FROM: "Maximum Frog" SUBJECT: a 90 deg obstical... DATE: Tue, 25 Sep 2001 09:50:18 -0700 ORGANIZATION: California State University, Fresno NEWSGROUPS: rec.autos.4x4 I'm curious about how a 4x4 would cope with a particular type of obstical...I'm sure someone here can help me out. If you were in a 2 wheel drive vehicle and tried to drive over something with a 90 deg side (cement step, for example), the vehicle could do it if the "step" is shorter than the center of the hub on the wheel. At that point the wheel would simply push against the object and you'd either break something or spin the wheels. If you were in a 4 wheel drive, the same limitation would happen untill you go into 4x4. At this time, would the front wheen turning lift the vehicle straight up and over the object? Or would you be more likely to damage something. Mostly curious, because there's a section of a trail that I go on..where I normaly avoid and leave it to guys with bigger wheels than I have. I know I can get through the rest of that section if I can get over the "gateway" rock...a big piece of..shale..or..some rock that's very cubical. I'd like to do it without re-locating my front suspension as well. :0) -Max Frog FROM: Roger Brown SUBJECT: Re: a 90 deg obstical... DATE: Tue, 25 Sep 2001 18:35:51 GMT ORGANIZATION: http://4crawler.cruiserpages.com/index.shtml NEWSGROUPS: rec.autos.4x4 Maximum Frog wrote: > > I'm curious about how a 4x4 would cope with a particular type of > obstical...I'm sure someone here can help me out. > > If you were in a 2 wheel drive vehicle and tried to drive over something > with a 90 deg side (cement step, for example), the vehicle could do it if > the "step" is shorter than the center of the hub on the wheel. At that point > the wheel would simply push against the object and you'd either break > something or spin the wheels. > > If you were in a 4 wheel drive, the same limitation would happen untill you > go into 4x4. At this time, would the front wheen turning lift the vehicle > straight up and over the object? Or would you be more likely to damage > something. > > Mostly curious, because there's a section of a trail that I go on..where I > normaly avoid and leave it to guys with bigger wheels than I have. I know I > can get through the rest of that section if I can get over the "gateway" > rock...a big piece of..shale..or..some rock that's very cubical. > > I'd like to do it without re-locating my front suspension as well. :0) Yes, it can be done to a point. If there is enough traction on the remaining wheels to push the tire against the vertical rock, it'll climb. Also helps to have a locker on that axle, in the picture below (from a recent Dusy/Ershim trip), I tried to back up the rock I'm on: http://4crawler.cruiserpages.com/4x4/CA-168/Dusy-2001/dusy_12.jpg but with open diffs, the other tire just would spin. I locked up front and rear and was able to climb the rock with ease. If you are open and try this the tire against the ledge has loads of traction, but unless there is enough traction on the opposite side to balance it out, that side will spin. In my case I was on sandy dirt with 3 tires, you can see some of the holes in front of my tires where I was spinning. Other limit is your approach (or departure) angle, If a bumper or part of your frame or suspension contacts the obstacle before the tire can get on it, you probably won't climb. Doing this does put a fairly high load on the drivetrain, since the wheel climbing up is not moving forward, while the other wheels are (trying to at least) If they can slip a bit, that'll relieve some of the stress, but if you are on a high traction surface, it'll bind things up pretty fast, but it'll loosen up once you get up the ledge. FROM: Stephen Young SUBJECT: Re: a 90 deg obstical... DATE: Tue, 25 Sep 2001 19:51:38 GMT ORGANIZATION: AT&T Worldnet NEWSGROUPS: rec.autos.4x4 Roger Brown wrote: > Doing this does put a fairly high load on the drivetrain, since the > wheel climbing up is not moving forward, while the other wheels are > (trying to at least) If they can slip a bit, that'll relieve some of > the stress, but if you are on a high traction surface, it'll bind things > up pretty fast, but it'll loosen up once you get up the ledge. This shouldn't be an issue - think of a vehicle travelling inside a ball. The climbing wheel isn't moving forward but neither is the vehicle - it's going forward & upward. This is one of the Hummer's claim to fame - it's can climb over a 24" vertical obstacle as part of it's normal capabilities. FROM: Roger Brown SUBJECT: Re: a 90 deg obstical... DATE: Tue, 25 Sep 2001 20:07:02 GMT ORGANIZATION: http://4crawler.cruiserpages.com/index.shtml