BRP Vehicle Project - Cars

[Atgxtg]

I7ve been working on Vehicle Design rules for BRP. I worked up a few formulas for calculating Move to the and kph speed, and thought I'd got in the works to see how it looks to everyone. Here is a sample from my spreadsheet of test vehicles.

It doesn't match perfectly for every vehicle, but considering all the things I'm m ignoring the results seem fairly decent. With a optional rule to account for streamlining and era/tech level, and the Model-T will work out alright. The AC Cobra is still way off, though.

The basic formula for automobiles is Move= square root (kW/metric tons) * 2, although I think I need to adjust it a bit to reduce the effect of mass/weight.

So far, it looks like, as far a cars go, STR corresponding to the weight/mass

of about 65kgxkW (metric)i , or about 108poundsxHp (English).

For example a Toyota Corrolla GT @119 hp would have a 62 STR (65kgx89kw = 5.785kg, or 108lbx119hp=12852 pounds).

How does this look to everyone else?

[rust]

It looks very good to me.

I will use your fomula for the next setting where ground vehicles are

possible ... right now the only ones in my campaign are crab-like wal-

kers on the sea floor.

[Atgxtg]

It looks very good to me.

I will use your fomula for the next setting where ground vehicles are

possible ... right now the only ones in my campaign are crab-like wal-

kers on the sea floor.

Man, you always come up with neat test cases.

Roughly, I think K halves for walkers, and about 1/8 or so for underwater, for about K=1.32.

But, as you know, I got a different formula in the works, that should handle water craft better.The current one doesn't hold (up to) water.

[rust]

Roughly, I think K halves for walkers, and about 1/8 or so for underwater, for about K=1.32.

Unfortunately I cannot test this formula, my setting's "Partan Walkers" only

have a description, but no stats. GURPS Vehicles did not work well for their

design, so I keep waiting for the relevant Mongoose Traveller supplement in

this case, too.

So, until July these walkers just keep walking - without power plant output,

mass and speed.

[Atgxtg]

Unfortunately I cannot test this formula, my setting's "Partan Walkers" only

have a description, but no stats. GURPS Vehicles did not work well for their

design, so I keep waiting for the relevant Mongoose Traveller supplement in

this case, too.

So, until July these walkers just keep walking - without power plant output,

mass and speed.

If you can give me some basic info, I might be able to get you some workable game data. I think the stuff I have for walking vehicles might work.

How big are your "partan Walkers", how advanced are they (TECH), how deep can they go, and what other things do they have to power? Not to mention just what would you like them to be able to do.

Stuff like that is kinda what I hoping the rules will help people build/stat out for BRP.

[rust]

Thank you very much for your offer.

The Partan Walkers are built at a low science fiction technology level. They

are civilian vehicles used mainly for sea floor construction and similar tasks.

They have a crew of 2 (pilot and marine engineer), their size is ca. 7 m long,

almost 4 m high and about 10 m wide, looking somewhat like a mechanical

crab. They have a weight of nearly 20 tons and can move at a top speed of

ca. 20 kph, with the average speed closer to 10 kph. The maximum safe

depth is 400 m.

The walkers are powered by radiothermal generators, but I have not yet cal-

culated their power requirements, mainly because I found it difficult to deci-

de which amount of the power they might need for the modular construction

equipment (power drill, power shovel, etc.).

[vagabond]

It doesn't match perfectly for every vehicle, but considering all the things I'm m ignoring the results seem fairly decent. With a optional rule to account for streamlining and era/tech level, and the Model-T will work out alright. The AC Cobra is still way off, though.

The basic formula for automobiles is Move= square root (kW/metric tons) * 2, although I think I need to adjust it a bit to reduce the effect of mass/weight.

