leegsr52

Hey superdog. What size rears were those 8.6" brakes on? Salesman is telling me the 7" are biggest you can get unless you go to 58's!?

Talked to Mack dealer. He said that he had already upgraded the brakes to the 7" on the 46k rears we originally speced, which I had not realized. He claims the 8.6" shoes are on the 58k rears. Anybody know it that is the case?

That crossed my mind superdog, when I read the brake specs. Unless it was a typo on the spec sheet. I'm meeting with the salesman tomorrow to try and finalize this truck. I'll inquire about the brake size, because I couldn't see how you could have a longer shoe. At least from my recollection from all the brake jobs I've done over the years. I'll let you know what I find out.

So Rob, Are you saying that the newer 46k rears are standing up better than the 52k rears are?

CORRECTION! Just heard from salesman at Mack. He called engineering, and they actually gave him the exact weights of the two rears. The 46k rears weigh 571 lbs The 52k rears weigh 869 lbs Difference of 298 lbs. The only caveat to that is that the standard axle spread on the 46k rears is 50", and on the 52k rears its 55". But we had already changed the spread on the 46K package to 55" anyways to accommodate the tall rubber. I'm trying the verify this with the salesman now. In which case, the weight difference would not be as much, because we would have had the longer, heavier springs already figured into the 46k rears. Hey. Long as I don't carry around some heifer in the passenger seat, I should be ok. I don't work by weight anyways, so no big deal.

Hey Superdog. I agree. So, I got the printouts from my dealer, showing the chassis axle weight distribution, using both 46's and 52's. Although they don't give you the actual numbers, when you subtract the two total weights of both chassis, everything else being the same, you get an added 171 lbs for the 52 rears. That's not much a weight difference, to get the beefier rears. Just as a little more added info, my body company gave me the weight distributions for my truck, with the body included, fully loaded to capacity, both with pusher up, and pusher down. Pusher down: Pusher up: Front axle - 15449 Front axle - 22047 Pusher - 20000 Pusher - 0 Rears - 47849 Rears - 61251 The 46k rears creep ratings are: Axles - 70,000 lbs Suspension - 73,200 The 52k rears creep ratings are: Axles - 70,000 lbs (yes, same as 46's) Suspension - 102,000 lbs It was interesting that the axle creep ratings are the same. As you can see, my fully loaded, pusher up rear axle loading is only 61,251, well under the creep rating of both axles. Some spec comparisons: The bearing sizes of both axles are the same. Inner bearing - 3.75" Outer bearing - 3.25" The spline diameters on axles are the same - 2.2" The spring leaf specs, with both axle ratings at at 55" load centers: 46k 52k Standard spring (# of leafs) 11 11 The brake specs (seems to be only difference between axle ratings): 46k 52k Lining diameter 16.5" 16.5" Width 7" 7" Lining area 880 sq in 1012 sq in (though it doesn't say it specifically in specs, shoes must be longer) Putting this all together, getting a little more carrying capacity and more braking capacity (cause you can never have enough of that), my thoughts are that for 171 extra pounds, and about $1600 more in cost, it might not be a bad choice to move up to the 52k rears. I wouldn't be overloading the axles at my fully loaded capacity, and would get better braking and longer brake life. Another quick note. It seems they offer an Anti-Sway option on the spring package. I don't know at what cost. Anyone shed light on anti-sway springs, if there is a reason or not to upgrade to them?

Just got the spec sheet for the 52s. It doesn't give weight of suspension. You'd think that would be an obvious bit of information you'd want in your spec sheet for a customer making a decision. Have to contact the dealer to see if I can get that info. Keep you posted

Not much of anything government does makes sense Macks!

