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Author Topic: 8x8 Discussion (Let's Talk Specs)  (Read 18439 times)

ZeosPantera

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8x8 Discussion (Let's Talk Specs)
« on: July 31, 2012, 02:17:23 am »

Lets use this as a general thread concerning JUST the current demo's and games 8x8.



First I will state that the physics are in no way final and the center of gravity is set to an immensely unrealistically low position. (like a weeble) If it were realistic you would be on your side constantly so assume that this is the Arcade version.

Despite this I have been in the testing version messing with the 8x8's new handling variables and have come up with something I feel best matches my impression of the vehicles climbing/acceleration possibilities.

-Vehicle.cfg---------------------------------------------------------

wheel = {
    suspension_rest_length  = 0.48
    max_suspension_travel   = 0.75
    stiffness               = 6.5
    damping_compression     = 0.10
    damping_relaxation      = 0.03
    friction_slip           = 0.57
}

max_engine_force        = 25000
max_braking_force       = 13750
redux_coef              = 0.2
max_kmh                 = 5

I timed it and it takes roughly 50 seconds to hit 28m/s (AKA 100km\h or 62mph) with an absolute salt flat max speed of 33m/s or 73mph. I have tried to find the "tatra 813's" 0-100km/h times but I don't think it is listed.

« Last Edit: July 31, 2012, 02:20:18 am by ZeosPantera »
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Peca

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #1 on: July 31, 2012, 10:05:36 am »

I have tried to find the "tatra 813's" 0-100km/h times but I don't think it is listed.
I it not listed because max. speed is 85 km/h for Tatra 813 (some say it can go faster, but this is from factory specs).
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cameni

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #2 on: July 31, 2012, 11:13:48 am »

First I will state that the physics are in no way final and the center of gravity is set to an immensely unrealistically low position. (like a weeble) If it were realistic you would be on your side constantly so assume that this is the Arcade version.
I've been trying to adjust the center of gravity but the effect was almost unnoticeable. After some digging around it seems there's an artificial roll prevention force that's strongly counteracting any rolling.

So yes it's arcadeish at the moment, but since the chassis doesn't collide yet, any roll would just make the truck fall under the ground.
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ZeosPantera

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #3 on: July 31, 2012, 11:17:06 am »

It is nice to know that you have at least looked. What aspect of this whole setup would put that erector force on? Is it hidden in the bullet code or something in the properties of the model?

I it not listed because max. speed is 85 km/h for Tatra 813 (some say it can go faster, but this is from factory specs).

I thought it was 100kmh max. I am pretty sure it can reach highway speeds.
« Last Edit: July 31, 2012, 11:19:06 am by ZeosPantera »
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cameni

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #4 on: July 31, 2012, 11:29:48 am »

It's in the vehicle code, I based it off some Bullet example initially and then modified and added to it. I remembered there was a "roll influence factor" mentioned, but that part I didn't modify yet.


I thought it was 100kmh max. I am pretty sure it can reach highway speeds.
Old T813 8x8 KOLOS was slow. Here are some specs (in Czech): http://www.tatra813.ic.cz/data.htm
The maximum speed with overdrive was 80km/h. Screaming like hell at that speed.

T817 (T815-7) can reach 110km/h, 8x8 variant probably less.
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ZeosPantera

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #5 on: July 31, 2012, 04:08:26 pm »

Those maximum speeds will have to be limited with an additional factor. If the engine power is lowered too much it can't climb realistically. Transmission emulation will need to be implemented at some point it seems.

Edit: Also I am not sure if this is a throttle handling bug or physics but I spent about 30 minutes driving the 8x8 across the russian tundra to try and crash it like it was (wasn't crashing) and when I let off the key throttle and alt-tabbed the truck drove for several minutes with only a moderate decrease in speed over the flat-ish terrain. Is their no/very low rolling resistance or is that some sort of keep alive throttle until brake that is implemented?
« Last Edit: July 31, 2012, 04:11:22 pm by ZeosPantera »
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cameni

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #6 on: July 31, 2012, 04:15:40 pm »

No rolling resistance. That's also related to the friction of the transmission I guess (tire + transmission friction).
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Timmo

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #7 on: July 31, 2012, 10:44:55 pm »

As a hack perhaps a general drag/resistance value could be added which summarises trans/tyre/aerodynamic retardation in one?

I'm assuming it will slow down more when travelling up an incline?
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cameni

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #8 on: August 01, 2012, 01:15:27 am »

It should not affect slow speeds as much as the higher ones, when the engine force is low. With the varying transmission you can accelerate until the engine torque cannot overcome the global rolling friction anymore.
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ZeosPantera

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #9 on: August 05, 2012, 01:14:14 pm »

OK. I know that some sort of internal combustion engine model is needed for "other" aspects of the OT engine modding career but I think we should take the Anteworld future vehicle in a different power delivery direction.

There is a prototype Jaquar that has been discussed on top gear and the youtube DRIVE channel that has peaked my interest in hybrids due to its sheer awesomeness. Essentially this Jag will have two small turbine engines in the rear 7"x30" each connected directly to electric generators and also thrusting the exhaust out the rear of the car for propulsion. These generators will spin at HIGH rpm and create a constant build-up of electrical energy for the drive-train-less individual four wheel drive electric motors. Some sort of reserve system of capacitors is likely going to be used to store energy for quick bursts and to make modifying the idle of the two turbines less dramatic and more likely determined by extended use or manual control. (Give me all she's got Mr Scott!)

