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Author Topic: best gear ratio selection?  (Read 3610 times)
robertglines1
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« Reply #15 on: February 19, 2010, 02:14:17 PM »

Stock tire size is 315 80r 22.5 .the new ones are running 365 80 r 22.5  I will do math as will have to buy all new tires and wheels..Thanks again..have 15000 lb front axle.
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Bob@Judy  98 XLE prevost with 3 slides --Home done---last one! SW INdiana
RJ
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« Reply #16 on: February 19, 2010, 06:22:28 PM »

Robert -

Not sure if you are aware of this, but TomC sells trucks, medium & heavy-duty Class 8 rigs.  In so doing, he is constantly putting together powertrain combinations to fit particular applications for his customers, and has lots of tools available to do so.  So he's a great resource to help guide you to obtain the best bang for the buck.  Might see if you can come up with the revs per mile for the 315 or 365 tires you're contemplating, to help him give you more accurate suggestions.

Are you planning any jaunts to the West Coast?  If so, do NOT expect your bus to climb the grades we've got out here like your car does.  Unless you've got over 800 hp pushing you along, figure you'll be pulling grades between 35 - 45 mph with most standard bus powertrains.  Lots of busnuts are shocked to find out their bus will not go up the mountain at 70+ mph like their Chevy, Ford, Honda or Toyota will.  It's called mass!

An example of our grades:  I-80 between Sacramento CA and Reno NV has over 70 MILES of 4, 5 & 6% grades, climbing to 7300 feet.  And, of course, you've got the infamous Eisenhower Tunnel on I-70 outside Vail, CO that's at the 11,000 ft level - wheeze city, even for the turbos!  So even if you've got it set up properly for decent fuel economy rolling across the midwest, it's going to fall flat on it's face going over RockyTop West. 

Just a little reality check for you.

FWIW & HTH. . .

 Wink

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RJ Long
PD4106-2784 No More
Fresno CA
robertglines1
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« Reply #17 on: February 21, 2010, 05:09:50 AM »

Our basic use will be midwest with occasional trip west of mississippi..I do have patients because my first coach was a 8v71 hound..Maybe Airzona for next winter...
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Bob@Judy  98 XLE prevost with 3 slides --Home done---last one! SW INdiana
TomC
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« Reply #18 on: February 22, 2010, 10:03:37 AM »

When regearing-remember-if you have only a 4 or 5spd manual transmission, your cruise speed is not the issue.  The issue is startability (the ability to start on a grade, or to be able to back up a grade).  Most buses with stock engines and 4 speeds have very poor startability.
If you want to figure your own startability, first take the total weight of your rig (including a towed or trailter) and multiply that figure by 10.7-and put it into memory.  Then take the starting torque of the engine (on a manual transmission it will be about 2/3 the rated torque, on an automatic, the full torque since the torque converter allows the engine to rev up into the torque curve), multiply that by the torque converter multiplication (this only applies to torque converter automatics-typically 2 to one), multiply that by 1st gear, multiply that by the rear end ratio, multiply that by the tire revolutions per mile, and divide the whole thing by the weight figure that is in memory.  You'll come up with startability in grade percentage.  For instance, Freightliner wants at least a 16% startability for over the road trucks, and at least 24% for on/off road.  Just something fun to play with.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
TomC
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« Reply #19 on: February 22, 2010, 10:12:05 AM »

For instance, on my bus, I weigh in at 34,750lbs-multiply that by 10.7 and put it into memory (371,825).  Take engine torque (1,175), multiply that by torque converter ratio (2.65 on V drive), multiply that by first gear (1.77), multiply that by rear end ratio (4.56), multiply that by tire revolution (476) and I come up with 11,962,688. Divide that by the figure in memory and I come up with a little over 32% startability-of which I feel confident on never getting stuck, since I have never seen a grade over 20% (typical interstate is 6%, sometimes maybe 8%).  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
bevans6
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« Reply #20 on: February 22, 2010, 10:41:49 AM »

I came up with 10.6%.  Interesting calculation!

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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