Oh, and by the way superdog, the Hendrickson is $300 less than than the Watson

The reason for considering the 52 rears, is that with the weight distribution I'll end up with fully loaded, I'll be at over 47,000 on the bogies. I won't be at full capacity all the time, but enough that I thought the added capacity of the 52's would help with longevity. I gotta get the weight specs for the two rears. I'll post that once I have it

Putting 20k steerable pusher on new Mack Granite. Truck will have 20k front axle and 52k rears. Looking for pros and cons of both axles

Just to give a little more info, I calculated the rpm at various speeds for both the 4.5 and 4.8 ratios. Here's the data: 4.5 Ratio 4.8 Ratio 50 mph - 1150 rpm 1227 rpm 55 mph - 1266 rpm 1350 rpm 60 mph - 1381 rpm 1472 rpm 65 mph - 1496 rpm 1596 rpm 70 mph - 1611 rpm 1718 rpm On average, there's about 100 rpm difference in each gear between the ratios. My current Mack RD with a 300 hp E6 and 10 speed transmission does 65 mph at 1550 rpm, in tenth gear. At that rpm, I often find myself downshifting a gear on the highways I travel, due to the usually commuter related traffic. So, I'm thinking that going to the 4.8 ratio is not going to kill my fuel mileage. There's only a 46 rpm difference between the two. I'm thinking the value of having a better low end will offset the slightly lower fuel mileage that I may see.

The whole discussion about fuel economy in my business is a total waste of time. When we get to a job, we sit there, running at 1250 rpm mixing concrete, anywhere from half an hour to two hours. How's that doing for your fuel economy?? So running at a little higher rpm on the highway isn't the end of the world. Matter of fact, I think I'm leaning more to the 4.8 ratio. The low end capability is more important to me than fuel mileage. Another question I'd like to pose, and I know this is an engine and transmission topic, and not drivetrain, but for the sake of keeping the discussion of this truck in one place, I'll proceed. After calculating my max loaded weight, I end up with 47,000 lbs on the tandems. My original spec for rears was 46,000. Now the amount of time that I will actually be loaded to full 47,000 will be minimal, maybe a couple of times per week, if that. But being as this is a new truck that will probably be around for 25+ years, I'm thinking on upgrading to 52k rears @ an added cost of approx. $1600. Seems like a cheap investment for peace of mind and longevity. Anyone?

I did some digging on line, and came up with the formula that is used for computing gear ratios, mph, etc, by entering the known entities. Here it is: engine rpm x 60 / transmission ratio / tire revs per mile / rear axle ratio = MPH Using this formula, you can calculate for an unknown, if you know all the other components Here are my fixed knowns: 12R24.5 tires - 458 revs per mile Allison 6th gear ratio - .67 My three unknowns are engine rpm, rear axle ratio and MPH What we are trying to determine is which axle ratio we should go with. The three ratios that Mack has available for the 46k rears, that are in the ballpark of what we might want are 4.19, 4.50 and 4.8 We do work in and around the Boston area. We are frequently on the main highways, 95, 93, 128 and 90 getting to our jobs. We don't travel for hours on end at 70+ mph to get to our jobs. We generally travel in a 25-30 mile radius If we don't hit traffic, our longest steady run is probably an hour, and even that is occasionally, not every day. Realistically, we are in a lot of rush hour traffic, road work , etc, and so if I had to give an average high speed that we might reach, I would say between 60-65 would be the top end. And most times, we are probably under that. And highway driving is maybe 50% of our driving, with the rest being on back roads. That said, I'm using 65 mph cruise speed in the above formula to come up with my calculations. Based on Mack Legacy's post above, and the Mack performance data for the MP8, I'm using 1550 engine rpm for my selected cruise speed. Plug all that into the formula: 1550 engine rpm x 60 / .67 transmission ratio / 458 tire revs per mile / axle ratio = 65 mph Rework the formula to get your unknown (axle ratio), and you get a 4.66 axle ratio Substitute 1600 rpm in the formula, leaving everything else the same you come up with a 4.81 axle ratio Now do the formula putting in the actual available ratios, leaving the 65 mph, and calculate the resulting rpm, and you get the following: 4.5 axle ratio @ 65 mph - 1496 rpm 4.8 axle ratio @ 65 mph - 1595 rpm I think the 4.5 ratio may be the best pick. It puts the engine in its optimum fuel economy rpm range for 65 mph cruise speed. Although, the 4.8 ratio is only 50 rpm outside the optimum fuel economy rpm range (1450-1550). It would give a little more torque on the low end for off road, and backing up grades. At least I'm able to narrow it down to two ratios. Let me know your thoughts.

Hey RFC. Did you see any advantage to having a retarder, as well as the engine brake?