 tires. 

 

[Gary RV_Wizard]

Most duals are rated at 20K #, and fronts can be rated for up to 12K #, so any RV over 32K # requires a second or tag axle

Just for the record, front axles can go well over 12,000 lbs; I've owned coaches up to 14,600 and have seen 16,000 lb front axles on others.  My last coach had a 36,600 lb GVWR and no tag (14,600 front and 20,000 rear).

 

[Kevin Means]

We're on our 3rd Class A in 25 years, and it's really an apples-to-oranges comparison to compare our current tag axle equipped diesel pusher to our previous two gas coaches, because they were so much lighter. The weight alone would partly explain why our current coach handles so much better, and why the ride is so much smoother, but I'm quite sure the tag axle has a lot to do with it too.

We bought a tag equipped coach primarily for its increased load-carrying ability (it's significant) but it probably wasn't necessary. Several 40' coaches would have been more than adequate. However, we have been very impressed wth the handling and ride, some of which can be attributed to the tag. There's virtually no porpoising on uneven roads, and crosswinds and passing semi trucks don't have nearly the affect on us as they did in our previous two coaches.

Here are a few other things... We boondock a lot, and have gotten the drive axle high centered a few times when trying to get over dirt berms - as Gary mentioned. Unless you have a steerable rear axle (we don't) a tag equipped coach won't be able to turn as sharp, but that hasn't been a problem for us. There's a little extra maintence associated with having a tag, but IMO it's negligible. When it's time to replace tires, however, it'll cost about $1000.00 more than if you didn't have a tag.

Kev

 

[Gary RV_Wizard]

As with many technical spec things in an RV, avoid overthinking this question.  Choose your coach based on your space and layout needs plus desired house amenities & features. The chassis designers will provide a tag axle if needed to handle the size of the coach.  You should be aware of the other pros and cons besides the weight-carrying capability, but it's rarely going to be a decisive factor in the purchase.  Your worry-time is much better spent making sure your wife is happy with the upholstery & cabinets.  ;D