The Cobra is way off due aerodynamics. The Cobra's shape is what limited

it's top speed. The Cobra Daytona was an attempt to use the same basic

internals as the Cobra, but give it a sleeker shape. The result was a top

speed of over 180 MPH. Even a mock up using a completely untouched

Cobra chassis, engine and transmission, but dropping a test Daytona body

on it yielded +25 MPH. The Morgan Super Aero would also have been

limited by aerodynamics (and possibly gearing - see below).

Your Dodge Viper numbers are off because it looks like you forgot to convert

top speed from MPH to KPH. The Viper can go much faster than 185 KPH (114

MPH). It should be 297 KPH which brings you to 94.91% accuracy, assuming

185 MPH is correct (it is actually over 192 MPH top speed). Actually, some

of your numbers don't look correct. The Ferrari 512 BB is listed as going

290 - 300 KPH in most sources I have read.

Another big factor is final drive ratio. Even though, in theory, a car may have

the power and aerodynamics to hit a certain speed, often the gear ratio

will not allow it.

Cars are a big part of my life, and I have done considerable research in how

they are put together and work. I hope to build a replica of the famed Ford

GT40 MkIb in the near future, from the beginnings of a tube frame.

-V

[Ian Absentia]

Excellent work! This looks very promising. All I'd need in addition to this is the boats & ships version of the calculation. Okay, and maybe helicopters.

!i!

[Atgxtg]

Vagabond tanks for the input.

The Cobra is way off due aerodynamics. The Cobra's shape is what limited

it's top speed. The Cobra Daytona was an attempt to use the same basic

internals as the Cobra, but give it a sleeker shape. The result was a top

speed of over 180 MPH. Even a mock up using a completely untouched

Cobra chassis, engine and transmission, but dropping a test Daytona body

on it yielded +25 MPH. The Morgan Super Aero would also have been

limited by aerodynamics (and possibly gearing - see below).

Yeah, that is part of it. I am still working on the Coefficient of drag stuff. Basically I am factoring it in, but don't want people to need an engineering degree to design cars for an RPG. As it stands the "basic" car is worked out a with about a 0.33 coefficient of drag, and I7ll had an optional text box with adjustments. After all, it isn't like we can do wind tunnel tests of out imaginary designs anyway.

Another part of it was my source. Te source I had listed the top speed as 225kph. Another, probably more reliable, source listed the top speed at 165mph (265kph) which cuts my margin of error in half for that car, and is reasonably close to the 15%-20% margin of error that I consider an acceptable cut off point. Factoring in for a CoD of about .44 or so and this thing is starting to get close enough for my purposes.

It was the CoD that is throwing off the Tatra, too. I picked it because one book I was using as a soruce had it listed, and it was a neat early design. A three wheeler and from the 20s. What I wasn't aware of, at the time, was that it had one of the lowest coefficients of drag in any production car. A CoD of 0.212 it incredible for a modern car, and unbelievable for a 1920s car. If the designer were still alive, he'd probably get some sort of honorary award.

Your Dodge Viper numbers are off because it looks like you forgot to convert

top speed from MPH to KPH. The Viper can go much faster than 185 KPH (114

MPH). It should be 297 KPH which brings you to 94.91% accuracy, assuming

185 MPH is correct (it is actually over 192 MPH top speed). Actually, some

of your numbers don't look correct. The Ferrari 512 BB is listed as going

290 - 300 KPH in most sources I have read.

Yeah, that is exactly what I did (or didn't do). I corrected that on the table and it is looking closer. A few of the entries have "improved" as I got more reliable data.

Another factor is my formula. Basically I'm use the square rot of the power/mass ratio, and it looks like I should be using a square root/4th root relationship. Since the mass for cars is low in terms of metric tons, the margin of error is low. I am trying the other formula and so far it is a little more accurate for most cars-including the Morgan.

For hp to speed it looks like a cube root function is the correct one, but the values won't work without factoring in a lot more details.

Another big factor is final drive ratio. Even though, in theory, a car may have

the power and aerodynamics to hit a certain speed, often the gear ratio

will not allow it.

