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Old 07-22-2006, 09:04 PM   #1
Dean A Van Peursem
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Rear End ratios - GVWR - Safety

I know this is a sensitive topic for some of us and sometimes creates heated exchanges but I'm trying to avoid all that and look at this topic logically w/o all the emotion.

GVWR is often used as a measurement for our TV's as something we shouldn't exceed. I agree with that but then I observe GVWR totally dependent on drive train elements like what rear end ratio is used. In some cases only a change of rear end ratio can change the GVWR more than 500 lbs or more with no other mechanical changes.

Now I understand a higher ratio will allow more pulling power for heavier loads, with some resultant loss in mileage. But what I don't understand is how that relates to safety going down the road or down the hill. I'm having trouble just thinking that using GVWR as a safety measure for weight may be misleading us. I just can't figure out why the rear end ratio should make my truck/5th wheel safer or not safer at a certain overall GCVW. What am I missing?
 
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Old 07-23-2006, 03:25 AM   #2
OntMont
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I have never seen a good explanation of how GVWRs are estabilshed, either for trucks or trailers.
I think it must just come down to what the engineers see as the weakest link in the vehicle. If it is the drive train, then maybe changing the rear end ratio is a legit. way of increasing the GVWR, but if it is the frame, brakes, tires, or axles, then changing the rear end ratio is not going to help the GVWR. It is just a matter of what the weakest link is in the design of each vehicle.

That said, I think if you look at Ford and GM trucks, you can see that this might be what is happening. Ford seem to have a heavier, probably stronger truck with larger brakes, derived from their bigger trucks, their weak link is the power train, hence they can play with the rear end ratio and wheel sizes, and come up with different GVWR. GM has a pretty solid engine and drive train that is capable of driving much bigger trucks, but their pick-up frame and chassis is lighter than that used by Ford, so for GM the weak link is something other than the drive train, so they don't fiddle with rear end ratios and wheel sizes.

This is NOT intended to mean the one is better than the other, just that the each has a different limiting factor determining their GVWR, and all of the this is just speculation on my part, others may disagree.
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Old 07-23-2006, 03:44 AM   #3
David and Jo-Anna
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Dean--a very good question. As a newbie here, I likewise have been a bit confused by the concentration on meeting the GCVWR and GAWR numbers and less focus on what else one should do in selecting a tow vehicle to be sure one has a reasonable safety margin in all respects (including the capability of safely handling panic stops, downhill braking, strong cross winds, etc) when towing one of the larger Montys like the 3400. I look forward to hearing what the experienced voices here have to share.
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Old 07-23-2006, 04:03 AM   #4
CountryGuy
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Again, I cannot address all these tech numbers, but David, you mention strong cross winds, and I can address that.

UNLESS you are driving Montana in 50 (or more) mph cross winds, I doubt you will feel em! With our TT and with the boat (both of which I pull(ed)) I can tell you, much over 35 mph cross and I am white knuckled, clenching jaws and very tense. Montana, being a 5er, and being Montana, tows very well in wind, even cross winds. I NEVER feel a 18 wheeler when they pass us on the xpressway, even if they are driving real fast, there is NONE of that push and pull that you experience with a TT or the boat. With the TT, it used to feel as if the 18 wheeler was going to whip the rear end of that TT around to meet the front end of the van/truck we were towing with. Did not like that. NEVER have had that sensation with Montana.

Now, you will feel it in your mpg of fuel useage. Many of us in the RGV, when asked when we are leaving for the north country, will respond, when the wind is out of the south!! We want it to push us out! One year we left with a 35 to 45 mph head wind, and it killed our mileage, from 7 to 8 average down to under 5.5! That hurt!

Also would suggest, that if you have any choice, that one might be wise to NOT run if the forecast winds are for more than 45 or 50. We have heard a few horror stories of 5ers getting tossed on their sides in 70 and 80 mph winds.

