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Old 03-04-2014, 11:39 AM   #1
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How much grip does a Class A have?

Having done some sort of amateur racing since I was 18, I am very familiar with vehicle dynamics and handling. Of course in racing, being able to feel the limit of tire grip is of consummate importance.

I have had my MH for 18 months and have about 8K miles under my belt (stop laughing all you old pros) and I have been wondering how much tire grip is really there. In the dry I don't expect to get close to the lateral limit, and under hard braking, it feels like I have more tire grip than braking ability (the opposite of most cars that I have driven).

I am more interested in the wet, as the limit of grip is much reduced. I assume that under increasing cornering input, the coach will understeer, but will it be a gradual increase, or once it starts sliding will it take a more dramatic reduction in speed or steering input to get the tires to hook back up again? Also the same under braking.

I ask these questions because this isn't something that I want to try in a parking lot, and I probably should have tested the braking when I had the coach empty. Of course, the last place to discover (or at least understand) the limits of your vehicle is in an emergency situation, so that's why I am bringing this up.
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Old 03-04-2014, 11:51 AM   #2
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Up until I was in my late 20s I think I found the limits of just about every vehicle I have driven. I have never heard anyone ask what the limits are for a class A motorhome.
I think your gonna put the rig on its side before the tire slip cornering on dry pavement.
Braking well,.......there is only one way to find out. Do you have air brakes? if so they should be able to lock the wheels with ease, as long as the slack adjusters are properly adjusted.
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Old 03-04-2014, 12:01 PM   #3
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Take her out to a wet parking lot and do some doughnuts like we did as kids!
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Old 03-04-2014, 12:30 PM   #4
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Truck tires have about 0.7 coefficient of friction on dry pavement and 0.3 on wet asphalt.

Don't worry about brake adjusters, automatic adjustment has been required by law on air brake vehicles since back in the 90's.

My gut reaction to your post was that a big motor home would fall over before it would slide sideways. Quick calculations based on crude estimate of CG height revealed otherwise.

At 0.7 G cornering (maximum possible with 0.7 tire coefficient):

Overturning moment = G force x CG height x Vehicle weight
0.7 x 4 x 30000 = 84,000 lb ft

Moment resisting overturning = Vehicle weight x Track/2
30,000 x 4 = 120,000 lb ft

Looks like it wouldn't tip until you cornered at 1 G, which the tires won't let you do so you will slide first.

CG height of 4 feet is just a wild guess, but probably in the ballpark. Working backwards, it says that it would take a cg height of 5.71 feet to start to tip at 0.7 G cornering.
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Old 03-04-2014, 12:40 PM   #5
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I've had an occasional time where I had to stand on the brakes to avoid idiots, and in both instances the anti-skid actuated on dry pavement. Both cases worked OK but no damage except that I emptied a cabinet or two. I can do that if I get too speedy on a normal turn, so I don't ever want to try it with an incipient skid. Good luck in your explorations, and please let us know what you conclude. I'd be very interested.
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Old 03-04-2014, 12:47 PM   #6
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Watch the movie Speed.
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Old 03-04-2014, 12:53 PM   #7
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Go watch the mythbusters episode where they were testing speed. They were using an old bus, but if I recall they had to add weight to the roof to get her to roll.

Bus Rolling Over While Turning At High Speed : Discovery Channel

So I am with John above, the CG is too low to roll.

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Old 03-04-2014, 01:02 PM   #8
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Just drive safely and defensively. Don't rush and keep an extra distance. Take advantage of Rest Stops to recharge your batteries and energize yourself. Obey the posted speed limits, and consider all weather conditions - wind, rain, fog, blinding sun.
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Old 03-04-2014, 01:39 PM   #9
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Thanks for the info so far, and for the cautionary advice, but as we all know you can only do so much if a squirrel cuts into your braking area when you are already stopping with some vigor. Being prepared gives you that much more of a safety margin, and my goal is to know just how much margin I have. In the car, I know if I can stop or if I have to swerve. I want to know the same thing in my MH.
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Old 03-04-2014, 01:41 PM   #10
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The "R" in RV does not stand for RACE.

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Old 03-04-2014, 02:01 PM   #11
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Quote:
Originally Posted by TXiceman View Post
The "R" in RV does not stand for RACE.

Ken
Unless you are kidding, I think that you missed the point here.

I'll give you the example of why I brought this up. This past February we were headed south on our big trip, cruising along at 50 mph in the right lane (55 limit) when a few cars came around the clover leaf to merge. There was traffic to my left so I was stuck in the lane. I started slowing to let the cars merge in front of me. There was plenty of room, provided that they just went. The third car (actually a pickup truck) was moving very slowly and it appeared that they were going to wait until I passed. The other 2 cars merged and were accelerating away. By this time I was probably down to 35~40 and this idiot waited until I was almost upon him to pull over in front of me and basically coast. I had to slow very hard and I hit the horn, showing my displeasure at his boneheaded move. This idiot was obviously offended as now he hit the brakes! I still had enough room to stop, barely, and was quite upset at this moron's thoughtless and dangerous actions. So you can do everything right (except hit the horn I guess) and still be put in a panic stop situation. After calming down, the thought kept going through my head about how much more stopping power I actually had, and was it as close as it seemed to be. This is why I put forth the question.
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Old 03-04-2014, 02:06 PM   #12
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Rollovers can happen at very slow speeds:
Terrible Bus Accident On Muddy Road - YouTube
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Old 03-04-2014, 08:37 PM   #13
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I don't see where the rollover video had any relevance to the post. Mud and a road giving way are completely different situation than speed, cg and traction.
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Old 03-04-2014, 08:56 PM   #14
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Quote:
Originally Posted by PushedAround View Post
Unless you are kidding, I think that you missed the point here. I'll give you the example of why I brought this up. This past February we were headed south on our big trip, cruising along at 50 mph in the right lane (55 limit) when a few cars came around the clover leaf to merge. There was traffic to my left so I was stuck in the lane. I started slowing to let the cars merge in front of me. There was plenty of room, provided that they just went. The third car (actually a pickup truck) was moving very slowly and it appeared that they were going to wait until I passed. The other 2 cars merged and were accelerating away. By this time I was probably down to 35~40 and this idiot waited until I was almost upon him to pull over in front of me and basically coast. I had to slow very hard and I hit the horn, showing my displeasure at his boneheaded move. This idiot was obviously offended as now he hit the brakes! I still had enough room to stop, barely, and was quite upset at this moron's thoughtless and dangerous actions. So you can do everything right (except hit the horn I guess) and still be put in a panic stop situation. After calming down, the thought kept going through my head about how much more stopping power I actually had, and was it as close as it seemed to be. This is why I put forth the question.
FMVSS 121 requirement is less than 280 feet stopping distance from 60 mph for loaded or unloaded buses. So that's what you have to work with. Definitely traction limited with non sticky truck tires. All modern trucks have enough brake power to lock the wheels on dry pavement.

Regarding rollovers, my calculations were based on flat surfaces and staying on the road. You can turn over anything if you slide off the road or into a curb.
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