NEWSGROUPS: rec.autos.4x4 Stephen Young wrote: > > Doing this does put a fairly high load on the drivetrain, since the > > wheel climbing up is not moving forward, while the other wheels are > > (trying to at least) If they can slip a bit, that'll relieve some of > > the stress, but if you are on a high traction surface, it'll bind things > > up pretty fast, but it'll loosen up once you get up the ledge. > > This shouldn't be an issue - think of a vehicle travelling inside a ball. The > climbing wheel isn't moving forward but neither is the vehicle - it's going > forward & upward. This is one of the Hummer's claim to fame - it's can climb over > a 24" vertical obstacle as part of it's normal capabilities. Yes, as long as all 4 wheels are climbing, this is true. If just the front wheels are climbing straight up and the rear are still flat, you'll get some drivetrain binding going on (assuming part time 4WD, or locked center diff). Say you are climbing the 24" ledge, straight up. So the front wheels turn 24" as they go up the step. The rear wheels are not going to travel that far, depending on wheel base maybe only a few inches over the same period of time. Once the rear wheels climb the ledge everything is back to normal. Sure it can be done, but just for fun, try climbing a 24" ledge and try to shift out of 4WD once the front wheels crest the ledge. Or even more fun is to watch a truck drop off a ledge and if you lift a front tire, it'll spin backwards. FROM: w.j.markerink@a1.nl (Willem-Jan Markerink) SUBJECT: Re: a 90 deg obstical... DATE: Sat, 27 Oct 01 18:05:46 GMT NEWSGROUPS: rec.autos.4x4 In article <3BB0CE87.B080ECB0@ieee.org>, Roger Brown wrote: >Doing this does put a fairly high load on the drivetrain, since the >wheel climbing up is not moving forward, while the other wheels are >(trying to at least) If they can slip a bit, that'll relieve some of >the stress, but if you are on a high traction surface, it'll bind things >up pretty fast, but it'll loosen up once you get up the ledge. Apart from binding: The forward force would have to be more than 1500kg (or better, 15k Nm) before it would be more than the downward/gravity force in normal conditions. Now, given that the friction coefficient between (rear) tire and road is limited to 1.0, you would have to put more than 1500kg weight on the rear axle before it could excert the same forward force on front axle. At the same time, this 15k Nm forward force, combined with a 1.0 friction coefficient between front tire and road/obstacle, means a lift force of 15k Nm. In other words: to lift weight X in front, you need at least weight X in the rear....:)) Different friction ratio's mess things up of course.... Note that this does not mean that the axle is similary designed to cope with forward forces as it is with downward forces, but the above is a static scenario anyway, the safety factors for dynamic forces are huge....otherwise it would never survive being slammed into the rubber blocks between axle and frame when hitting something at high speed....and at slower speeds, that means an almost equal force in horizontal direction as well. -- Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!] FROM: w.j.markerink@a1.nl (Willem-Jan Markerink) SUBJECT: Re: a 90 deg obstical... DATE: Sat, 27 Oct 01 18:10:17 GMT NEWSGROUPS: rec.autos.4x4 In article <3BB0E3E6.46979E84@ieee.org>, Roger Brown wrote: >Stephen Young wrote: >> > Doing this does put a fairly high load on the drivetrain, since the >> > wheel climbing up is not moving forward, while the other wheels are >> > (trying to at least) If they can slip a bit, that'll relieve some of >> > the stress, but if you are on a high traction surface, it'll bind things >> > up pretty fast, but it'll loosen up once you get up the ledge. >> >> This shouldn't be an issue - think of a vehicle travelling inside a ball. The >> climbing wheel isn't moving forward but neither is the vehicle - it's going >> forward & upward. This is one of the Hummer's claim to fame - it's can climb over >> a 24" vertical obstacle as part of it's normal capabilities. > >Yes, as long as all 4 wheels are climbing, this is true. If just the >front wheels are climbing straight up and the rear are still flat, >you'll get some drivetrain binding going on (assuming part time 4WD, or >locked center diff). But, given my previous calculus, that wouldn't cause more binding 'force-wise' than when the vehicle was turning on the same grippy surface (remember that the rear tires can't rotate much either, until the front tires start to rotate upwards, so even rotation-wise it's not the same as driving a corner!) -- Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]