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Author Topic: Figuring Startability  (Read 3503 times)
TomC
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« on: February 15, 2011, 10:04:32 AM »

To figure your startability, first you need to know the difference in torque between an automatic and manual.  When figuring with a automatic, you use the full torque rating of the engine.  When figuring with a manual, about 2/3rds of the torque should be used-or what is called clutch engagement torque-which is what the engine can produce around 8-900rpm.  So for a 6-71N you'd be around 400lb/ft torque, 8V-71 and 6V-92TA would be around 530lb/ft (turbo engines actually produce less engagement torque since the turbo is usually not boosting much).  8V-92TA around 930lb/ft torque.

So to figure your startability, first take the complete weight of your bus and if you do-the towed and multiply that by 10.7 (it's the multiplier to make the equation work) and put that figure into memory.  Then take the full torque rating of the engine for automatic, or the fore mentioned torque for manuals, multiply that by 2 for the torque converter on automatics only, multiply that by first gear, multiply that by the rear end ratio, multiply that by the tire revs per mile and then divide that number by the figure in memory.  You come up with the percent grade that the rig can start on. 

With big rig trucks, Freightliner likes to see 14% for highway, 16% for in town and highway, and 24% for off road.  I think some of the 4104's and such with 6-71's and only 4spds will be in the 10-12% range, while automatic, like my bus with 1125lb/ft torque with the V730 and 4.56 ratio rear end will be more like 28%.  Have fun with it.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #1 on: February 15, 2011, 10:28:17 AM »

  Using your math and 530 lb ft, a 26000 pound -5 with an 871 and 495 tires has a startability of 14.8%.

  But what kind of grade can they climb with a rolling start? Will the same formula work? Using 800 lb ft (figuring its rolling with clutch engaged, I get 22.38%. 900 lb ft (turbo) gives 25.18% grade. FWIW.

  Thanks for posting this Tom. I dont know who figured out the formula, but I hope its right. Or more right than wrong (wrong in the right direction?). You know what I mean (fudge factor).
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HB of CJ
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« Reply #2 on: February 15, 2011, 02:50:11 PM »

Great post.  Now can you do it again and show us how to figure gradeability with different conditions such as altitude, temperature and moisture and the difference (if any) between mulitispeed manuals and fewer speed automatics?  Thank again.  HB of CJ (old coot)
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RoyJ
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« Reply #3 on: February 15, 2011, 04:43:19 PM »

Great post.  Now can you do it again and show us how to figure gradeability with different conditions such as altitude, temperature and moisture and the difference (if any) between mulitispeed manuals and fewer speed automatics?  Thank again.  HB of CJ (old coot)

Altitude will lower your engine power, therefore clutch engagement torque. For turbo charged engines, this is negligible. Temp and humidity can also affect torque, but very minutely. In other words, if your bus can't start on a hill at noon, don't hold your breath that it'll make it by midnight  Grin

The number of speeds does not matter, only the 1st gear ratio, as Tom mentioned.

I'm not sure if Tom's 10.7 constant takes into account rolling resistance or not. If not, then keep in mind on high resistive surfaces (deep dirt, sand, snow) your startability is further reduced. We're also assuming that traction is not limited. On wet grass or snow, you may run out of traction first, and that'll limit your startability.
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« Reply #4 on: February 15, 2011, 11:12:18 PM »

We're also assuming that traction is not limited. On wet grass or snow, you may run out of traction first, and that'll limit your startability.

  Thats why God made tire chains, lol
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HB of CJ
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« Reply #5 on: February 17, 2011, 09:45:54 AM »

Thank you for the clarification.  What I meant to ask was, what is the formula to determining at speed the percent of gradability in each gear at various altitudes, temps and humidity and how changes in road gearing might affect speeds on hills.

An example would be a Bus Conversion with a RTO-910 Roadranger, how fast could one expect to crest some grade.  One would have to figure out coach weight, road hp and the mechanical multiplication of each gear.  HB of CJ (old coot)
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TomC
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« Reply #6 on: February 17, 2011, 12:27:13 PM »

You could make a graph that would be fairly close.  If we use the same way of figuring startability as hill climbing, you just have to plug in the various gear ratios.  I believe the RTO-910 would have the same first ten ratios as the present day 15 spds do.  1st-7.83; 2nd-6.00; 3rd-4.63; 4th-3.57; 5th-2.82; 6th-2.19; 7th-1.68; 8th-1.30; 9th-1.00; 10th-0.79.  So using as an example, a 30,000lb bus with a 8V-71N putting out 800lb/ft torque, 12R-22.5 rubber (485rpm), and 3.73 rear end, then the rolling climbability will be- 1st-35.3%; 2nd-27%; 3rd-20.8%; 4th-16%; 5th-12.7%; 6th-9.87%; 7th-7.57%; 8th-5.86; 9th-4.5%; 10th-3.56%.  By these figures, to climb say the Grapevine which is a 6% grade, you'd have to be in 7th-which at 2,100rpm would be 41mph-which sounds about right.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #7 on: February 17, 2011, 12:59:22 PM »

Tom, I had Allison work some numbers for me and they were looking for 15%. Which is right were you said. I came in at 25.3 for my change to a  3200SP.  Tom Y
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« Reply #8 on: February 17, 2011, 09:34:35 PM »

We're also assuming that traction is not limited. On wet grass or snow, you may run out of traction first, and that'll limit your startability.

  Thats why God made tire chains, lol


Yes, but apparently he failed miserably at creating humans who enjoy putting them on   Grin
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« Reply #9 on: February 18, 2011, 07:16:02 AM »

When figuring with a manual, about 2/3rds of the torque should be used-or what is called clutch engagement torque-which is what the engine can produce around 8-900rpm.  So for a 8V-92TA around 930lb/ft torque.

  Thats a lot of starting torque. But do I understand correctly that the 92 series doesnt work well with a 4 speed manual? I thought I read somewhere that they dont live long due to thier having a narrower operating band.
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TomC
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« Reply #10 on: February 18, 2011, 07:29:26 AM »

I think the typical 4 spd has a torque rating of 1200lb/ft torque, and the 8V-92TA at 475hp has about 1450lb/ft torque.  You'd have to go from the 90 injectors down to 80 or even 75 injectors to make a 8V-92TA weak enough for the 4 spd.  With the small injectors, the 8V-92TA would have more than enough rpm range for the 4 spd to work.  I would use a 7spd overdrive with the 8V-92TA. Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #11 on: February 18, 2011, 07:51:58 AM »

  Its also my understanding a 7 speed wont fit an MCI 5 with a V8? gears or power, but not both, lol
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