| Author | Message | ||
| Mr Bill Rating: N/A |
The question I have is: I have a Gillig bus with a 3208 Cat and the alison auto trans. My top speed at 2800 RPM was about 60 MPH. My rearend ratio (when I jack up one wheel of the bus and turn the wheel) is about 3:1. I pulled the pumpkin out of another bus and rotated it on the bench--ratio is 5.86:1. What kind of gear ratio do I need to get the bus to go 70 MPH? Tires are 1100x22.5. Both buses at ontime where standard tranny. Thank you in advance! Bill | ||
| Earl-8-Ky Rating: N/A |
If your Gillig is a city bus you are more than likly geard low. The 5.86=1 sounds about right for a transit. You need to go to a371=1 or maybe a 411=1. 2800 RPM will not be good for your engine. Try to get it down to about 2100. It woll last a lot lomger. | ||
| Ron Swanson (Swaneh)
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In order to achieve 60 MPH at 2800 engine RPM with direct drive in your transmission you must have a rear axle numerical ratio of about 5.89:1. In order to reach 70 MPH you will need a ratio of aprox. 5.1. If you were to change your drive tires to 12x24.5 you could reach about 66 MPH with out changing the differential gears. This assumes that you have enough HP available. With out knowing your gross vehicle weight, engine HP and bus frontal area I can't tell if you have enough HP to over come wind and rolling restance encountered at 70 MPH. | ||
| FAST FRED Rating: N/A |
If you go to this website, http://www.freewebs.com/darisb/ there is a computer that will answer all your questions on speed rear end ratios and tire diameters. FAST FRED | ||
| Stan Rating: N/A |
You don't say what model of Allison you have. Do you know the high gear ratio in the transmission? | ||
| David Hartley (Drdave)
Rating: N/A |
Boxer shorts.  | ||
| Henry R. Bergman, Jr. (Henryofcj)
Rating: N/A |
Hello Mr. Bill...try www.roadranger.com and click on the rpm/road speed calculator. I'm sure the site includes your high rpm and desirable higher road speed. Does your 3208 have a turbo? If sooss, then your probably (may) have the 640/642 Allision 4 speed which has a lock up convertor. If your Cat does not have a turbo... ....then you may (may) have a 540/545 Allision, which does not have a lock up convertor. This will make a difference in your top road speed somewhat. Or..you can crawl... ....under your Gillig and look at the tranny and tell by the ID plate or the general size of the auto transmission. Your 3208 will be more happy turning a slightly lower top rpm. | ||
| Tom Caffrey (Pvcces)
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Mr. Bill, what no one mentioned above is that if you jack up only one wheel and rotate it only once to check the ratio, the result will be only half of the true ratio. If you use chalk marks both at the pinion and the wheel, you can get the ratio a bit closer. Then, multiply by 2 and you will have it. The reason you are only getting half of the ratio is that the ratio is defined as the sum of the wheel revolutions divided by the pinion revolutions. To obtain accuracy when measuring this way, use multiple revolutions of the wheels. This spreads a fixed error over the several revolutions. To simpify, rotate the wheel twice and count the pinion turns. The result will be the ratio of the axle. For what it's worth. Tom Caffrey PD4106-2576 Suncatcher | ||
| Tom Caffrey (Pvcces)
Rating: N/A |
Correction: I misstated the definition of the differential ratio in the above post. I should have said that the ratio is defined by the pinion revolutions divided by the quantity which is the sum of each wheel's revolutions divided by two. Wheel L turns 2 times and wheel R turns three times while the pinion turns 11 turns means that the ratio is 4.4:1 I'm sorry if I misled anyone. Tom Caffrey PD4106-2576 Suncatcher | ||
| Brian (Bigbusguy)
Rating: N/A |
With one wheel off the ground and the other down you will have the same ratio as your gear ratio. if its 4:1 drive shaft turns 4 times wheel turns once. Tom the big words lost me. Brian 4905 Oregon | ||
| Richard Bowyer (Drivingmisslazy)
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Earl, is the 2100 for a DD? I thought the Cats could operate at a higher RPM. right? Wrong? I do not know. Richard | ||
| Tom Caffrey (Pvcces)
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Sorry, Brian. The trouble was that the differential complicated trying to explain it. The main thing that I was trying to say was that both wheels had to make a full turn to get the right ratio or one wheel would have to make two full turns. If you only turn one wheel, you will get half the actual ratio. Maybe that will make it easier. Tom Caffrey PD4106-2576 Suncatcher | ||
| Brian (Bigbusguy)
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Still wrong, Turn one wheel and the other dont turn you get the actual ratio. The the spider gears are 1:1 and dont change the final ratio or every time you went in to a turn you would have the ratio change. I see what your trying to say in corning the wheels turn at differnt speed but your ratio stays the same. But if one wheel is not turning and the other is, the ratio of the wheel spinning will be the axle ratio. Jack up your car or truck and see for your self. If you have 4.10:1 you will not turn the one wheel 8.4 times per drave shaft turn. It will be 4.10 if the other wheel not turning. If your going to be at the rally I will show you if you want to put some money on this I have $100 that says Im going to be right. Or maybe Im not reading you post right? Brian 4905 Oregon? | ||
| Tom Caffrey (Pvcces)
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Brian, I disagree. You can keep your money. You've seen the post and you will likely remember it, so whenever you get around to checking it, you will know which way it is. I've already done this a number of times. Tom Caffrey PD4106-2576 Suncatcher | ||