Yup. Fortunately, for my purposes I can ignore a lot of the real world engineering aspects. I only want something that will give good "ballpark" figured so peoplec an use the formulas to design non-existent vehicles. For real vehicles, I am simply going to use the real data. But I needed real world examples to test my formulas and see if they are good enough to use.

One benefit of this will be that when the rules are finished, I should have a table with stats for a bunch of real vehicles. A hundred vehicles that I design are a hundred vehicles that other people won't have to design. And 100 "benchmarks" to use to analogs for other cars. In BRP Terms, the differences between a Aston MArtin DB5 and DB6 are fairly minor.

Cars are a big part of my life, and I have done considerable research in how

they are put together and work. I hope to build a replica of the famed Ford

GT40 MkIb in the near future, from the beginnings of a tube frame.

-V

Cool, does that mean I can "draft" you and take advantage of your real experience to help with the R&D? I can give you some of my formulas to look at.

P.S.> Ironically, I don't even drive. I just like to design things.

[Atgxtg]

Excellent work! This looks very promising. All I'd need in addition to this is the boats & ships version of the calculation. Okay, and maybe helicopters.

!i!

I'm testing boat& ships now. I got something that seems to work, but is a bit more complicated, since I had to use a square root/4th root function and adjust the drag/resistance factors for different hull types.

Thanks to rusts, off the wall choices for test cases, I think we might have it working for hydrofoils and submarines too.

Piston driven aircraft seem to be working out okay, and I suspect rotary wing aircraft (helicopters) are probably going to be alright (I'll check a few designs on the spreadsheet).

Jet aircraft are proving somewhat more problematic. But then, if were really this easy as my formula makes it, people wouldn't need to go to school and get engineering degrees.

Maybe I should make the formulas available so people can try them out?

[vagabond]

Sure, feel free to bounce ideas off me.

Three wheelers are primarily motorcylces with a sleek fuselage , so the Cd is

not far fetched at all - look at the Morgan Super Aero It actually hit a top

speed of 115 MPH (185 KPH). And that brings your accuracy to 92.97%.

It seems you are off to a solid start. Some other points to consider - modern

cars will most likely have some sort of electronic speed limiter/governor. So,

when you calculate things out, that may be the cause of some discrepancies.

You could always modify the max speed to equate to the given printed specs,

and place an asterisk stating "Electronically Governed" or similar.

Here is a great online tool/site to calculate various auto performance numbers:

Car Performance Calculator

You'll note - you should actually use Torque to derive speed. I'll

see if I can find a decent formula, most of the ones I have seen

fall a bit short.

-V

[Atgxtg]

Thank you very much for your offer.

What's to thank? I need test cases like this to build my design system. Consider it a symbiotic process.

Partan Walker Design Data Prototype 1.0

STR 78 (347kW)

SIZ 77 (19.7mt)

Move 23 (20.7 kph/11 knots)

AP 24

Design Info:

Move: Was calculated using the same formula I used for the Swiftsure-class submarine. This might be a bit generous if the walker is less streamlined than a sub, but then it might have a better propulsion system. To get 20kph, 11 kts, I got a power requirement of 12kW.

STR: Was calculated by converting the kw into a STR score using the SIZ table, with 1kW = 65kg. So 12kW=780kg for STR 39.

Based on previous experience using the Mecha design in the BRP core book, I assume that the final STR is about twice the STR rating needed for motive force. 2x39= STR 78. Using the kg/65 rule and keeping the same proportion in the range, I got 347kW. Leaving you 335kW left over to power all the other junk. So roughly you got a 462 hp engine, equivalent to the Barbus GLK V8 auto,save that is is some SCi-FI high tech version, and thus probably a bit smaller, with a mass of say, half a metric ton.

SIZ: Assuming you meant 20 long tons (measure used for naval vessels) that works out to 19.7 metric tons or so. Looking on my expanded SIZ chart, I got SIZ 77. Nice since STR is 78 and the two values tend to be close to each other for creatures in BRP.

