Axle ratios

[Swadian]

Please explain axle ratios to me and their effect on fuel economy. With a Series 60/B500 powertrain, I would like to know the absolute best possible axle ratio for fuel economy. I do not need to tow.

Thanks.

Cheers.

[Lee Bradley]

What speed do you plan on travelling?  Any mountains in your plans? What does your bus weigh?

[HB of CJ]

Engine tune?  Wind resistance?  GVW?  Rolling resistance?  Duty cycle?  The same engine may work in one application but not in another.

Back in the day, our old DD fire apparatus was geared very short.  The application required maximum acceleration and not much else. 65mph @ 2400..

Same with inner city transit buses?  I drove one that would go a max of 45 mph.  That was all that was needed.  It sounded like max rpm.  2 stroke DD

My old Crown Supercoach Small Cam Cummings ran at 1500 rpm @ 60 mph.  The sweet spot.  Gearing depends upon many factors and needs.  A DD 60 ...

MIGHT, (might) like 1200-1400 rpm IF the application would allow it.  Pick a reasonable cruising speed.  "Lugging" hurts.  "Running Free" wastes fuel. 

If one had a 10 speed Roadranger and all the dash gages including boost, pyro, fuel pressure, fuel flow, etc., one could pick the correct gear for the situation.

In a Bus Conversion one may, (may) not have that liberty.  One just drives the thing and has fun with it.  Pick a cruising speed and enjoy the ride.

Some help here please.

[RJ]

Swadian -

Simply, the larger the axle ratio number, the faster you'll accelerate, but your top speed will be lower.

Example: A transit bus with a 5.36:1 ratio will accelerate to 40 mph faster than the same bus with a 3.33:1, but will top out at 57 mph.  The 3.33:1 bus will be slower to 40 mph, but will end up doing 80 mph. 

Another way to look at it is that for any given road speed (say 50 mph), the bus with the 5.36:1 ratio's engine will be turning higher rpm than the bus with the 3.33:1 ratio.  Obviously, then, the higher rpm engine is going to burn more fuel at the same road speed, thus lower economy.

If you have an S-60/B500, chances are good that the factory engineers specified a rear axle ratio that gives the best compromise between decent performance and acceptable fuel economy.

Of course, the weight of the operator's right shoe has a big influence on fuel economy, too!

FWIW & HTH. . .

[luvrbus]

The most common ratio for the series 60 with the B500 to utilize both overdrives was 4:30 to 4:56,some Prevost were 4:78.The new generation Allison offers different overdrive ratios you see 3:78 ratios now to utilize the torque range at low RPMs.
I have several friend that tried to use 3:73 gears with the older B500 and the transmissions just hunt between 5th and 6th so all they use is 5th (1st overdrive),my MCI has 4:56 gears with the B500 I am 7.8 at 70 mph.Can you tell us what bus and year model you own   

[Swadian]

It's a 102DL3. Out here, the roads are straight and flat and the speed limit is 80 mph. I don't use it in the mountains (it bottoms out too easily), so the standard compromise probably doesn't apply. I specifically want it to be able to go 80 mph on the flat as efficiently as possible, so I want the RPM to be as low as possible at 80 mph. Don't call me a speed demon; if the speed limit is 80 and I can safely go 80, I'm going 80.

What is the numerically lowest axle ratio possible for this vehicle? Is it 3.58:1?

[luvrbus]

What 60 series 11.1 or 12.7 and HP MCI is not big on high HP engines 

[buswarrior]

As asked, there are a bunch of different engines that cover the 102D, all of them are called "Series 60" but the evolving emission controls changed their operating characteristics, and the sweet spot for fuel economy. Two different displacements, 11.1 and 12.7. Later, the 14 litre came along.

And the B500 of that vintage has a minimum rpm required for its lubrication pump, (look it up, but up 1500 -1600 someplace???) and it will not shift to 6th gear without the necessary RPM, defeating the whole exercise...

That RPM is significantly higher than the typical fuel economy RPM of today's engines, so busnuts are going to get into trouble mixing and matching their found treasures.

That said, the S60 of those older days was happy spinning at whatever the B500 needed, they were a good pairing.

4.56 differential went out the factory door in many, many of them.

80 mph in a motorcoach and seeking fuel economy in the same thought, I expect there is little to no wiggle room, it will cost what it costs, with little chance to improve or worsen it with the available stock bits?

happy coaching!
buswarrior 

[lostagain]

80 mph is not safe in a bus. Too heavy. It may feel good and steady, but stopping it will be tough. What if you blew a tire? 65, 70 is lots.

