Hard downshifting? Really? Or is it you?

[frioman]

This may be common knowledge, but just in case it's not, I'll take the risk and post this.

When I first got my M923A1 I thought the truck downshifted pretty hard -- lunging forward a bit too much. But then I remembered something I read years ago while studying how to drive emergency response apparatus (fire trucks, etc.). When some large trucks downshift and lunge forward, it can also cause your brake foot to move forward and thus press on the brake even harder -- accentuating the forward lunge even more.

To solve this, one should actually push much harder against the floorboard with their LEFT foot than the right foot on the brake. By pressing the floorboard with your left foot you can greatly improve the precision of your right foot and remove the tendency for your right foot to move forward on the brake pedal when the truck lunges into the next lower gear. I hope this helps.

(Update) swbradley1 made a good observation. In describing the problem I used the term "lunging forward". The truck in fact does not lunge forward, but more accurately stated abruptly momentarily slows.

 

[Flyingvan911]

Very interesting.

 

[swbradley1]

I understand what you are saying but if a truck lunges forward unless your foot is glued to a pedal it is going backwards, not forwards. Newton's Laws.

Now if you are saying the foot goes back and then forward that's different.

 

[frioman]

I understand what you are saying but if a truck lunges forward unless your foot is glued to a pedal it is going backwards, not forwards. Newton's Laws.

Now if you are saying the foot goes back and then forward that's different.

When slowing down (braking) a M923 will obviously downshift, and this can occur pretty abruptly. This downshifting will of course cause the truck to suddenly slow, but initially your body (and foot) will continue to move forward (Newton's law -- an object in motion remains in motion unless acted upon by another force) . If you find your truck downshifting unusually hard, you might try this technique. It could be that your right foot is pressing even harder on the brake accentuating the annoying hard downshift. The purpose of pressing against the floorboard with your left foot is to prevent the right foot from suddenly moving forward and thereby maintaining an accurate "touch" with the right foot. You should be able to feel the difference, but its easier for me to actually see the softer "downshift" by observing the passengers before and after I use this technique.

Give it a try and let me know if it helps. What have you got to lose?

 

[swbradley1]

Now I get it. Your first statement in the OP was wrong.

"When I first got my M923A1 I thought the truck downshifted pretty hard -- lunging forward a bit too much."

Newton's Laws still apply exactly the way I said they did. You said the truck lunged forward, not slowed down too much and caused your foot to hit the pedal. Big difference.

I like the way a M939 series shifts and feels. Tells me it's working.

 

[frioman]

Now I get it. Your first statement in the OP was wrong.

"When I first got my M923A1 I thought the truck downshifted pretty hard -- lunging forward a bit too much."

Newton's Laws still apply exactly the way I said they did. You said the truck lunged forward, not slowed down too much and caused your foot to hit the pedal. Big difference.

I like the way a M939 series shifts and feels. Tells me it's working.

You're right. I worded it wrong, but the technique helps me come to a smoother stop.

 

[swbradley1]

Next time I will try it and we agree.

The M35A3s are horrible for that problem by the way.

 

[quickfarms]

Just drive it like a truck. I was taught to downshift to a stop and just use the brakes for the final stop.

 

[Another Ahab]

Good thread; I'm in.

 

[frioman]

Just drive it like a truck. I was taught to downshift to a stop and just use the brakes for the final stop.

I like downshifting manually when coming to a stop whether I'm riding a motorcycle or driving a truck, but in a 5 ton with an automatic transmission there are times when one wants to stop faster than simply allowing the truck to gradually coast to a stop with your foot off the brake. It is times like this that I'm referring to. If your foot is on the brake during the speed reduction, each time the truck downshifts your foot may inadvertently press the brake a bit harder. That's why I use my left foot to stabilize myself and maintain a constant deliberate pressure on my brake pedal.

This is not that different from those times a M923 with super singles begins "bucking" back and forth on the road. If you leave your foot on the gas pedal while the truck is "bucking" forward and back, the unintentional intermittent pressure applied by your foot can encourage the bucking even more.

 

[Suprman]

Lower the trans pressure via the cable adjustment it may help a bit

 

[hgun]

I have a m923a2 my truck lunges after i drive it, Then park and shut it off restart and after the air pressure builds up i put it in 1-5 and hang on it jumps forward or if select reverse it will jump backwards. I have checked the tms and cant seem to find this issue or the cause. im wondering are my RPMS to high. or my fluid it is still the mil spec motor oil. Any help would be great thank you

 

[Artisan]

The "fix" for a hard shifting MT654Cr is to
Just tow another 5ton around for about 100 miles. It takes
the edge off them. Ask me how I know... Glad I did it.

 

[Suprman]

What is your idle rpm? There is a hand throttle adjustment it may be out a bit far.

 

[hgun]

My idle is at 500-1000

 

[hgun]

the throttle is not out

 

[Suprman]

Idle should not be all over the place it should be in the 500 rpms range. At 1000 rpms putting it in gear would make it jump. I would start with checking the throttle linkage assembly make sure nothing is hanging up anywhere.

 

[hgun]

I can do that i have messed with that b4 I thought i had a issue with the accelertor pedal being very hard to press. it was like doing calf raises in a gym just to drive down the road. Well thank you for giving me a the rpm speed I will look after work tomorrow I will post what i find thanks again

 

[Suprman]

I have seen the gas pedal pivot get rusty and stick. You have to take it off, soak with wd40 and work it good and loose.

 

[hgun]

ok thanks i did not know that def will do that