teleman Posted March 26, 2011 Share Posted March 26, 2011 Just killing time looking up tranny ratios for my B 61 T with a trt 720 with a 5:78 rear end.On the build sheet they crossed out 57 mph and put in 66 mph..What was the reason or advantage for ordering the truck like this .I talked to Larry(freightrain)and he has a trt 72 with a 4:72(62)? rear end.When comparing ratios thru the gears it seems my tranny is geared a little higher in each gear up to 4th high(over) then shift to 5th high...dont use under or direct in 5th(13 speed) and our top end is the same.Trying to understand why use the trt 720/5:78 over the trt 72/4:62...was it to use the lower 5:78 gear ratio and still have the same top end or was it just the buyers preference.I think ive confused myself and am going to quit thinking now...thanks Jim Quote Link to comment Share on other sites More sharing options...
Mike Posted March 26, 2011 Share Posted March 26, 2011 Just killing time looking up tranny ratios for my B 61 T with a trt 720 with a 5:78 rear end.On the build sheet they crossed out 57 mph and put in 66 mph..What was the reason or advantage for ordering the truck like this .I talked to Larry(freightrain)and he has a trt 72 with a 4:72(62)? rear end.When comparing ratios thru the gears it seems my tranny is geared a little higher in each gear up to 4th high(over) then shift to 5th high...dont use under or direct in 5th(13 speed) and our top end is the same.Trying to understand why use the trt 720/5:78 over the trt 72/4:62...was it to use the lower 5:78 gear ratio and still have the same top end or was it just the buyers preference.I think ive confused myself and am going to quit thinking now...thanks Jim Jim, Yes the ratios can be confusing. I have a 10 speed duplex and a 5:39 rear with a top speed of 47 mph. The truck was used for local city driving and did not need the speed as the highway tractors did. The B Models as you probably already know, only had about 180 HP and used gearing to get the power and speeds they needed for a low horse power engines, not like todays trucks with 4 and 500 HP. As you probably noticed, the tri-plex and Quadra Plex was used more on cement or dump trucks. All of these trucks were geared for mainly grunt power and not for higher speed use. You will get more or better feed back about this subject soon on this site. mike Quote Link to comment Share on other sites More sharing options...
teleman Posted March 26, 2011 Author Share Posted March 26, 2011 Thanks Mike...My truck was ordered as a single axle tractor to haul steel.I know they needed those gears to make up for the lack of HP but was wondering what the logic was.The build sheet says that Elmer C Breuer co. Cleveland Ohio ordered two exact trucks in July 1963.I was wondering about the difference in the splits between the set up I have and the trt72/4:62 combo.I think my tranny has over in the aux. and direct 4th,5th over in the main box. I came up with a road speed comparison and found that the trt720/5:78 had slightly higher speeds in all gears with 4th Direct 45 mph and 4th over 54 mph then drop into 5th over 65 mph.My 4th direct and over are the same as my 5th low and direct.It seems that my set up gets about the same results and uses two less shifts...It says not to split 5th but could you split 5th then go 5thL to 4thL???I just find it interesting all the different combinations you could get.I had know idea how it was geared when I bought it and Im glad I will be able to cruise pretty easy at 50-60...Jim Quote Link to comment Share on other sites More sharing options...
Freightrain Posted March 30, 2011 Share Posted March 30, 2011 I think it gives you the grunt, plus the "get home" gear when empty. Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment Share on other sites More sharing options...
peterj Posted March 30, 2011 Share Posted March 30, 2011 I'll try to set the stage and give some factoids..When the B model came out, there were very few interstate highways. The 4 lanes then had stoplights. Speed limits were mostly 45 for trucks/55 cars. The first 60 mph limit that I can recall was the New Jersey Turnpike.Overdrives in those days were not for pulling. They were for come home empty, or running off of the hill to get speed for the next up hill.It takes about 200hp NET to pull a truck at 55-60 on a level road. These trucks only had 150-160 NET hp. Each set of gears reduced your horse power. A set of gears only transmits 85-90% of the input HP. Therefor you want the transmission to be running in direct where you are going to do most of your pulling. A dump or a heavy tractor would be geared lower in direct (both boxes) than a highway freight tractor, but have the same top speed. Also , its easyer on the drive line to have the reduction as close to the wheel as possible. Its better to have the reduction in the rear than the transmission... the driveshaft doesen't have to carry as much torque...If i've confused any one ask questions.... Some more factoids; In those days if you could climb the hills on the Pa turnpike in high range (not high gear) you had a big horse.. many times you were going 10-15 mph...Most trucks were running wide open all the time, including down hill..... except down mountains. See ya later................................ Quote peterjhttp://forums.aths.org/Instantforum2010/topic72786.aspx57 mack h67 Link to comment Share on other sites More sharing options...
Rob Posted March 30, 2011 Share Posted March 30, 2011 I have both a TRTL72, and a TRTL720 and like them both. The double over triplex will be behind an E6-350 in my 58 B61 with 5.05 rears, on 10.00X20 rubber. The truck right now will run right at 72mph on flat ground with the original 170hp Thermodyne. Just a little bit of a hill and your downshifting a couple of gears to keep up momentum. Never have pulled anything with the truck so can't answer as to loaded.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
Freightrain Posted March 30, 2011 Share Posted March 30, 2011 Most trucks were running wide open all the time, including down hill..... except down mountains. That's how I drive my B right now when pulling my trailer. Flat on the floor, for all it's worth. At only 27+K lbs it still barely goes fast enough to get a speeding ticket LOL!! Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment Share on other sites More sharing options...
randyp Posted March 30, 2011 Share Posted March 30, 2011 Killer B has so much horsepower and mind boggling torque, to save transmission, sometimes even loaded or empty, I just slip her in neutral, rev engine way up, and let fan pull me around, kinda like airplane. Have to watch bout pulling "compact" cars or pedestrians into radiator though. (sorry, too much caffeine lately) randyp Quote Link to comment Share on other sites More sharing options...
Rob Posted March 30, 2011 Share Posted March 30, 2011 Killer B has so much horsepower and mind boggling torque, to save transmission, sometimes even loaded or empty, I just slip her in neutral, rev engine way up, and let fan pull me around, kinda like airplane. Have to watch bout pulling "compact" cars or pedestrians into radiator though. (sorry, too much caffeine lately) randypSounds very similar to what I call the "Giant Sucking Sound" when I walk around my trucks. It is so powerful I have to use both hands to contain my wallet from taking flight.Rob 1 Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
randyp Posted March 30, 2011 Share Posted March 30, 2011 Sounds very similar to what I call the "Giant Sucking Sound" when I walk around my trucks. It is so powerful I have to use both hands to contain my wallet from taking flight.Rob Heh heh heh,,,Rob,,been there, doing that, I hear you buddy! randyp Quote Link to comment Share on other sites More sharing options...
fifth wheel Posted March 31, 2011 Share Posted March 31, 2011 A triplex direct is better suited to the power band of the thermodyne engine, a triplex over helps with slower rear end ratios for top end but 1st and rev are to high. These single countershaft gear boxes don't like the low end torque of the maxidynes. I had encountered this problem in the mid 70's came up with , a quadruplex, add a browine box, or 2 speed rear. It ended up 673t,triplex direct ,4.24rear and 22' rubber.FW Quote Link to comment Share on other sites More sharing options...
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