As far as driving down hills, we have the Allison, Al has been teaching me how to brake enough to get the Allison to kick in and roar and brake us down the hill. The way I understand it, you need to learn the tricks of driving your truck, gas with or without Allison, or diesel with whatever trans you have. I am still learning, but I can tell you, I LOVE that ALLISON, specially on the 75 heading north into Cincinnati! Goodness it roared, but, I held to 55 mph with very little application of the truck brakes! Whaoooo, I am ready to try that again!

Again, this does not directly address the subject of the original question/post, and is only CAROL's experience, granted, Al does most of the driving, but I am doing more and more, as we find that we don't arrive at our destination as tired when we share the driving load.

Carol
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Old 07-23-2006, 04:17 AM   #5
richfaa
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As I have said before I am not at all brand loyal.I buy what is best for us at the time.We got the Ford in 05 because it was the toughest built/We saw the frames of the big three side by side at a auto show and the Ford was awsome. Next time who knows.. Out last Tv was a Chevy, before that a Ford.
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Old 07-23-2006, 04:22 AM   #6
trukdoc
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I will try to help. Gear ratio is simular to leverage. 4:10 gears have more leverage that 3:79.
Old school was 4:10 or higher, keep in mind even up into the eighties trucks had nowhere near the pulling power of todays engines, particularly diesels. Onboards electronics have made incredible changes.
The higher gear ratio was needed to simply pull the load, anyone having an old Ford 6.9 or GM 6.2 can tell you what I mean.
But today with HP much higher and advances in transmissions the lower ratios pull very well. Not to mention better fuel economy.
As far as safety goes the ratio has little to do with it, with the exception of mountain driving. The lower ratio will help with engine braking. My Ford has engine braking through the transmission as part of the tow package. Now when I say mountain driving I am not talking about going down the interstate and going over a pass. It is up the middle of the rockies. But with some common sense Is still is not an issue...slow down.
As fas as CGWR and GVWR if you stay within specs the braking and mechanical needs are met. I will ruffle some feathers here but any vehicle will safely tow more than they are rated at due to the built in safety margin. Rule of thumb is 20%. With that said the closer you come to the limit the closer you come to problems. Again common sense.
GVWR is total weight of the truck, CGWR is combined load of truck and trailer. One more thing to factor in is the load rating of the tires.
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Old 07-23-2006, 04:22 AM   #7
Wrenchtraveller
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What you are missing is that your Gross Vehicle Weight Rating is not effected by your rear end ratio. It is more model specific. GVWR is the ability of your suspension and tires to handle a certain payoad. If the pin weight of a large Fiver overloads your GVWR, there might be a safety problem.

Your Gross Combined Weight Rating is effected by engine choice, rear end choice, and model choice and it is more of a performance issue but both weight ratings come into play and the foot notes on the bottom of the Ford Fifth wheel chart explain this the best. I have quoted these footnotes in the 3400 post above but if any one wants me to email them a scanned copy of the Ford Fifth wheel chart for the 05 model year, let me know.
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Old 07-23-2006, 05:04 AM   #8
David and Jo-Anna
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Glad to hear that the Monty holds up well to big rigs blowing past. Last year I rented a 32 foot class A to travel around Arizona and southern Utah. On the last day of our trip. we were on a stretch of Interstate with a 70 mph speed limit. Because of moderate rain, and my limited familiarity with the rig, I was going about 60-65 mph while the tractor-trailers were blowing by at 75-80. Between getting blown sideways by their wind and having my windshield covered by heavy spray as they blew past, it was not a fun ride. Had it not been necessary to get my brother-in-law home for a medical appointment the next morning, I would have pulled off the highway and found someplace to plant myself for the day, which is what I'd do if it arose again once we are full time. But I like knowing that I won't get blown all around in a Monty if and when I do find the big rigs zipping by me.
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Old 07-23-2006, 05:12 AM   #9
Cat320
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Wrenchtraveller is right on. GVWR has nothing to do with rear end ratios...it has to do with what it will carry (cargo capacity).

