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Old 07-01-2009, 12:15 AM   #1
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Exceeding gCrw/trailer weight with MANUAL trans, risks??

Half on topic, does anyone know what the risks would be of exceeding the gross combined weight rating or trailer rating on a vehicle with a manual transmission?

Automatics have been mentioned, especially for non lockup the converter heats up and shears and it burns out transmissions. What about clutches in a manual? I guess my thinking is that torque in should matter more than torque out, and if I keep the power down (even if i'm hauling at 40-45mph) the risk of burning a clutch or damaging a gear from hauling something like TWICE THE RATED WEIGHT should be less...

Or am I dead wrong??


Btw i'm not so much planning on exceeding the GCRW rating of a particular bus (i'm aware of the safety issues primarily in brakes, starting torque and liability) rather what i'm referring to is swapping in a different manual transmission from something rated for lighter duty. Like an NV4500 (used in dodge cummins diesel trucks but I dont think they meant to haul 26-33k lbs with one) or even an SM465, both are used in 1 ton trucks. I've heard these are rated around 14,500lb weights or so and yes i'm aware that i'm talking twice that. If I kept it at 2/3 of normal highway driving behavior why would the tranny die?? Others have told me this but they couldn't explain specifically why or where it would break.

Two reasons for considering this swap are first it gives very wide ratios giving good acceleration and starting torque even if you swap in a higher ratio rear end, and second if you need even more gear you can use 1 ton transfer cases to drop the gear even more. (ie - I think it's the NP203 which has a divorced 'underdrive' unit of 1.96:1 that can be used separate of the 4wd and is extremely strong) I've heard of SM465 transmissions with transfer cases going for $200 complete...
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Old 07-01-2009, 09:27 AM   #2
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Re: Exceeding gCrw/trailer weight with MANUAL trans, risks??

The SM465 found its way into some DAMN big trucks. MAybe not bus big, but big. The same goes for it's father, the SM420. I've heard of people having some problems with NV4500's and even the MUCH larger NV5600 when their diesel pickups spend their lives close to the GCWR, but I have never witnessed it myself.

The NP203 does indeed have the underdrive in a separate portion of the case from the chain, but I've never heard of someone using just that. You can't just unbolt the underdrive and run it as is. It does require an adapter on the backside which most guys end up mating to an NP205 giving them a 4 speed (ok, technically 5 speed, but 1.96:1 and 2.00:1 are too close to matter).

SM465's with NP203's attached are very rare. It is far more common to find an NP205 or even an NP208 hanging off the back. Plan to spend some money to get the proper adapter for a 203 to an SM465. With that combination guys are ripping apart the output shafts of the NP205 with the granny low of the SM465, the underdrive of the 203, and the 205 in low. I'd love to blame it on the big tires, but offroad guys run ridiculously low axle gears which takes some of the strain off the upstream drivetrain components. It's all about size and weight though. I'm sure it would be fine in a short bus. You'd just be limited to one range set (high or low) from a standing start because the underdrives do not shift while moving.
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Old 07-01-2009, 10:51 PM   #3
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Re: Exceeding gCrw/trailer weight with MANUAL trans, risks??

Quote:
Originally Posted by macgyver
I think what you'll run into is not burning your clutch out so much as stripping gear teeth from the gears internally... I've seen it happen in pickups, volkswagen bugs, cars, etc...

There's a reason they've got a maximum rating... The strength of the internal components is the factor here... Not your clutch. You could be shifting from 1st to 2nd and the shock of all that weight on the tranny might even bend the internal guide rods, crack gears or simply shatter them...
I guess my thinking is whats different from the shock (aka load) vs a hot rod driving under high aerodynamic load for instance, or a drag racer using full throttle massively exceeding engine torque ratings for a transmission because they aren't doing it very long.

The only load the transmission should be seeing is the torque of resistance on the output shaft. And say changing from say 6.18 gears to 3.08 gears with the same chassis would double the forces and resistance on the transmission all else being equal. Or for that matter driving at 70mph instead of 50mph is about double the aerodynamic load, and that's a constant load, instead of a more temporary load which weight is while accelerating (unless youre on a grade). So I guess i'm making a few assumptions like the typical light truck is expecting to bomb up grades at full throttle usually and should have a fair bit of safety measure. I mean aren't I right that most GVWR are basically a guess from the engineers typically assuming worst case conditions?

I can fully see breaking gears if you are climbing uphill with double the rated load with the foot to the floor all the time. But once the load is moving isn't it basically static except for aerodynamic resistance?

I'm also curious about what the difference is between the load placed on the back instead of the front, an SM465 is designed to take 400lb/ft basically all day long from what I understand, even in granny gear.

Am I wrong in saying "its fatigue from the constant LOAD that breaks the gears, not the weight" and thinking weight should be more flexible as long as you arent worried about grades and are mostly flat towing? (or using limited throttle/power, not full throttle, on any grade you meet even if it means going 25mph) And shouldn't the load on the output shaft and through the gears be directly determined in proportion with my right foot? (I mean I should think in my mind that an SM465 would move 1 million lbs for the Shuttle Orbiter, if I had 6000:1 ring and pinions in the axle because the axle load/leverage is what you are actually resisting afterall)

(a part of me has always just wondered about things like why the transmission torque rating is say 457lb/ft last year and this year is upgraded to 468lb/ft, those differences seem so miniscule I have to wonder who computes or decides them based upon what formula or simulation or what the improved weak point is that made last year's model 11lb/ft different)


I suppose if there's room I could stick a big rig transmission under there but I wouldn't know how to hook that up to the engines i'd usually find in busses without spending 10x as much for an adaptor plate. :P This line of thought is because i'd seen a pair of manual transmission gas engined busses with tranny quirks for sale in the area and I know the SM465's are cheap for swaps and give alot of gear in 1st.
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Old 07-02-2009, 01:23 AM   #4
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Re: Exceeding gCrw/trailer weight with MANUAL trans, risks??

The issue with the SM465 (and any other transmission) has to do with the helical cut on the gears. Because the teeth on the gears are cut at an angle they are not only forcing each other to counter rotate, but are also putting axial loads on each other trying to slide apart. The solution to this is to put larger bearings on the torque face and to also beef the case up in that area to support the bearings. Because the transmission will spend the majority of its time going in one direction you can determine which surfaces are going to see the greatest average loads. Sure, it will be able to handle loads in the other direction, but is not designed for that constant load.

In terms of wear on a transmission from exceeding it's rated capacity, I forsee a lot of wear on the dog teeth of the sliders. The increased constant load is also going to wear heavily on the surfaces of the helical gears which might result in excessive lash. More than anything I think the bearings will suffer.

You better believe those transmission load ratings are more than just a guess by engineers. There is a tremendous amount of mathematics involved. It's all a balance of capacity versus longevity. In modern transmissions it often IS just a replacement of a singular "weak link" that makes that small difference in capacity. Also, never underestimate the powers of electronics. Long gone are the days of manuals getting better mileage and being faster in the quarter mile. Computers are only as good as their programming, but they're damn good and getting better all the time.
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