Gearing for a S/A dump

[Orchard Ex]

I'm trying to spec out a S/A dump.
I was looking at getting it with an ISC @ 315 hp and a 10 speed.
I was using the Cummins Powerspec website to work out the gearing.
http://www.powerspec.cummins.com/site/home/index.html
Cummins is suggesting gearing that will spin the engine in the 2000 - 2200 rpm range at your selected "target cruise speed" for a balance of economy and power. That seems high to me. Does this sound normal for a local dump application? I am targeting 65mph. The nearest road with more that a 55 mph limit is an hour away so...

Also, does the rear ratio have any bearing on strength? Will a 4.10 be stronger than a 3.7?
I've never torn up a drive line, what usually causes snapped driveshafts/rears etc.? We were beaten with a stick if we spun the wheels fast when bogged down etc. I've always heard that automatics don't break the driveline as bad but didn't know exactly why. Is it a cushioning effect of the automatic? (I can't afford and auto in this truck so I need to know what not to do)

[Steve Frazier]

The trucks I drove forever had the NTC series Cummins in them. They ran at about 1800 rpm at highway speed and tached out at about 2150. I don't know much about these newer engines, but I'd tend to agree with you that the specs they give seem high for rpm.

[Jeff D.]

That does seem high for RPM,but that particular engine may spin higher than others.What is the governed RPM?I think the cruise rpm is figured about half way between max torque RPM,and max HP RPM.So most of the big diesels have max torque at 1200RPM,and max HP at 1800rpm,giving a range around 1400-1600 RPM for opitmum cruise RPM.They do vary depending on application,in that range.

I would strongly consider what Cummins recommends for gearing,even if it does seem odd.I'm sure there's a reason why they recommend that.I think a target cruise of 65mph is good.I wouldn't raise it anymore than that,but you could reduce it if your tranny choice won't give enough torque in low gear to move the truck in worse case scenario(the Cummin's program may have a value as to how much torque is required for a dump truck to get moving in difficult situations)

The rear ends won't make a difference for durability in the ranges your looking at(3.36-4.56)The weight carrying rating would be a better guide to how heavy an axle is(How large of bearings,etc.)I've heard that when they drop into the 2.90 range and lower,some changes in the tranny area are recommended,i.e. higher geared OD. Again,I'd go with Cummins recommendations.

I think the drivelines get torn up by driver abuse,not so much because of the fact an auto or stick tranny is used.It's the fact that an auto won't shock the driveline like a poor driver will with a stick,when they mess up shifts.If a driver is a good shifter,I don't believe there would be much difference in driveline dependability.But even an experienced driver can mess up.I've accidently dumped the clutch before,due too diesel fuel on my boots,and my foot slips off the pedal.Very hard on driveshaft!!

[Jeff D.]

OrchardEx,I tried that program out and here's what I came up with using these components:

ISC@315hp@950ft/lbs torque@1300rpm and 2200rpm rated gov speed
500ft/lbs engagement toque (cruise RPM 1900-2100 recommended@balanced performance & economy)

Eaton FR11210B 10speed w/direct top gear,rated at 1150ft/lbs max torque
(had a 14.? 1st gear reduction)

Eaton 23070S rear-end 3.55 ratio rated @23,000lbs/single

11R22.5 Michelin HDXT drive tires.

They'll give this:
startability 35% (above Cummins recommended 28%)
55mph@1627rpm
65mph@1923rpm
75mph@2200rpm(max. speed)

I would think this would provide a useable truck,and meets Cummins recommendations too,although the revs still seem high.
(That small motor must NEED to spin.)

[Orchard Ex]

Thanks guys,
Jeff - The spec that you ran is just about the same as the one I was running. The recommended rpm range just seemed high. The one of the salesman at the dealership also commented that the gearing would top out at around 75 and hinted that I wouldn't be happy with it. I'm 99% on local roads so I don't really think that the top speed will be that big of a deal. I'm more interested in being able to carry the load easily and being able to tow a tag trailer in the future. Am I overdoing the HP? If I baseline @ 260 = base cost, I can get 285 for about $940 more and the 315 is about $1570 more. Is it worth the extra money now or wait a couple years and do an upgrade? Seems like a ripoff when its just a change to the computer program, but the salesmen say it's because the warranty is more likely to be used at the higher output - so Cummins raises the cost. I guess it makes sense - no free lunch.

That is a pretty handy little program. I'm glad that Cummins put it out on the web instead of only in the dealerships or a pay for access policy.

One more question - the S/A #33k dumps that I have looked at have 10k fronts and 23k rears. This dealer likes 12F/21R and says that I'll never overload a 21k rear on a 10 foot dump, but will build anything I want. What do you think? Take the 12/21 in case I ever want to plow? (not that it ever snows here anymore...)

[Jeff D.]

It's all in what a guy wants,or expects it too do.

I drove all over the U.S.A. with a 9670 International,that had a 310hp cat,and 1250 ft/lbs torque,often grossing 80,000lbs,and it had a 9speed.It did the job just fine.The thing is,is that HP(and torque) are addictive.Once you get use too having it,it's hard to do without.

I'd think 75MPH is fast enough.You go much more than that and bad things happen too fast.