AP: Calculated by the safe depth, number of atmospheres, using the natural log of the safe depth (in meters) x4 AP. I think that is what I used in my Quick Vehicle Write-Up rules.

That's about as far as I can go with the data given and my notes.

Is it in the right ballpark/aquarium?

[rust]

Is it in the right ballpark/aquarium?

Thank you very much, it fits perfectly - especially because it gives the

walker lots of "surplus" power for heavy construction equipment and the-

like.

[Atgxtg]

Thank you very much, it fits perfectly - especially because it gives the

walker lots of "surplus" power for heavy construction equipment and the-

like.

Thank Jason,

What happened was that when trying to "reverse engineer" the Mecha from the BRP core rulebook, I had a hard time keeping the MOVE down to 15, with a STR 100 engine, about 1400kW with my design rules. Even with legs.

Then I though of all the other stuff that the mech had to power, like arms and a laser cannon. Since the 15 Move worked out with around a 46-50 STR, I decided that only about half the STR was used to power the design. So the big mech only uses about 50 STR (31kw) for movement and the other 1470kw for the other goodies.

I just designed the Partan Walkers along the same parameters. If only the sub formula holds up.

One thing I'm kinda please about is that their 9D6 damage bonus lets them get past each other's AP, usually.

[Atgxtg]

Here is a great online tool/site to calculate various auto performance numbers:

Car Performance Calculator

You'll note - you should actually use Torque to derive speed. I'll

see if I can find a decent formula, most of the ones I have seen

fall a bit short.

-V

How short? Since I'm going for about a 15% margin of error and a easy formula, maybe you have something better than what I'm using.

[vagabond]

How short? Since I'm going for about a 15% margin of error and a easy formula, maybe you have something better than what I'm using.

I put in the numbers for a car (450 HP, 2100 lbs) and they gave a top

speed of ~140 mph. Using the actual top speed (205 mph) and

weight (2100 lbs), they returned over 1000 HP. So, not very accurate.

I found one good possibility, but I have to solve it for velocity.

-V

[Atgxtg]

I put in the numbers for a car (450 HP, 2100 lbs) and they gave a top

speed of ~140 mph. Using the actual top speed (205 mph) and

weight (2100 lbs), they returned over 1000 HP. So, not very accurate.

I found one good possibility, but I have to solve it for velocity.

-V

Let me check my formula here for the "mystery car".

450hp is about 337.5 kW

2100 lbas is about 0.95 metric tons

using square root (Power/Mass)*21, I get a Move of 396.

kph is about 90% of Move or 356kph (221mph).

With an actual top speed of 205mph the formula is only about 8% too high.

If the "mystery" car happened to be a Factory Five replica of a Shelby Cobra, with a drag coefficient of around 0.45 I think it would work out to a speed of about 327kph/203mph.

[vagabond]

Let me check my formula here for the "mystery car".

450hp is about 337.5 kW

2100 lbas is about 0.95 metric tons

using square root (Power/Mass)*21, I get a Move of 396.

kph is about 90% of Move or 356kph (221mph).

With an actual top speed of 205mph the formula is only about 8% too high.

If the "mystery" car happened to be a Factory Five replica of a Shelby Cobra, with a drag coefficient of around 0.45 I think it would work out to a speed of about 327kph/203mph.

The numbers are from an approximation of the 1968/69 Gulf Wyer GT40

that won 24H Le Mans. Cd is 0.39.

After some research, when talking absolute top speed, wind resistance

and drag is actually more important than weight. I have some interesting

formulas that seem OK, but they are probably more complex than you want.

-V

[Atgxtg]

The numbers are from an approximation of the 1968/69 Gulf Wyer GT40

that won 24H Le Mans. Cd is 0.39.

Ah-ha. So that's the beast. With that Cd I think it would probably come out with a speed in the 210-215mph range by my formula. A bit high, but not too bad considering.