Richfaa...I just made post on a different thread re Ford weights. I compared a Ford F250 and Chevy 2500HD, both diesels, 4x4s and CCs. The Ford had about 100 lbs more cargo capacity and an 800 lb higher GVWR. Bottom line the Ford was 700 lb heavier.
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Old 07-23-2006, 05:43 AM   #10
CountryGuy
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David

A class A and a Montana ain't even in the same league. NO WAY. Your experience in a MH will be an asset in the Montana, you will be soooooooooo relieved! (Apologies to all MH lovers, I exercise my rights to my own opinion here! HA HA)

Carol
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Old 07-23-2006, 06:13 AM   #11
Dean A Van Peursem
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Ok, thanks for all the responses. There still is confusion as far as I see it. It has been stated more than once in this thread that Rear End Ratio has nothing to do with GVWR. Yet, TV manufacturers change their published GVWR with only a change in rear ends ratios used. That tells me we are not all singing out of the same hymnal.
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Old 07-23-2006, 06:20 AM   #12
richfaa
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Quote:
quote:Originally posted by Cat320

Wrenchtraveller is right on. GVWR has nothing to do with rear end ratios...it has to do with what it will carry (cargo capacity).

Richfaa...I just made post on a different thread re Ford weights. I compared a Ford F250 and Chevy 2500HD, both diesels, 4x4s and CCs. The Ford had about 100 lbs more cargo capacity and an 800 lb higher GVWR. Bottom line the Ford was 700 lb heavier.
Oh yea..If you saw the difference in the frame, Etc you would understand whay the Ford weighs more.We understand the GM's have caught up 07. It is always a ..lets keep up guy..thing and not always good for the towing consumer.. You know..Mine is bigger than yours thing.
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Old 07-23-2006, 06:57 AM   #13
Dean A Van Peursem
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Here is another anomoly for consideration. Lets say the TV Manufacturers struggle with a compromise between GVWR vs comfort of ride when empty. And I think they do. And lets postulate that only one mechanical change could change GVWR and that change is adding an additional leaf to the springs, or adding a heavier overload spring. No other mechanical changes, no rear end ratio change, no drive train changes, no axle changes. I think we could site a real live example of that. Now that determines or at the minimum influences the GVWR on the door sticker. Now if this is true then why cannot a TV owner do the same thing with aftermarket parts and effectively increase the GVWR with full knowledge that the door sticker hasn't changed? But the effective TV GVWR has really increased for safety considerations. I ask this question to generate discussion, not that it is necessarily true.
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Old 07-23-2006, 07:17 AM   #14
David and Jo-Anna
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I'd like to followup on Trukdoc's comment about a 20% safety margin. Is this a suggestion that the OEM manufacturers may be providing a 20% margin beyond their ratings or, to the contrary, that we should be cautious about getting closer than 20% to the OEM's posted ratings? To me, that's a big difference. I have seen references by some on other forums that one should only load up to 80% of an OEMs GVWR and GCVWR numbers, but the posters seldom provide any support for their postings.

And is there any feeling as to whether one or two of the big three have provided a bigger margin, in terms of safety, in terms of their vehicle's ability to meet and exceed posted GVWR and GCVWR?
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Old 07-23-2006, 07:31 AM   #15
Dean A Van Peursem
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A follow up to the previous postulation. When one looks at the sum of the GAWR of the front and rear axles on the TV in some cases there is a a significant difference between that sum and the GVWR for a particular truck. Is it possible that the only cause for that difference from one truck to the other is what springs were factory configured for that specific truck? For example: the sum of GAWR may be 10,000 lbs but the GVWR may be only 8500 lbs because lighter springs were used. If configured with heavier springs the GVWR may be published at 9,000 lbs or higher with no other mechanical change having been made.

My suggestion here is we may have to dig deeper than just looking at published numbers.

Now heavier axles, larger brakes, heavier transmissions, higher rear end ratios, higher torque engines also comes into play but I don't think we fully understand how certain numbers are being established by the manufacturer. I truly beleive that some of the numbers have changed due to competitive pressures rather than real structural or mechanical enhancements that have caused the TV GVWR to change. I'm prepared for being corrected on this premise.
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Old 07-23-2006, 07:41 AM   #16
richfaa
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Although We never fully believed it we were told many times by TV tech reps at RV shows that we worked that there was a 1000lb "safety" margin built into the tow rating of the trucks. I never told a prospective buyer that.
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Old 07-23-2006, 07:47 AM   #17
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I know that in the '05 Fords, they beefed up the chassis, suspension, and whatnot, in order to add 3,500 pounds to the GCVWR.