So now look at this 8x8 from the future we are all driving. Isn't it just BEGGING to have an M1abrams style/sized turbine 16"x50" (that will run on any combustible liquid at nearly any ALTITUDE...) strapped sideways behind the cab churning away at a huge electrical generator. Subsequently feeding either a simple High/Low gearbox placed before the standard tatra drive-train  OR  Four motors/reduction, one for each pair of left right wheels therefore still using the differentials both in-line and side to side but with detachable clutches so that 8 wheel drive can be disabled for road driving economy or all locked for extreme up hill climbidness(new word pat pending)!

The torque curve will be a bit of a guess and this system would require some special coding to handle initial power buildup times and reserve generation/holding but just think of the sound it will make instead of a rattling old diesel.. Plus higher speeds might then be possible with the simpler drivetrian even if just slightly.


As an added gameplay mechanic you could use the parked vehicle as a power source at remote locations.
« Last Edit: August 05, 2012, 01:23:23 pm by ZeosPantera »
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cameni

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #10 on: August 06, 2012, 08:30:17 am »

I like that idea, we knew something had to be done about the truck and the futuristic setting. Since nobody knows what fuel will be available, it's important to have an engine capable of burning almost anything, while being reliable.

Torque can be easy, actually similar to the simplistic model there is now. Electric motors have a neat torque curve. Like this:



Think locomotive :)

We could, for example, assume that there are going to be modern electric motors directly in the wheel hubs, propelling the wheels individually.
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ZeosPantera

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #11 on: August 06, 2012, 02:12:01 pm »

It will also be a good place to showoff any heat haze distortion effect with the exhaust of the turbine out the top of the truck.



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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #12 on: August 26, 2012, 11:02:00 pm »

Without knowing what “redux_coef” does (can guess), the 8x8 likely has lots of torque multiplication through gearbox and differential.

Modeling gearbox (transmission) need not be excessively complex at this point, but a understanding of gear ratios would help allot. With that said, so should be power transmission losses:
- Good manual transmission on passenger car (rear wheel drive) is ~85-88% efficient.
- 4x4 (full time) is 65-70% on a good day.
- 8x8, wouldn’t want to guess, because tire condition (varying diameters in contention) and many other variables come into play.

A simple mathematical equation that factors in gear multiplication (torque) versus speed (m/s or KPH or MPH ) would help. Highly loaded vehicles like 8x8 likely have many gears to keep engine in optimum power band, so  step changes encountered during shift are minimized.

Also a 8x8 should not coast for that long, too many tires engaged, too many fictional losses.

Also, last thing you want is a turbine for a power source. A pig to say the least, especially at low power. The M1 tank has the lowest range of any modern tank. As RPM drops on turbine, so does compression ratio, with that efficiency. Turbines do best being pushed hard and not idling.
 
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ZeosPantera

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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #13 on: August 27, 2012, 12:02:45 am »

Also, last thing you want is a turbine for a power source. A pig to say the least, especially at low power. The M1 tank has the lowest range of any modern tank. As RPM drops on turbine, so does compression ratio, with that efficiency. Turbines do best being pushed hard and not idling.

And that would be the case if we were direct powering the wheels with a transmission and the turbine. In this rendition we are going for a more diesel locomotive setup where the turbine is just there to generate electricity. A storage system whether batteries or capacitors will build up and be tapped for the electric motor driven wheels. Which can be switched on and off as needed. (ie smooth road 30mph) you can probably get away with 2 wheel drive. In a high use scenario like climbing mountains to escape packs of wild cheetah's the turbine would need to be spooled up to replenish the capacitors faster. Using more fuel.
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Re: 8x8 Discussion (Let's Talk Specs)
« Reply #14 on: August 27, 2012, 02:17:05 pm »

First, hope you didn’t take my comment about last thing you want is a turbine personally. It was directed at how poorly turbines do with respect to fuel consumption and how it gets worse the slower you run/turn it.

Now I used the term compression ratio because most people can relate to that. In actuality it is pressure ratio that is correct term, so let’s start with a turboshaft (turbine like in M1 tank) and say it has a pressure ratio of 10:1 at full power, at 50% RPM it will have a pressure ratio ~5:1, this results in a lowering of efficiency  and with that higher fuel consumption.

Now I’m not a huge fan of Wiki but refer to link http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption and scroll down to table.

The aircraft turbo prop is a good example of what I was describing.
-aircraft engine at idle = 2390 g/(kW•h)
-aircraft engine at ground roll = 1270 g/(kW•h)
-aircraft engine @ 0.30 Pmax = 508 g/(kW•h)
-aircraft engine @ 0.70 Pmax = 328 g/(kW•h)
-aircraft engine at Pmax (100% RPM) =294 g/(kW•h)

The reason most people don’t talk about this, is turbomachinery (jet engine, turbo fan, turbo shaft) usually spends most of it’s time at high power levels in which it is working in efficiency sweet spot.

I’m going to switch to standard engineering for now (not metric).
0.70 lb/hp/hr = Turbo shaft (good modern design)
0.39 lb/hp/hr = Stratified combustion (good modern gas design)
0.32 lb/hp/hr = good modern diesel design

So let’s say you are on a flat/smooth surface and it takes 100HP to maintain 62 MPH (100 KPH), turbo shaft will consume 70 pounds of fuel, gas engine 39 pound and diesel 32 pounds in one hour.  So regardless of drivetrain type you are burning twice the fuel (same fuel) for turbine vs. diesel.

Turbines have great power density and are very reliable, but a small application like 8x8 a diesel or gas engine would be more practical. Especially if truck was used as generator for people when not on the move (this is what several militaries have as a future goal).

Not sure if you want to get into drivetrain conversation, but there are also losses when going from generator-to-battery-to-electric motor. 

 :)
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