6BT, CUCV Axled M715 Build

[ryanroo]

Isn't 8 degrees a bit too much caster for such large tires?

with 8 degrees i would be completely happy with it. when i set up steering axles i try to shoot for 7 degrees. 6 is acceptable, but i would want more.

with my axle swap and shackle reversal/suspension redesign i ended up with 10 degrees of caster. i will likely drive it that way for a while and see if it bothers me. if so i will add a 3 degree shim to it and be at 7. i may just add the shim before its driving too.
ryan

[Barrman]

I just used the specs for a mid 1980's K30. 7.5-8.5 degrees is what the factory says it should be at. The axle as the perches were set up was at 3.9. The previous owner of the axles got rid of them because it wouldn't go down the road right. The difference between 8.0 and 7.0 is very small. The difference between 8 and 4 is more than I want to shim.

The camera I use at work is owned by the State. I am the only person here who actually can make it load pictures to a computer, so I get to keep it normally. It is out being used to document a welding project this week. Maybe some more pictures in a few weeks.

[FLASHMAN]

RPM calculator

http://www.csgnetwork.com/multirpmcalc.html

A question about the calculator...

How does it account for specific engines? 4, 6, or 8 cylinder engines will turn at different RPM with those same input calculations, will they not? Or is this calc specific to the 6BT and I missed that somewhere?

[sermis]

RPM is RPM 2,500rpm with a 4, 6, or 8 is 2,500rpm. Now the horse power of the 4,6,8 might not be the same and a 4 might not have the power to turn 43" tires where the 8 will turn them just fine.

[FLASHMAN]

RPM is RPM 2,500rpm with a 4, 6, or 8 is 2,500rpm. Now the horse power of the 4,6,8 might not be the same and a 4 might not have the power to turn 43" tires where the 8 will turn them just fine.

Ok, I see in the calculator description that they use a small block v-8 as the default... I also understand that RPM=RPM... But maybe I'm missing something here... With equal final drive ratios, different engines will turn at different speeds, will they not? Regardless of horsepower, a 4-cylinder engine will turn a higher RPM than a 6, and a 6 will turn higher than an 8, right?

I'm not trying to be argumentative, so please don't take it that way. I'm beginning a 6bt swap project myself on an M1008, and I'm just trying to understand the concept, and how I can account for the difference in engines... Thanks...

[sermis]

Maybe Tim or someone can explain better than me.

Motor RPM will be the same on all motors, ie 2,500 is 2,500 RPM
Think of a drill press. Change the belts to a different size pulley and the chuck turns faster or slower. The motor is still turning the same speed.

Gear raito, transmission ratio, transfer case ratio, tire size will all effect the final motor RPM. Over drive will lower the motor RPM, larger tires will also lower the motor RPM, lower gears will, increse motor RPM at the same speed.
axle tire speed tranny
4.56 40 65 1 = 2490 rpm
4.56 40 65 .70= 1743 rpm
5.89 40 65 1 = 3216 rpm
5.89 40 65 .70 = 2251 rpm

The motor still has to turn the same RPM weather 4,6,8 to get to the same speed. The gears, tire size, overdrive will change how many RPM's the motor turnes to reach that speed. Now the hores power the motor has will effect weather or not the motor can reach the rpm needed to get to the speed.

[FLASHMAN]

Maybe Tim or someone can explain better than me.

Motor RPM will be the same on all motors, ie 2,500 is 2,500 RPM
Think of a drill press. Change the belts to a different size pulley and the chuck turns faster or slower. The motor is still turning the same speed.

Gear raito, transmission ratio, transfer case ratio, tire size will all effect the final motor RPM. Over drive will lower the motor RPM, larger tires will also lower the motor RPM, lower gears will, increse motor RPM at the same speed.
axle tire speed tranny
4.56 40 65 1 = 2490 rpm
4.56 40 65 .70= 1743 rpm
5.89 40 65 1 = 3216 rpm
5.89 40 65 .70 = 2251 rpm

This part I get. This is general rule type stuff... Changing final drive ratio whether done by changing ring and pinion, changing tire size, adding OD, or a combo of all of the above will always effect engine RPM. This part is clear as a bell...

The motor still has to turn the same RPM weather 4,6,8 to get to the same speed. The gears, tire size, overdrive will change how many RPM's the motor turnes to reach that speed. Now the hores power the motor has will effect weather or not the motor can reach the rpm needed to get to the speed.