It's the only reason I'm not driving a Dodge, actually, because we were 3,000 pounds over the GCVWR of our old 2002 F-350 dually.

The suspension changes means that I hardly feel the jerking that we sustained and got sort of used to in the 2002.

I’d still love to have free rein at a set of scales to measure all the possible weights, with the Monty, and without the Monty.

I only knew that our 2002 truck, fueled up but not fully loaded, weighed in at 8,010 pounds! Together with the Monty, the total was 22,970 pounds.

I know that this Monty has less storage room, and we’ve since shed loads of stuff we had carried all the way from Alaska, and put it into my brother-in-law’s free storage until we need it once again.

But, we’ve just added a 5.0 Cubic Foot freezer.

I’d love to get this thing weighed at each axle, too.

I’d also love to lose about 85 pounds, myself, and grow ALL my hair back!


Steve
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Old 07-23-2006, 08:39 AM   #18
Dean A Van Peursem
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Steve:

I'm not sure where you live but out here in Washington state many of the WA DOT scales are available for RV weighing. When the sign says "closed" it is a weighing opportunity. Many of the scales leave the scale active and the weight display on and in recent experience the weight display was visible directly from the drivers seat out the front windshield. Very handy! I weigh almost every opportunity I get.

When hauling equipment trailers I have also weighed at commercial scales such as those that sell bulk feeds, etc and in some cases had to pay a small fee. Many have done it free and even provided a certified scale receipt. Some of the larger chain gas stations along the Interstates that cater to cross country truckers offer weighing for a small fee. They are out there just look for them.
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Old 07-23-2006, 09:19 AM   #19
Montana_1240
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Dean,

I’ll have to look into that. Sounds like a good deal!

In Fairbanks, the scales were at a truck stop. You would have to ask before driving onto them, because the read-out was inside the store, rather than at the scales. They did that so that people would tend to buy certified scale print-outs. Something I’m guessing were fairly mandatory for commercial vehicles. But they would let you know the weight, and you could jot it down for free, if they weren’t busy with paying customers. I wanted the print-out for my records. That’s when I learned how heavy the truck was. The next visit, when we left town, I learned how far over the GCVWR we were. Kimmrg still has that sheet, I believe.

I’d love to have the time to weigh each axle of the TV, (without the Monty,) but with it loaded. Then weigh the Monty’s added weight as it stood with the wheels off the scales, but hitched up to the TV. Then I’d like to weight just the Monty. And then the Monty’s wheels, with the TV off the scales.

I’m guessing that all those figures would allow me to calculate all that I’ll need for feeling good about weight.

With as often as the scales I’ve seen are closed, I’m guessing that all that wouldn’t likely hold anyone up.

We’re in Alabama, right now. But with as many states as we pass through, I’m guessing a few will have much the same policy as does Washington.

Thanks,
Steve

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Old 07-23-2006, 10:35 AM   #20
The_McCullochs
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While this may only apply in Texas, the DPS couldn't care less about the manufacturers' GCWR numbers and, in general, are unaware that they even exist.

They determine the GCWR of a towing/towed vehicle combination by simply adding the GVWRs together. [If the License/Title information is available they'll use that data but, otherwise, they'll use the door/sticker numbers.] If the Sum of the GVWRs is over 26,000 pounds then a Class A DL [Non-Commercial] is required.

We were also surprised to find that any towing/towed combination where a trailer's exceeds 10,000 pounds requires a Class A DL! [Non-Commercial if you're not towing for pay/]

Considering that our 2003 Dodge 3500 has a GVWR of 12,000 and our 2003 3655FL has a GVWR of 14,100, that meant that DL upgrades were in order on both counts.
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