Why are the rear axle housings narrower ?

[marv_springer]

I think it has to do with rear tire scrub and leverage. The rear axle is a fixed unit (i.e. the rear wheels track one direction...front to back). If they were mounted on casters and the front wheels were driving the vehicle, it wouldn't matter how wide the rear axle was because the front wheels would travel in there natural path and the rear would follow. Because the rear axles drive the vehicle forward and have to travel a wider arch with a wider axle there is more resistance and leverage of forward motion. ...

Leadfoot,

Scrub would be a factor if you're comparing a "locked rear end" to an open. With an open diff, there is no scrub. I've pushed around an open rear axle on tires by hand before, and believe me.... it will pivot on it's inside tire. By chance was your "RC" experience w/ a locked rear axle?


I appreciate your input, and I respect your opinion.... but I still disagree. I'm hoping that someone can provide a satisfactory explanation. I'm not trying to be argumentative.... just trying to learn something.

Funny thing too.... This all goes back to that ? about why GM made the rear track narrower. This is what I believe:
Back in the 60's when GM first started making production 4wd trucks, they used drum brakes. The front/rear track widths matched then. Disk brakes became the norm in the early 70's, and GM just bolted them on to the front housings - and the fronts got wider..... Never fixed it - never was a big problem... No rocket science to it.... just the way it worked out.

Also, when the IFS rigs were introduced, they went back to even track widths....

Marv

 

[ncgamedog]

most all 4wds have uneven trac widths. sfa toyotas are the same way so are heeps. scout II's however are the same. and i dont care how many hairs you split it affects turning radius. when i did the sas on my toyota the factory ifs rears are wider that sfa rears so you had to add wheel spacers to the front just to make it be equal lengths. it turned like ass, worse than our 2wd long box chev. i also have an all original sfa toy and the turning is acceptable even w/ push pull ( rear narrower). when i got my k5 i cant belive the turning radius its great and belive me you will be messing up makeing the trac equal. the reason its not too noticeable w/ spacers out back is b/c of the short wb. try it on a long box or even sb truck. just my .02c

 

[dyeager535]

Leadfoot,

Funny thing too.... This all goes back to that ? about why GM made the rear track narrower. This is what I believe:
Back in the 60's when GM first started making production 4wd trucks, they used drum brakes. The front/rear track widths matched then. Disk brakes became the norm in the early 70's, and GM just bolted them on to the front housings - and the fronts got wider..... Never fixed it - never was a big problem... No rocket science to it.... just the way it worked out.

Also, when the IFS rigs were introduced, they went back to even track widths....

Marv

BUT, if it were solely the simplicity/cost effectiveness of not changing what they already had (which GM loves to do) why did they run Van's with wider rears during the same time frame? It would make sense from every standpoint to use the exact same axle under as many vehicles as possible.

If I'm not mistaken, new body style, 1973, ALL the fronts were disk, so a body style change (in regard to how far the tires stuck out) would have been easy enough to do if required.

After looking at how far my rear tires "stick out" going with an '88+ rear axle that makes my front and rear the same width, I wonder if the narrower axle width doesn't have to do with how junk gets thrown off the tires. I haven't had a chance to try my truck offroad with the wider rear, but I really didn't get much dirt on the sides of the truck from the rear wheels. Now it will probably throw it as high as the body line.

That could be a safety thing for other drivers, or it could be a paint/body thing for the vehicle itself. (like the special rocker treatment they started doing)

 

[guido666]

It also effects stability in cornering and braking. Since a majority of your breaking force is applied by the front brakes (a discussion of how much would be for another thread, because numbers vary), when you brake, if the rear end were wider it would want to have the rear end spill out around the front tires. By narrowing it, it adds stability by making the rear end want to push inside the front track width.

Same applies to power, it will push inside the front track width.

All this becomes extra important in low traction environments where power is not going to both wheels equally.

 

[marv_springer]

when i did the sas on my toyota the factory ifs rears are wider that sfa rears so you had to add wheel spacers to the front just to make it be equal lengths. it turned like ass.

I don't know the details here, but my guess would be that the "ackerman angles" were not correct for the wheelbase....

belive me you will be messing up makeing the trac equal.

Making track width equal is not my desire - I was just trying to under stand the logic. ... which I still don't - and so I respectfully disagree w/ your statement.

when you brake, if the rear end were wider it would want to have the rear end spill out around the front tires.

Spill out...?!? Do you mean induce a spin?.... Boy you've lost me on that one. No matter how wide the rear track is, provided the center of mass of the vehicle is near the left/right center, I should think it would brake the same.

I just don't get it... Maybe you guys are right, and I just can't see it - but I still don't believe it.

Marv

 

[neverendingproject]

So try to imagine that your rear axle is 20 ft wide. if you tried to make a small turning radius, your inside wheel would be going backwards because it would be on the other side of you pivot point. If you had one wheel in the back, there is no hinderance.

Same idea on the difference in axle widths, just not such a big difference.

 

[muddybuddy]

how strong are the spacers?where do they go between? dont seem safe to me , but i like the idea off even track width, even if it is just for looks

 

[marv_springer]

So try to imagine that your rear axle is 20 ft wide. if you tried to make a small turning radius, your inside wheel would be going backwards because it would be on the other side of you pivot point. If you had one wheel in the back, there is no hinderance.

