Full time 6x6 question

[Sikorsky502]

I've been working on a custom Halftrack project and the axle ratios are different for the front and rear axles, which has caused a huge headache, but I've found a way forward at the moment. In the process of doing the math for different tire sizes and the axle ratios needed I got to thinking about the LMTV trucks and their all time 6x6. What happens if the front tires are flat or have lower air pressure from the rear? This would change the tire diameter and cause binding in the system. Is this a problem on the trucks or does some component allow for this difference?

 

[tobyS]

What type of transfer case are you using? The older ones with a spragg will allow the back to over-run the front (but not the opposite). Put up your math and include the OD of your track drive sprocket that includes the track on it.

 

[Sikorsky502]

Are you on g503? Cause it would be easier to give you the link to the whole conversation on this. If not I can explain best as possible.

 

[Oxyacetylene]

I *think* with the FMTV's it is just a function of the transfer case that handles this difference. I was actually thinking about this a couple days ago. It occurred to me that because of the weight on the front axle, the front tires squat more than the rear tires (truck unloaded), and I wondered about this same issue. The FMTV transfer case has a special "MODE" that locks it to a 50/50 front-rear power split and the manual says not to engage this mode on hard surfaces...maybe for this reason. The normal power split is 70/30 and obviously fine on hard surfaces.

 

[tobyS]

Are you on g503? Cause it would be easier to give you the link to the whole conversation on this. If not I can explain best as possible.

Since I don't know what it is, I guess the answer is no, not on g503.

Was this conversation on another thread, I remember thinking about it but not seeing any conclusions?

 

[Sikorsky502]

It's another military vehicle forum. Here are the copied conversations containing the appropriate information:

IHC M9: Front axle ratio 7.16:1; track drive ratio 4.22:1; drive sprocket pitch diameter 22.918 inches, 18 teeth; track pitch 4 inches, 58 pitches/track; front tire size 9.00x20.

Note: tire size is given as section width by rim diameter. So a 9.00x20 has a section width of 9 inches and mounts on a 20" diameter rim.

White M2/M3: Front axle ratio 6.8:1; track drive ratio 4.44:1; drive sprocket pitch diameter 22.918 inches, 18 teeth; track pitch 4 inches, 58 pitches/track, front tire size 8.25x20.

 

[Sikorsky502]

The following is the math to find the ratio:

The computed loaded radius of a 9.00x20 tire is 19.44 inches; that of an 8.25x20 is 17.55 inches (not 18.46" as in my previous post - not sure what happened there, sorry).

If the rear ratio in the IH were the same as the White, what would the front ratio need to be and still run 9.00x20 tires on the front?

Loaded diameter of 9.00x20 / Sprocket Pitch Diameter = Front Ratio / Rear Ratio

2*19.44 / 22.918 = ? / 4.44 or ? = 4.44*2*19.44 / 22.918 = 7.533.

The ratio of 7.53 to 1 is not possible with any single reduction ring / pinion ratio. I computed ratios with a range of pinion tooth count from 4 to 12, against a ring gear tooth count of 10 to 60. The closest ratio is 45 tooth ring gear, 6 tooth pinion, or 7.50:1.

Why not use 7.50:1? The error between 7.53:1 and 7.50:1 is 0.44%. Sounds pretty small why not go for it?

The circumference of a 9.00x20 tire is 2*pi*r = 122.14 inches. That is how far the HT travels in one full rotation of the front tire. 0.44% of that is about 1/2". Or, with a ratio of 7.50:1 instead of 7.53:1 in the front differential with the White rear ratio of 4.44:1, the front tire or the tracks must slip/skid 0.5" for every full single revolution of the front tire. That might not seem so bad, but expanding on this, the front tire must slip one full revolution for every 1/2 mile that the vehicle moves forward, assuming the tracks don't slip at all. When put into that context, this rate of mismatch would lead to unacceptable tire and/or track wear when the front axle is engaged.

 

[Sikorsky502]

This is the combinations of available axles and their ratios:

That's the thing though, I really don't want to run the old NDT tires. I'd really like to put a pair of Michelin XL or XZL tires on it. The Loaded radius is 20.1 and 20 respectively. That equals 40 inches for total loaded diameter. Pretty close to the 9.00-20 tires on the IHC models, but way off for the white models. Being that I can get an axle with a ratio of 6.8:1, but not one with the IHC ratio I was hoping to adjust tire size to make up the difference in revolutions to align the front and rear. I'm not opposed to using another axle, but I'm limited if I want to keep it modern. It's questionable if a deuce axle would bear the weight, but it has a ratio of 6.72:1 the m809/m939 series trucks axles would carry the weight and have a ratio of 6.44:1 the lmtv/mtv series have an axletech axle with a ratio of 7.8:1 and the option of changing the pumpkin out with MRAP caiman ones for a ratio of 6.14:1 the RG-31/33 axletech axles that are more desirable have various ratios, but the ones I've been able find are either 5.64 or 6.857:1 tire information can be found by googling "military tires". Goodyear and Michelin both make excellent tires. Whew that was a lot. So, with those options do you see a combo or option to go with?

