Why do some AFV tracks sag and some don't?

Hey Bish,

simple reason could be that you are most likely thinking Army.  I was in the U.S.A.F. Security Police, Law Enforcement. In S.E.A. during alerts, we would blend into the Air Base Defense section so I learned to drive the 113 and the 706.  

We were not trained to be mechanics.  All vehicle repairs were turned over to motor pool who had the equipment and expertise to fix it.  Then we had to wait for them to get around to releasing our vehicles, which they held up on purpose except in an emergency.  

These armored vehicles carried a Quick Reaction Team with a G.I. driver and sometimes a Second G.I. as  turret operator or backup up team leader  The rest of the 13 man unit were made up of local nationals except in a very rare circumstance.  With the exception of Town Patrol and a couple other special problems, our vehicles never left base.

Given the Jpz IV is a pz IV chassis then the same track behaviour would be broadly expected.

The tracks are most definitely 'dead' not 'live', so completely tight tracks make little sense when in use.

I would suggest that the impression of not so much sag on the jpz is partly just an effect of a small sample of pics and partly because the pziv return rollers are relatively close together so it is less marked visually than on a III. That said a late J or a jpz iv/70 has 3 rollers and more visible sagging!

LOADS of sag is pretty much unlikely because a well maintained tank would be usual for a relatively short lived Wehrmacht vehicle. You see more sag on pictures of wrecks I think.

If it was me I would go for a small but noticeable amount.

You may also wish to consider that many museum display vehicles are stripped down shells. No innards like engines, ammo etc. This makes them ride a little higher than an operational vehicle and the extension on the suspension tends to tension the tracks a bit more.

Thanks much. If wear and tear is an issue and the GB is 1945 and I don't want a mud-monster (just did one of those) we'd have to thinking of somewhere near the Rhine in March. I'll check production figures, but I'm not sure many new AFVs were headed to the front after January 45 or so.

Eric

It boils down to two reasons: 1) design, and 2) maintenance. A live track with a particular bushing design will give some counter action to natural sag weight and help spring the track back up, while a "dead" track does not. As far back as the the Lee/Grant, Stuart, and Sherman, US tanks have used a "live" track. The only time you saw real sag on those, like on other countries contemporary tanks is when the rubber bushings burned away. Maintenance of keeping proper track tension and replacing worn shoes/links will minimize throwing track due to maneuvers or terrain.

Live or dead tracks are not dependent upon suspension type such as return rollers, Christie, Torsion Bar, etc.   In the late Cold War era, the Soviets began to refit their older tank designs such as the T-55 and T-62 which originally had a dead track with the same type of track as on the T-72 which was a live track. 

Ikar now that's a dio idea.

ikar01

Maintennance does have a lot to do with it.  Here's what can happen if someone isn't watching for problems:

i213.photobucket.com/.../scan0139-1.jpg

i213.photobucket.com/.../scan0138.jpg

i213.photobucket.com/.../scan0137.jpg

We actually had one that lost its left tracks as it was starting to make a right turn to go into the armory parking area.  When we got there the completely intact track was laying on its edge near the vehicle.  It was still completely linked up but very loose.  The team commander was sitting on the track waiting for motorpool to get around to show up and fix it.

I have to ask. Why didn't he and his crew just get off there butts and fix it themselves.

EBergerud;

There are two basic types of track systems as well. With variations of course.

One type is like you show. The track returns on idler rollers to the front of the tank.

The other type is like you'd see on that M113. The track returns to the front of the tank over the tops of the same road wheels that ride over it.

WW2 aircraft have similar issues with their control wires. They need to be kept tight.

Maintennance does have a lot to do with it.  Here's what can happen if someone isn't watching for problems:

i213.photobucket.com/.../scan0139-1.jpg

i213.photobucket.com/.../scan0138.jpg

i213.photobucket.com/.../scan0137.jpg

We actually had one that lost its left tracks as it was starting to make a right turn to go into the armory parking area.  When we got there the completely intact track was laying on its edge near the vehicle.  It was still completely linked up but very loose.  The team commander was sitting on the track waiting for motorpool to get around to show up and fix it.

Yeah what Doog said sums it up pretty good.

You'll get two kinds of answers to this question: one that tells you to ignore "evidence" and model it how you want; the other will tell you that sag is ALWAYS waaaay overdone and it's "incorrect". Both answers are right and wrong at the same time. lol

Sag comes about because of track wear, and to some degree the suspension design of the tank affects that. If you put a new chain on a dirt bike, it's tight until you ride it for a few hours, then in has noticeable play in it. No different than tank treads, really. Tankers have to take the time to tighten it, so yes, it is a maintenance issue to a great degree.  What determines the "evidence" in the photo you choose as your guide simply has to do with when in that timeline the photo was snapped. Build it as you want it.

As far as I know, WWII tanks have a FAR greater degree of sag in them due to improvements in tank technology and tracks. Some have some degree of "automatic" sag adjustment I think, but others will give you better information.