Pros and cons of 8*8 against 6*6 wheeled vehicles

[morris9791]

Dear Experts,

Does any know of a source to get info regarding the advantages/disadvantages regarding the ride/handling, weight transfer comparison of 6*6 against 8*8 etc?

Thanks
eddie

 

[patprimmer]

Well for one you hit the same bump 4 times instead of 3 times or maybe even 8 times instead of 6 if it's a wide bump at n oblique angle, however each bump might be about 25% softer or 33% harder depending on spring rates and axle mountings.

Front end alignment will be a lot more complex especially if you want to maintain Akerman on all 4 steering wheels, that is presuming 4 wheel steer which was not actually specified.

It might turn in better or under steer less if high speed handling s a consideration on a military type off road type vehicle if that is what it is which was also not specified.

Regards
Pat
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[morris9791]

patprimmer

I am trying to get a picture of how a 6*6 compares to an 8*8 in terms of ‘fatigue loading’ of the suspension components.
Question: Would the 6*6 experience more fatigue loading if each wheel station on both 6*6 and 8*8 has the same sprung mass? I am asking this from a purely ‘geometric configuration’ point of view, if that makes any sense?

I am guessing that the 6*6 would be more susceptible to squatting and diving due to it CoG being in close proximity to the central axle. So from this I am visualising a rocking motion about this point as the vehicle accelerates and decelerates.

For the 8*8, I think the effects of squatting or diving would not be as significant. Although I am not sure.

I am surprise I couldn’t find and articles relating the performances between 6*6 and 8*8 heavy vehicles in terms of ride and handling.

Any thoughts on this? thanks

 

[morris9791]

patprimmer

Just to add...In addition to the above, the 6*6 has a higher CoG and shorter wheel base relative to 8*8

 

[patprimmer]

So your question is how will two quite different vehicles react and even though there are several significant variables can we pin the difference on one variable.

Maybe I am hard nosed, but it seems a bit absurd to try to compare unless you tie down the variables or just compare vehicle to vehicle and don't try to attribute all the differences to only one cause.

Regards
Pat
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[morris9791]

ok,

Its sounds like an unreasonable question. I was just trying to get an idea of differences etc. Best option would probably compare the 2 vehicles using dynamic models.

 

[morris9791]

All,

I’m currently in the process of building a dynamic model of a 6*6 and an 8*8 both of which have different axle rating, springs, dampers, CoGs and wheel travel.

I would like to extract some useful information/characteristics to compare the wheel station of both for a given road profile like a speed bump, pothole.

I am interested in magnitude for forces on the suspension such as wishbone pivots and particularly king pin forces.

For example my first thought is to normalise the king pin force against its wheel force so that a direct comparison can be made between the vehicles. Other ideas include finding the energy absorbed by the wheel station by integrating the force displacement curve and comparing.

Does anyone have any other useful characteristics that could help to demonstrate differences between a 6*6 and 8*8?

thanks

 

[GregLocock]

For overviews like this we mainly look at the peak g levels, ie the peak force seen in a component divided by the static weight on that wheel.

Unless you have spent a million dollars or a couple of years your pothole forces will be indicative at best. A step(kerb impact) is easier to model.

Frankly I think the most likely outcome is that the leading axle gets most of the damage, and the other axles are along for the ride.




Cheers

Greg Locock


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[morris9791]

Greg

I currently have modelled a step as one type of manoeuvre.

I was also considering a 200mm high 7m sine wave to see the effects of weight transfer of both vehicles as it traverses over the course.
What you think?

 

[sbozy25]

Build the vehicles, ride in vehicle, hit 10" half rounds at 30mph... you will have your answer when done!

 

[GregLocock]

sbozy - that is exactly the poiint. building a decent proto costs 300k+, doing this stuff up front costs 50k.

morris- yes once you hav a model throw it at lal sorts of raod surfaces, when you find a significant difference between the 6 and the 8 wheeler investigate that and see if it is reasonable.

We have something 15 different road profiles and 5 different sorts of potholes to play with.

Cheers

Greg Locock


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[morris9791]

OK,

thanks guys.

Greg, are these road profiles and potholes based on some standard?

Thanks

 

[GregLocock]

No, they are proprietary, although there are some Asutralian standards for kerbstrike I think.

Cheers

Greg Locock


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[patprimmer]

Greg

I am not quite sure where Asutralian is. is it anywhere near either Australia or Austria. ;-)

Regards
Pat
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[dfoxengr]

there are some road profiles and of course half rounds that are US Mil standards. Look for aberdeen and NATC documents.

Aside from the model, look at some video of 6x6 and 8x8 mil vehicles riding on courses if you can find any. The 8x8's that I have seen basically just glise all over the courses since they have a more stable platform. think of a centipede and how it's many legs make the load supported by each less. you can have softer ride sprigns on the 8x8 (assuming same mass like you mentioned) and if you have enough wheel travel then IMO the 8x8 is going to be a better choice for ride.

if i remember correctly, the front wheels of some multi (>2) axled vehicles sometimes are more highly damped due to them being the one that hit an obsticle first. this also depends on weight distribution and spring rates though.

what simulation program are you using?

 

[1MarcoD]

One issue with 6x6 -v- 8x8 is that, in order to evenly distribute the axle loads, CG has to be towards the rear, i.e closer to the rear bogie. This forces the CG height up relative to an 8x8 where the CG is pretty much in the middle and can be lower down because payload can fit in the gap between the wheel wells/suspension spring mounts. That's my experience with fire trucks and military vehicles. Not sure what vehicle you're looking at.

 

[nobog]

Go find an old (and I mean old) snowmobile that had bogie wheels for a rear suspension and gang the bogies in any amount you like. Put some weight on it and run it over a proportion "bump" (or bumps) and video the entire process. If nothing else it would be cheap!