Even in this extreme case (20' wide rear axle - and turning radius <10'), if the rear diff was open - it would provide no resistance to turning. Your right in saying that the inside wheel would be rotating backwards, but the outside wheel would be rotating forward on a nearly 20' radius.

Marv

 

[mofugly13]

Is the track width difference only on 4wd trucks? If so, this just popped into my head. Having different track widths makes sure that your rear tires aren't riding "exactly" in the track of your front tires. Maybe for a little more traction? A little bit of the rear tires gets a fresh "bite". I donno, just a guess.

 

[dyeager535]

I had thought about that previously as well, but the track is the same 2wd/4wd. It would still be a traction advantage in most situations, but I doubt that was the reason. It could be that everything being brought up was a factor.

Then again, everything now (GM) has the same track width, no?

Heck, maybe it helped accomodate the dually rear setups...less stickout/fender flare/width necessary without having to change the actual vehicle construction SRW or DRW.

 

[az-k5]

when i got my k5 i cant belive the turning radius its great and belive me you will be messing up makeing the trac equal. the reason its not too noticeable w/ spacers out back is b/c of the short wb. try it on a long box or even sb truck. just my .02c

Speaking from experience, I call BS. The small width changes we make to track width hardly change the geometry of even the short blazers. I have an even width by using a Dodge D60 up front with a 14FF rear, and noticed no difference over my wide 3/4 ton D44. I also had a crew cab that started with a DRW D70 in the back and a converted SRW front D60. The rear end was a good 4" wider than the front. With normal wheels on it (the way it was bought from the PO) it turned like crap. Put on a normal 14SF and it still turned the same, drove the same, handled the same. Chop 17" out of the middle of the frame (for the short bed conversion) and it didn't change how the truck behaved. It did turn a little better but still barely made it into my drive way.

 

[dubbyx]

Having different track widths makes sure that your rear tires aren't riding "exactly" in the track of your front tires. Maybe for a little more traction? A little bit of the rear tires gets a fresh "bite". I donno, just a guess.

That's what I tell my kids.... They buy it... and can't complain much because it makes a bigger rut for them to bounce their bikes around in.

 

[Leadfoot]

Leadfoot,
I'm not trying to be argumentative.... just trying to learn something.

No problem

I'm trying to learn too.

I did not even think about whether or not the R/C was open or locked (I just ASSumed it was open, but don't know for sure).

This would actually make a cool experiment for the Mythbusters. Put a full width van 14FF underneath a jeep and see if it changes turning radius (assuming open diffs on both).

Hell there are several members here who could do it with parts/vehicles in their yard. I'm sure Alan (Mountainexplorer) has a spare rear-end or two....if only he had a jeep layin' around

I understand that with an open diff one tire can move forward and the other backward, but would that happen "voluntarily". I can't put it into the words I want, but I'm thinking that both tires want to turn forward with the input provided at the pinion, and although one tire could move backward it would have to be due to resistance (i.e the front wheels forcing it to) and that force is what causes the front end to understeer off it's ideal course. Now if the rear end were on a zero traction surface and the front wheels were not (not a real world situation for an entire turn most of the time) then maybe...........

Besides, now I'm interested in finding out what the answer really is and all this thinking is making my head hurt

 

[BigOrange90Jimmy]

Hell there are several members here who could do it with parts/vehicles in their yard.

I've got a Zuk with a fragged rear axle.....

 

[Twiz]

It kinda impoves turning radius, but not in the way you would normaly think.
It does, however, help your wife or girlfriend from curb-checking your truck every time she drives it.

Useing Marv's picture;
Put a curb right by the outer side-wall of the L/Front tire. Then continue the turn. The L/Rear tire will eventualy go right over the curb, or even inside it. Because the rear tires fallow a tighter turn than the front.
The actual turning radius doesn't change, because the R/Front follows THE widest radius in a the turn, regardeless of how wide the rear axle is. But the over all turning radius is tighter. (The actual ground covered, between all 4 tires, is less)

 

[jimmy88]

From: 971584
Introduction to Formula SAE
Suspension and Frame Design
Edmund F. Gaffney III and Anthony R. Salinas
University of Missouri - Rolla

http://campus.umr.edu/fsae/library/sae_paper/paper.html

When selecting the track width, the front and rear track widths do not necessarily have to be the same. For example, track width is typically wider in the front for a rear wheel drive race car. This design concept is used to increase rear traction during corner exit by reducing the amount of body roll resisted by the rear tires relative to the front tires [4].

Also mentioned elsewhere that a wider front track relative to rear track contributes to understeer. May or may not have any relevance to why GM did it.

 

[fireplug]

I don't know dick squat but I've always been told it was for improved tracking. And I've heard this time and again. I'm surprised it took so long for it to be mentioned. The extra width of the van rear end may be to counter the higher center of gravity of a loaded van vs a loaded blazer/p/u

 

[Pow06er]

I have 1" spacers on the rear beause I have a 14bolt and wanted to stay 6lug. My truck dident track or turn any better before or after, It looks a lot better IMO and does cover my bedsides with alot more mud

Is it unsafe to run adapters to go from 8 lug to 6 lug? I cant afford to convert to 8lugs and if I can build up a 14ff instead of a 14sf I will.