 

[Sikorsky502]

And here's my math with the only option I could find that works:

I've finally had a chance to sit down and run the math for the various options. The only one that I can get to come close is to use the Michelin 335/80R20 with a loaded radius of 18.6 inches. The math is IHC axle with 4.22*2*18.6/22.9183=6.849 and the closest ratio is an axletech 4000 with a ratio of 6.857:1 final drive. It's not exact, but very close. Correct me if I'm wrong, but this may be an acceptable difference if used only off road. The only other option would be to use the Goodyear with a slightly higher loaded radius and shave some tread off, but don't see the value in that. In the process of doing the math I got to thinking about how the tire wearing down its tread would affect things. The original Halftrack should experience an issue, if there is one, in this situation since the ratios are of the axles versus time diameter and track pitch. Has anyone experienced issues after their tires wear down and they engage 4x4? Also along this line of thinking I got to thinking about how CTIS may affect this also, since adding or removing air in the tire changes its diameter. If I were to use a CTIS setup the tires are more inflated for on road driving and less inflated for off road use. Should I get the slightly larger tire since it would have a smaller diameter when in the off road CTIS setting? What are your guys thoughts? To answer the braking question, yes I would have air brakes up front and hydro in the rear. I guess technically it's air over hydro for the fronts though. It'll be a quirky machine for sure, but unique and fun to play with.

 

[Sikorsky502]

You mentioned track thickness and OD, but this was discussed and it's not relevant. The track pitch is though, since that's the distance between centerlines of the track bars an in relation to how the sprocket moves the track around.

 

[Suprman]

The lmtv transfer case has a locking center differential, which is normally unlocked for road driving. This the front and rear driveshaft to turn at slightly different speeds so you dont buck when turning and for other minor differences. In 50/50 mode or low range the transfer case locks the center diff, forcing both driveshafts to turn at the same speed for maximum traction.

 

[tobyS]

It's another military vehicle forum. Here are the copied conversations containing the appropriate information:

IHC M9: Front axle ratio 7.16:1; track drive ratio 4.22:1; drive sprocket pitch diameter 22.918 inches, 18 teeth; track pitch 4 inches, 58 pitches/track; front tire size 9.00x20.

Note: tire size is given as section width by rim diameter. So a 9.00x20 has a section width of 9 inches and mounts on a 20" diameter rim.

White M2/M3: Front axle ratio 6.8:1; track drive ratio 4.44:1; drive sprocket pitch diameter 22.918 inches, 18 teeth; track pitch 4 inches, 58 pitches/track, front tire size 8.25x20.

Yes, I understand and remember the PD part now. Okay, two questions here;

Do you have some parts that you are wanting to use specifically? If you said in the next post, I'll get there.

I take it, these are examples of the M2/M3 and the M9. Are you using either of them as a starting point?

 

[Sikorsky502]

I haven't gathered parts for the Halftrack portion yet, but with the math I've done it's looking like the rear axle will need to be an M9 IHC variant. Interesting about the FMTV having different spin ratios. I wonder if this would be something I could use to compensate for variations in the final drives between the front and rear axles on my project. It's very, very close at the moment and I o my really plan on using the front axle when off road, but I'd like to add a CTIS system for the front tires. This of course changes the front tires diameter, further compounding the different ratio issue. If I had a way of allowing for variation like this, then it seems like it may negate any problem. Thoughts?

 

[tobyS]

There is a good chance the original halftrack used a spragg clutch to relieve a difference in rev's per mile.... or at least YOU could. Suprman would know about the SS, which might work also

I see so many combinations you have went through that we need to start a bit more basic, like what parts you already have and want to use. How far along in the project are you? Do you have the sprockets, track, front or rear ends or transfer case that locks us into specific numbers?

 

[tobyS]

Interesting about the FMTV having different spin ratios.

I think he is talking transfer of torque in the 70/30 thing. If they are going different speeds, like cornering, there is something accommodating the difference in that transfer case in the 70/30 mode where they are locked to equal torque in the 50/50 mode. But I don't have one of them to have studied it. I have the spragg.

 

[Sikorsky502]

I haven't gathered any parts for this part of the build because of this reason. I'm looking to start acquiring drive train stuff, but want to make sure I get what I need to have the best outcome.

 

[tobyS]

I haven't gathered any parts for this part of the build because of this reason. I'm looking to start acquiring drive train stuff, but want to make sure I get what I need to have the best outcome.

What parts do you have? Are you starting with a deuce or 5 ton? Age would matter on the 5 ton, since older ones have a spragg and manual transmissions. On the other side, the late 5 ton has the turbo Cummins 8.3 and auto that would be sweet. Price and availability favor starting with the 5 ton.

 

[Sikorsky502]

I'm not starting with anything. Having the frame custom made at a shop here in town. The engine will be either a surplus CAT 3126 or Cummins QSB 6.7 mated up to an Allison automatic transmission. Do you see I'm not limited by anything. I've been collecting various other parts, but no drivetrain parts until I have a firm understanding of what direction to go in. Looking like it'll be an axletech front axle, with MRAP rims with 335/80R20 tires mounted and the IHC M9 Halftrack rear drive axle. Now I'm into what transfer case to use. If the MTV transfer case has the ability to let the front tires have some "slop" in them from the rear drive sprocket that seems advantageous. Is this a divorced transfer case?

 

[Suprman]

No its attached to the trans

 

[tobyS]

With the MTV you are moving out of what I have experience with. I thought MTV's used Meritor axles? I take it you have located a source for the IHC M9 axle and the Axletech that is the ratio you want (above).

On an 809 series there is a 13/14 ratio in the transfer case that makes the back try to over-run the front and the spragg relieves it. On a 939 series it's an overdrive .7? to 1. The front and rear are equal 1/1 on the 939 series with no provision to relieve the drive-train stress of unequal final ratios and you surely don't need an overdrive.

That MTV 70/30 torque divider needs more research. It does not seem to be the same as what you are asking for...which is exactly what the TC with the spragg is intended to do. Note it would be all time 6x6, just like any other 809 series.