Hi,I replaced the springs and had a leaf added on each side.There were air bags that may have been improperly installed from the get go--7 inch bags in a 5 inch space, so the brackets torn away from the frame.The air bags have been replaced with timbrens, which do not touch the frame--but might if I had full water tanks (66 gallons).I put on new monro shocks at the rear.Results (big surprize).The RV rides 2.5 inches higher at the rear.Ride is improvedAnd now the shock. It appears from early results that gas mileage has gone up by about 1.5 to 2 mpg. I've asked one mechanic and he is totally puzzled.Anyone care to offer an explanation?

Could raising the rear have lowered the front, OR changed the angle of the wind over the front? would be my wild guess.

Weigh the axles/ wheel weightAnd compare to previousI think the center of gravity has movedPutting more weigh forward and less on the rear drive wheels

NOt a mechanic but have spent a lot of time on the forums over the years. One thing that is common and often reported is noticeable mpg improvement after wheel alignment. Did the bad springs contribute to poor alignment? But 1.5-2 that is a 20% improvement for most, far beyond anything I have heard. Start with checking your math LOL

The wheels had been aligned less than 1000 miles ago.

You were going uphill before :)Seriously, I would bet aerodynamics relating to the underside of the coach.

first report new springs with a surprize result

Harvard
Explorer
Nov 20, 2019
pianotuna wrote:
The wheels had been aligned less than 1000 miles ago.

If the wheel base were 158 inches, raising the rear end by 2.5 inches would reduce the front end caster by about 1 degree.
JaxDad
Explorer III
Nov 20, 2019
philh wrote:
JaxDad wrote:
As a pilot with a degree in aerospace engineering I can tell you that “parasitic losses” can manifest themselves in many weird and wonderful ways. Likewise the gains created by eliminating them, witness the little air tabs (originally designed for aircraft) that can measurably increase fuel mileage on a coach.

In this case, I’d say an overloaded suspension created excessive (even if you weren’t aware of it, the slow creep is imperceptible) motion which created the losses you’re now recouping.

At Motorcoach speeds, parasitic losses on a brick aren't going to amount to much. OP said 1.5 to 2 mpg improvement. There was either something very seriously wrong with the alignment or the OP's calculations are a bit off.

On he contrary, any improvement in the aerodynamics of a ‘brick’ result in pretty substantial gains. Bear in mind also that as speed increases the drag does NOT increase proportional to the gain. According to Bernoulli's equation, drag is proportional to the SQUARE of THE speed.

Look up “AirTabs” and the gains that can be made by just adding some little plastic stick on tabs to a ‘brick’ and changing the vortices that form BEHIND it.
pianotuna
Nomad III
Nov 20, 2019
Hi,

I've now driven about 600 miles and I continue to see improved mileage.

I'm on a long haul to Florida--and tomorrow instead of driving my "economy speed" I need to drive the speed limit.
landyacht318
Explorer
Nov 19, 2019
Agree that one data point is nearly useless. Especially when human emotion is is fed through right foot.

I can see a sagging coach body creating more air resistance in between axle and road that is relieved somewhat when raised, but a 2.2mpg increase seems highly unlikely. Report back once you get to Florida and watch the roadside flags for wind direction and make a note of it on your miles per tankful log.
pauldub
Explorer
Nov 19, 2019
One data point is not statistically significant, but we always want it to be when we like the data.
Gjac
Explorer III
Nov 19, 2019
pianotuna wrote:
Hi Huntingdog,

It drives pretty much the same way. The front end has always been on the light side. Once, the folks who rotated the tires put 85 lbs in the fronts. That made it quite uncomfortable to drive--when I went back down to 65 it was back to normal.

I've toyed with the idea of lowering front wheel pressure, but I think that might increase the major cost of my RV which is fuel.

It is early days as I've only driving it about 200 miles.
I think your answer is the 200 miles driven. One tank is not enough to prove a mileage increase like you asked about. I track every tank of gas and tried a lot of different thing to improve MPG. Went from 8.5 to 12.5 mpg one time, after the next tank it went back to 8.7 MP.
Huntindog
Explorer
Nov 18, 2019
I have read of cases where bad alignment can cause poor MPGs.... Because the tires can act almost like a brake... But the tires would be wearing pretty fast in that case
philh
Explorer II
Nov 18, 2019
JaxDad wrote:
As a pilot with a degree in aerospace engineering I can tell you that “parasitic losses” can manifest themselves in many weird and wonderful ways. Likewise the gains created by eliminating them, witness the little air tabs (originally designed for aircraft) that can measurably increase fuel mileage on a coach.

In this case, I’d say an overloaded suspension created excessive (even if you weren’t aware of it, the slow creep is imperceptible) motion which created the losses you’re now recouping.

At Motorcoach speeds, parasitic losses on a brick aren't going to amount to much. OP said 1.5 to 2 mpg improvement. There was either something very seriously wrong with the alignment or the OP's calculations are a bit off.
pianotuna
Nomad III
Nov 18, 2019
Hi Huntingdog,

It drives pretty much the same way. The front end has always been on the light side. Once, the folks who rotated the tires put 85 lbs in the fronts. That made it quite uncomfortable to drive--when I went back down to 65 it was back to normal.

I've toyed with the idea of lowering front wheel pressure, but I think that might increase the major cost of my RV which is fuel.

It is early days as I've only driving it about 200 miles.
Huntindog
Explorer
Nov 18, 2019
pianotuna wrote:
Hi,

I replaced the springs and had a leaf added on each side.

There were air bags that may have been improperly installed from the get go--7 inch bags in a 5 inch space, so the brackets torn away from the frame.

The air bags have been replaced with timbrens, which do not touch the frame--but might if I had full water tanks (66 gallons).

I put on new monro shocks at the rear.

Results (big surprize).

The RV rides 2.5 inches higher at the rear.

Ride is improved

And now the shock. It appears from early results that gas mileage has gone up by about 1.5 to 2 mpg. I've asked one mechanic and he is totally puzzled.

Anyone care to offer an explanation?
Raising the rear will change the alignment. Even a low tire will have an effect. In fact the first step a good mechanic does is setting the tire pressures to spec, when doing an alignment.... Having said that. I don't think it would change so much as to cause the results you are seeing. How does it drive? Monitor the tires for unsual wear. If that all checks out, it might be a good idea to get the alignment checked (not changed) so you can record the specs as to where it is set now... And use those specs from now on.

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