JC

[TomC]

B500 top 3 gears are 4th-direct 1:1, 5th- .74 overdrive, 6th-.64 overdrive. You want to gear a S60 to cruise at 1400-1600rpm. If you have 12R-22.5 rubber, that is 485rpm (revs per mile). For maximum fuel economy, then you want to be turning 1400 at 80mph-or 1050 at 60mph. Divide by .64 and that gives 1,640 in direct. Divide by the tire 485 and that give 3.38 ratio.
How much does your bus weigh? Do you pull a towed? At 3.38, startability could be a factor (slow off the line, hard to start up a big hill).
Personally, I would gear for running at 1600 @ 80. That works out to be more like a 3.90 rear ratio, that will give exactly 1614rpm at 80mph. Good Luck, TomC

[Swadian]

I'm not going to do 80 mph all the time, only when it is legal and I deem it safe in my best judgment to do so. I want the ability to go 80 mph.

The vehicle weight is 35,100 lb, GVWR 48,000 lb. I don't tow and I don't have slides. Tires are 315/80R22.5, diameter 42.3". Engine is a Detroit Diesel 6067BK28 12.7L rated at 430hp. I understand fuel economy will be poor at high speeds, but I'm looking to save whatever fuel I can. Again, I usually only run flat routes.

[buswarrior]

a quick search turned up this 2005 brochure.

You want the HP/Torque graph for the engine in question, same as on the second page, or it is all a guessing game.

http://www.schneidertrucks.com/pdf/DetroitSeries60EngineSpecs.pdf

Notice on this example, there's maybe 475 HP of the advertised 515 available at 1500 rpm.

If you are starting with a 375 HP 11.1 litre S60....

In the old days, the trucker types said it only took around 280 HP to keep 80 000 lbs moving well on the highway, the rest was just profit blowing out the stacks as they roared up the hills.  Must be something to it, witness the fuel economy the 4104 owners enjoy with their smaller HP?

Anyway, the engine has to have the power available at the rpm you are designing towards, or you will be unhappy with the performance.

Interesting exercise, keep going!

happy coaching!
buswarrior

[luvrbus]

The 4000 and 4000MH transmission are more forgiving on lubrication than a B500,the B Allison were for bus use and that is the only place you find those,very high dollars to rebuild or replace.
A friggn rebuilt ECU cost $3200.00 (I know that for sure) and you are probably going to need one special programed to except the higher gearing you want and IF you can find one or somebody to build it.There is a lot more to it besides the just gearing,MCI,Detroit and Allison engineers knew what the were doing IMO you are not going to save enough in fuel cost to offset the cost,why not just have the ECU's programed for 80 MPH mine will top out at over 80 mph    

[Branderson]

I must have the slow version b/c I have to use a sun dial to time how long it takes to get up to speed.  However, it feels like the sweet spot is around 68.

[bevans6]

According to this BSFC (Brake Specific Fuel Consumption) chart, https://www.nap.edu/read/12258/chapter/5#36 that engine (a 1994 440 BHP 12.7 L Series 60)has a sweet spot of .312 lbs/BHP HR (lbs of fuel used to produce a Brake Horsepower for an hour) between 1400 and about 1675 rpm, producing between 360 bhp and 420 bhp.  That is at full throttle, so part throttle at those engine speeds will produce less horsepower and use less fuel.  But this shows the engine is quite efficient at those engine speeds.  To use this kind of chart you need to know your load, so how many HP you need to produce to move your bus, at whatever weight you carry, at whatever speed you choose.  This chart was for an 80Klb tractor trailer running at 65 mph geared for 1500 rpm, the yellow dot, and needing around 200 hp to do so.

Now lets guess a bit.  Your bus is lighter than a tractor trailer but has about the same drag.   Lets pretend it takes the same 200 hp to cruise at 65 mph.  It will need around twice the hp to run at 80 mph, so around 400 hp.  That chart shows (and it's for a 1994 440hp 12.7 litre Series 60, as it happens) that you need around 1650 rpm to develop 400 hp, so if you want to run 80 mph, you should gear for 1650 or maybe 1700 rpm.  If you can find out what the actual hp/mph requirement for a bus is, you might be able to tweak that.    It's interesting to note that you get the best economy out of the engine by gearing it so you can run within a window of RPM and as close to full throttle as possible.  Small engines working very hard will usually beat a big engine not working hard at all, in efficiency.  But big engines have the ability to do a lot more work in a shorter period of time, accelerate to speed faster, etc. so it can come out in the wash.

Brian