M1 Abrams Tank Wheel Spacing?

[avadon77]

Hi all,

Got a quick question. Why is it on many main battle tanks the foremost/front idler wheel is further ahead, or has more space between it's following wheel than the other idlers behind it? Some tanks seem to have fairly consistent spacing in between all idlers but the Abrams and several others have more space behind that first idler. Is that to accommodate the fact that the front/leading wheel has a different arm angle than the following wheels?

From some rough drawings it appears that the first idler arm on the Abrams is orientated much higher like ~40dgs and the other arms are much more laid down like 15dgs. So I'm guessing there has to be more space behind that front wheel because it takes the brunt of all obstacles and consequently swings a wider/larger arc then the wheels behind it. Also the front idler arm is mounted near the front of the hull which has the inclined angle and the arm couldn't be set at the same angle unless the hull was extended much further forward.

In other words it appears to me that the arm is angled as such because of where it's mounted. But maybe there is also a gain in having it at that angle because it rides over obstacles better than having it at a lower angle.

Can someone please elaborate why the Abrams was designed this way? And if i'm correct about the spacing behind the front wheel? Thanks
Attached Thumbnails

[BarryC]

I am no expert in tank chassis design phylosphy but here are a couple of pictures that show the complete installation.

The first road wheel is linked to the idler via a "slack adjusted" and between them they act to maintain track tension.

Barry

[Woz]

Weight distribution.

You have a heavy engine at the back, heavy turret in the middle and a driver up front.

As for the swing arm. The hull bottom is sloped at the front so swing arm can't be at the same level as the rest.

[avadon77]

Woz,

I think you're right, I think it's simply that the hull bottom is sloped so literally there is no room to put the swing arm at the same axis/position as the others. There's probably also some significant gain by having it raised up which allows that foremost wheel to take the brunt of the first obstacle much better if it's at 40dgs.

@Barry, yes I noticed this in my research too that many track vehicles have that slack adjustment connection or simply the idler and the road wheel where on the same pivot axle. Thus when the front road wheel rotates back (climbing an obstacle) the idler actually comes down and rotates with it thus keeping the track tension correct. Otherwise the road wheel would create a huge hiccup of slack that would transfer to the road wheels following it and possibly throw the track.

[43rdRecceReg]

Woz,

I think you're right, I think it's simply that the hull bottom is sloped so literally there is no room to put the swing arm at the same axis/position as the others. There's probably also some significant gain by having it raised up which allows that foremost wheel to take the brunt of the first obstacle much better if it's at 40dgs.

@Barry, yes I noticed this in my research too that many track vehicles have that slack adjustment connection or simply the idler and the road wheel where on the same pivot axle. Thus when the front road wheel rotates back (climbing an obstacle) the idler actually comes down and rotates with it thus keeping the track tension correct. Otherwise the road wheel would create a huge hiccup of slack that would transfer to the road wheels following it and possibly throw the track.

I've often wondered about tank running gear configuration too; relative to weight distribution, track tension, grip and dynamics..etc. Oddly enough, one of the best explanations of lead wheel positioning
is in a short UK Government film produced in 1942, and incorporated in the worthy 3-hour long collection of wartime MOD vignettes: (British Tanks of the Second world War: expanded edition):-
http://www.ebay.co.uk/sch/i.html?_from= ... r&_sacat=0
(and yes, you're right about the effect of hull shape.. partly..but it's also due mostly to the need for the tank to be able surmount obstacles..)

[BarryC]

Woz,

I think you're right, I think it's simply that the hull bottom is sloped so literally there is no room to put the swing arm at the same axis/position as the others. There's probably also some significant gain by having it raised up which allows that foremost wheel to take the brunt of the first obstacle much better if it's at 40dgs.

@Barry, yes I noticed this in my research too that many track vehicles have that slack adjustment connection or simply the idler and the road wheel where on the same pivot axle. Thus when the front road wheel rotates back (climbing an obstacle) the idler actually comes down and rotates with it thus keeping the track tension correct. Otherwise the road wheel would create a huge hiccup of slack that would transfer to the road wheels following it and possibly throw the track.

I have been considering modifying my M1 chassis with "working" slack adjusters. I wonder if it would prove to be useful, I would think it would.

Barry

[avadon77]

Woz,

I think you're right, I think it's simply that the hull bottom is sloped so literally there is no room to put the swing arm at the same axis/position as the others. There's probably also some significant gain by having it raised up which allows that foremost wheel to take the brunt of the first obstacle much better if it's at 40dgs.

@Barry, yes I noticed this in my research too that many track vehicles have that slack adjustment connection or simply the idler and the road wheel where on the same pivot axle. Thus when the front road wheel rotates back (climbing an obstacle) the idler actually comes down and rotates with it thus keeping the track tension correct. Otherwise the road wheel would create a huge hiccup of slack that would transfer to the road wheels following it and possibly throw the track.

I've often wondered about tank running gear configuration too; relative to weight distribution, track tension, grip and dynamics..etc. Oddly enough, one of the best explanations of lead wheel positioning
is in a short UK Government film produced in 1942, and incorporated in the worthy 3-hour long collection of wartime MOD vignettes: (British Tanks of the Second world War: expanded edition):-
http://www.ebay.co.uk/sch/i.html?_from= ... r&_sacat=0
(and yes, you're right about the effect of hull shape.. partly..but it's also due mostly to the need for the tank to be able surmount obstacles..)

Thank you for the information. I'll check out your links
Cheers.

[keno]

Those slack adjusters as you call then are track tension adjuster. That is where the tanker will manually adjust for track tension. When the front road wheel is flexing up the idler compensation roller, (wheel) is pushed forward keeping the track at the proper tension. I had an answer for the reason there is a gap but my memory is not cooperating...I'll ask Ernie to remind me. Ernie is a retired field service rep with GD. We worked together while fielding the Abrams. I'm sure you are aware of the staggered road wheels from side to side. This facilitates in the placement of the tortion bars for the suspension.

[keno]

Ok, Ernie said the wider gap is to facilitate the rotary shocks at the two front locations...

[keno]

Remember 3-6 just have torsion bars and only need to be indexed to a point where the torsion bar can go in. The rotary shocks need more travel because the shocks are indexed at a higher degree be cause of more movement. That's the purpose of the spacing...