A guy could always spec out the "ultimate" truck,that has everything,but it has too be paid for too.If this is what you can afford,then go with it.I'd also recommend a 12K front/21k rear axles.Ive never needed heavier rears than 20k/axle,but many guys seem to recommend heavier for the dumptruck,so??

As long as you're sure it has a low enough first gear,too pull it out of the mud,the rest will just be how fast you get from A to B,and how long it last.I think your looking at a comprimise of cost and performance with this truck,with these specs,but your in business too make money,not too have the biggest,badest truck in your area.

30 years ago these spec's would have been "sweet",but today guys are use to having it all.They still got the job done back then though,with what they had.

You'll be able to too!!Just gear it low enough,and don't be in a hurry.
(I sill think it'll perform pretty decent,with those spec's for a S/A.)
Good Luck.

[Jeff D.]

FWIW,the truck that I had the 9speed changed into a 13 had gearing so low,that 65MPH was on the gov.(1850RPM).It ran 65,on the gov for over 600,000miles before I changed it.That RPM never killed that motor(Detroit60/ 325hp).I still see it running on the roads here today,pulling a log trailer.The guy says he's put injectors in it.It has to be well over 1mil by now.

I'd better fly,or I'm gonna be late.

[John Banks]

my setup is 10k/23k and the front springs are 12k (not sure why that is though...)

[Ford LT-9000]

If you are going with a single axle go with a 23,000lb rear the 21,000lb is too light you may run the risk of snapping a axle shaft. With a single axle its hard on the driveline so heavier the better. You look at the hub size on a 21 and on a 23 you will notice the size difference.

If you are going to use this truck for a front mount plow I would also consider the heavier axle there are options for a 14,000lb steer axle.

[Bob Horrell]

My International has 12K front and 23K rear. I like it that way for the few times I am overloaded on short trips when I know I won't get stopped. I think that for the extra you would pay for the 23K rear, you would never be sorry. I have always felt that you benefit in the long run with equipment set up for more than it sees on a daily basis.

[Jeff D.]

OrchardEx,on your question this morning about the HP/torque,and extra cost,my opinion would be go for the 315HP.It has higher torque,and that's really what's doing the work in these trucks.

I can't understand why the salesman thought you'de be unhappy with a top road speed of 75mph.Maybe I'm alone on thinking that's plenty fast enough?
If you do decide that you want the top road speed higher you may need too get more gears to maintain your start-up torque in first.Otherwise just gearing the rear-end lower(higher ratio)with the same 10speed is going to drop the start-up torque.Perhaps a 13OD would be in order?What did the dealer recommend,after mentioning they thought you'de be unhappy with 75mph??Is there a 10speed that has the same 1st gear reduction,and an overdrive 10th gear,that wouldn't have too great of jumps between gears for that motor??

I looked in my Ford S/A tonight at the axle ratings.They were 12k front,22k rear.The axle ratio said 6.50???:eek:The 3208 does turn higher R's though.I have to get a run too tug a pickle off a plate,and I'd think I should be able to climb trees with those gears!!Still,187hp of wheezing V8 Cat can move 15k of dirt,with a backhoe behind it,down the road......albeit slowly.(IF I kick at the road with my foot on the hills):rolleyes:Your truck would be a sports car in comparison.

Steve,if you read this,I found the elusive "L" you spoke of(as in L8000)on the build sheet(from an early post of mine in "old iron").It's not on the truck anywhere else though,only 8000 is???So,you were right.....So there!!

[Orchard Ex]

What did the dealer recommend,after mentioning they thought you'de be unhappy with 75mph??Is there a 10speed that has the same 1st gear reduction,and an overdrive 10th gear,that wouldn't have too great of jumps between gears for that motor??

He was trying to nudge in the 8LL direction. The deep reduction gets down to 14.56 and the top end is .74. A pretty good $ difference too. I think that I'll stick with a straight 10 speed. I'm not sure of his experience level on smaller trucks like this. I think that he may be comparing to the many road tractors and triaxles that they sell. He does lots of the build quotes on the computer for the actual "salesman" that I'm dealing with. (2 guys - 1 is just cranking out the computer stuff, the other is making the deal - both have been good to work with).

Your truck would be a sports car in comparison.
Your's is probably a sports car compared to what I'm in now. 22k# 4700 with a 175hp T444E and a 4 speed automatic.:crying The first day with the new truck will be spent remembering how to shift and the next month will be trying to remember to keep my foot out of the "profit removal pedal" when it's not needed.

Regarding axles: I'm still not sure which way I'll go, probably decide at the dealership when ordering.
Am I correct in thinking that I can only scale 20k# max per (single) axle anyway? MD is a bridge law state. I know that some cushion is good for times like Bob mentioned. Just wondering.

[Jeff D.]

Am I correct in thinking that I can only scale 20k# max per (single) axle anyway?

Yup,that's right.

[KeithRA]

I drive a peterbuilt tri-axle dump with 375 cummins and a 9 speed it hauls 15.80 tons and grosses 55,500 it runs 1450 rpm at 60 mph.Every 100 rpms it gains 5 mph in speed.I have Hauled a load and pulled a 20,000 pound trailer no problem.It tops out about 100 or so.