| Brian (Bigbusguy)
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I dont want to keep my money I want to take yours . Im sure some elese here can chime in and let you know Im right. Look at it this way both wheels up and spinning the same speed constant RPM. Now stop one wheel . And what your saying is the other one will now slow down to 1/2 speed? It just cant do that. I know I counted the gear ratio with one wheel down the other one in the air and if it is 2 times the ratio my bus would have a 2.?:1 ratio and I would be doing 140 mph in top gear. If more say Im wrong I will do it when I get back to my truck. Brian 4905 Oregon? | ||
| RJ Long (Rjlong)
Rating: N/A |
Brian - It's been my experience that when you stop one wheel side, the opposite spins approximately twice as fast as when they were both turning together, due to the action and ratio of the side and spider gears. Good example of this is shown here, third illustration down: http://auto.howstuffworks.com/differential3.htm (Note that in this illustration, what I referred to above as "spider" gears, are called pinion gears - shown in red. Side gears are the gears on the ends of the axles, or what the axles slide into inside the differential.) Just looked at a Chevrolet Corvair parts book, and it mentions that the pinion (spider) gears have 10 teeth and the side gears have 16. Doing the math gives you either a 0.63:1 or a 1.60:1 ratio, so there is some multiplication taking place. With a non-limited slip (open) differential, if you pick the entire axle up off the ground and hold the drive shaft so it won't turn, when you spin one side's set of tires, the other side will spin in the opposite direction at the same rpm. Same scene, but with a limited-slip (Positraction) differential, when you try to rotate one wheel, nothing will happen - the limited slip will not let the side/spider gears rotate the opposite wheel. You MIGHT be able to get the opposite side to move with enough leverage on your side's wheel, but it will take a lot of effort to overcome the Posi clutch pack. Same scene, open differential, this time with drive shaft free to rotate. Turn drive shaft, both sides will turn equally the same direction. Turn one wheel, and both drive shaft and opposite wheel will rotate, but opposite side wheel may, or may not, rotate the same way or possibly the opposite - depends on driveline resistance. Hold one wheel and turn drive shaft, opposite wheel will turn approximately twice as fast as both turning together. Same scene, limited-slip differential: Turn drive shaft, both wheels rotate equally the same direction. Turn one wheel, and opposite wheel will turn the same direction at same rpm as one you're turning, and drive shaft will turn at rear axle ratio speed. Whew! Now that I've thoroughly muddied that up. . . I wonder if Mr Bill, the fellow who started this thread, has had his question answered??? RJ PD4106-2784 Fresno CA | ||
| Stan Rating: N/A |
RJ provided information that I never considered before. In the small number of differentials that I have worked on, the spider gears and side gears appeared to be the same size. I never had ocasion to count the teeth and cannot think of any reason for them to be different. Tom and Brian are probably both correct using their experience. Tom must have checked a differential with a 2-1 ratio between the spiders and side gears and Brian checked differentials with 1-1 gears. Maybe someone like Sojourner can tell us why a manufacturer would use differnt sizes. | ||
| Richard Bowyer (Drivingmisslazy)
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Ever notice when a car is stuck and one wheel is spinning, how fast it appears to be turning. Same principle as RJ shows, it is spinning twice as fast as it would be if both wheels were turning. One other phenonoma that you kids would probably never have seen but it you are pulling a car, with manual transmission, trying to start it by pulling it with a team of horses and there is snow on the ground, then one wheel (that has the most traction) will turn forward, the other will spin backwards, and the engine will not turn over. The wheel turning forward will be at the same rpm as the wheel spinning backwards. Richard | ||
| Brian (Bigbusguy)
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2-1 1-1 Mmmmm. Brian 4905 Oregon? | ||
| Tom Caffrey (Pvcces)
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Sorry, this post was a duplication of the following one. Tom Caffrey PD4106-2576 Suncatcher | ||
| Tom Caffrey (Pvcces)
Rating: N/A |
That's what you get, Brian, for trying to take my money!!! For what it's worth, I didn't even consider the ratio of the different gears inside the carrier; I was just looking at the action required in a differential. I still don't see what the ratio of the pinion gears to the side gears has to do with figuring out the overall differential ratio. It looks to me like they would be independent of each other. Brian, watch out for that whopper of a ditch right outside the fairgrounds at Rickreall. It's almost deep enough to swallow that 4905 if you try parking on the shoulder. Tom Caffrey PD4106-2576 Suncatcher | ||
| Brian (Bigbusguy)
Rating: N/A |
Tom After rereading yours and the others Its is more clear what you was saying . I think I was just not reading it right. Im sure you was right but the way I read it it looked wrong. As for the ratio of differnt pinion gears I have no idea what that will do or if the front diff on a 4X4 is for the final ratio with one up and one wheel down. I will just go back and sit down and be slient on this before I give my money away. I going to try to park in side all depends on when I have to leave for Kanasas city and if I can pull out early. Brian 4905 Oregon? |