This is the where I get confused. Let's apply this to three different vehicles, all with the same driveline from trans to tires. If we assume 4.56 gear ratio, a .73 overdrive final drive trans, and 38" tires, one truck has a 4BT, one with a 6BT, and one with a 6.5 detroit. Wouldn't all 3 engines have different RPM at the 65MPH? Since the calculator uses a small block Chev gasser as it's base example, if I'm I'm correct, what math do we use in conjunction with that calculator to account for the differenmce in engines? If I'm not correct, maybe some more experts can chime in to explain it to my dumb ass, because I'm

[Barrman]

Sermis explained it pretty good. My big Block turns 2000 rpm at 54 mph. I could pull that engine out and put in anything else. If all I did was change the engine, it would still be 2000 rpm at 54 mph. I really doubt a 4 or probably not even a V6 could generate enough umph to get that fast however.

That might be what has you confused. The smaller something is, the more it has to work to do the same job compared to something big. The way to get more work out of a smaller engine is to have it turn faster.

The only vehicle I can think of that came with a 4, a 6 and a V8 was the Chevy II back in the 1960's. (Maybe the Mustang in the 1980's) The V8 car had different rear axle gears to it was around 2000 rpm at 60 mph. The I-6 engine needed different gears in the rear axle so it could run 60. The rpm was around 2500. The old "Iron Duke" I4 needed to spin faster to hold the same road speed. The rear axle ratio was changed to allow it the engine to turn around 3000 rpm at 60 mph.

That make sense? Car companies and modifiers will change gears ratios around to better fit a different engine in a car. Or, the mission of the vehicle.

For your M1008 swap you really won't have to worry. You have 4.56 axles in the truck and a TH400 transmission. The 6.2 has a red line around 3000 rpm. The Cummins going in has about the same red line. You will just get there faster with the Cummins and stay near it on hills.

[FLASHMAN]

Sermis explained it pretty good. My big Block turns 2000 rpm at 54 mph. I could pull that engine out and put in anything else. If all I did was change the engine, it would still be 2000 rpm at 54 mph. I really doubt a 4 or probably not even a V6 could generate enough umph to get that fast however.

That might be what has you confused. The smaller something is, the more it has to work to do the same job compared to something big. The way to get more work out of a smaller engine is to have it turn faster.

The only vehicle I can think of that came with a 4, a 6 and a V8 was the Chevy II back in the 1960's. (Maybe the Mustang in the 1980's) The V8 car had different rear axle gears to it was around 2000 rpm at 60 mph. The I-6 engine needed different gears in the rear axle so it could run 60. The rpm was around 2500. The old "Iron Duke" I4 needed to spin faster to hold the same road speed. The rear axle ratio was changed to allow it the engine to turn around 3000 rpm at 60 mph.

That make sense? Car companies and modifiers will change gears ratios around to better fit a different engine in a car. Or, the mission of the vehicle.

For your M1008 swap you really won't have to worry. You have 4.56 axles in the truck and a TH400 transmission. The 6.2 has a red line around 3000 rpm. The Cummins going in has about the same red line. You will just get there faster with the Cummins and stay near it on hills.

Ok, I see what you're saying now... It just wasn't clicking, (sorry sermis)... Thanks for the help.

For my build, I'm changing trans too... I'm going with a Destroked-built Allison 1000 6 speed and stand alone controller to give it better highway manners... The Cummins I'm going with is an altogether remanufactured unit too, so I'm really starting from scratch...

That said, after the 1008 build is complete, my next project WILL be an M715. I haven't decided whether to go with a 4bt or 6bt yet, but that project is far enough off that I don't have to think about it yet... I'm going to make that one my daily driver...

[Mikel]

with 8 degrees i would be completely happy with it. when i set up steering axles i try to shoot for 7 degrees. 6 is acceptable, but i would want more.

with my axle swap and shackle reversal/suspension redesign i ended up with 10 degrees of caster. i will likely drive it that way for a while and see if it bothers me. if so i will add a 3 degree shim to it and be at 7. i may just add the shim before its driving too.
ryan

8 degrees is not unreasonable, just seems to be at the high end of what is normally used. I have about 4.5-5 in the Dana 60 in my J300 and it tracks and self centers very nicely on the road.