After some research, when talking absolute top speed, wind resistance

and drag is actually more important than weight. I have some interesting

formulas that seem OK, but they are probably more complex than you want.

-V

Yeah, I've been finding out that the drag force is the major component too, and that most of the energy applied by a car is just to overcome drag.

As drag is based on shape and frontal area, and both of these are indirectly tied to mass (larger area, shape typically means a heavier vehicle with more inertia) the mass value kinda factors this in.

Send my any formulas you think are interesting. I can always eliminate or standardize some of the variable factors to get some approximate figures. THat is what I have done with the formulas I use now.

For BRP purposes, a couple of kph don7t matter much. Especially as the values increase. By the time you scale things to keep the cars on the "battlemap" differences less than 20% or so don't matter much. A car going Move 396/225mph is probably going about 9 or 10 "spaces" a turn if you want to keep it on the map for a couple of turns.

[vagabond]

One of the better formulas is:

V = {[(P - R)(76716)]/[(Cd)(A)]} ^ (1/3)

Where:

V = velocity in kph

P = power in Kw

R = rolling resistance - you can use 6 - 8 Kw

Cd = coefficient of drag

A = frontal area in sq m

and the constant 76716 is a conversion factor accounting for air

density at sea level and converting from m/s to Km/h

But it gives about the same number as your formula.

Oh, and apparently, the GT40 had a Cd of 0.43. So, your

formula might be better. And, you can drop HP to 425 -

sources vary on race trim.

-V

[Atgxtg]

Thanks for the formula, I'll try it out on a spreadsheet, and see how it compares to what I'm using.

I'm surprised at how well my simple formula is holding up for cars and other ground vehicles.

[Atgxtg]

This table gives a correlation between STR and horsepower/kilowatts, by way of the SIZ table.t It can be useful is determining just how "strong" a vehicle is in game terms, as well as how much power a human, horse (or other creature) can produce for a machine (like a bicycle, galley,or even Fred Flintstone's car.). The conversion factor is that every kW equals 65kg on the STR table, and that every hp equals 107 pounds on the STR table.

For vehicles just look at the engine's power rating and read across to find the STR.

Example: Mister Muscles, is attempting to stop some bank robbers from escaping in their getaway car. Mr. Muscles has a STR of 70, while the Sedan used by the robbers is powered by a 120 hp engine. Looking up on the chart, we see that 120hp is around STR 62. So ignoring little things like mass and inertia (like in the comics!), Mr. Muscles would match his 70 STR vs. the car's 62 to get a 90% chance of stopping it.

For creatures, assume that the power is peak power than can only be sustained for the short time (costs fatigue points), and use 1/4th STR to determine the sustainable effort.

Example: A character with STR 12 is manning the oars on a rowboat. He would be able to sustain a STR of 3 (about 0.33hp or .25kW) over a long term, but could exert himself to STR 12 ( about 1.5hp/1.13kW) for short bursts.

If Mr. Muscles happened to be in the boat, he could work the oars at STR 18 or about 2.5hp/1.9kW aafor hours, or at STR 70 for brief periods, producing the same output as a 225hp/170kW motor!

[soltakss]

So, an average horse (STR 28-29) would produce between 5 and 6 horse power! Ah, just seen the 1/.4 STR rule, so a STR 28 horse uses STR 7 over a period of time, relating to 1 horse power. Clearly, there are no flies on you.

It looks like a pretty good reference table. I like the simple way of converting between STR and HP when stopping vehicles.

How would you figure teams of animals? What STR would a team of four horses (STR 28 each) use?

Looking at the two tables, it would be very useful to include STR and SIZ in the Vehicle Table. But, thinking about it, would you use STR or STR/4? Maybe using STR/4 as the base STR but STR for acceleration purposes, might be a good way of doing it.

Certainly, what I'd like to see is a set of tables that relate Characteristics (STR/SIZ) to objective measures (Horse Power/Mass) and also a set of tables giving sample stats for vehicles with both Characteristics and Objective Measures. That way, I could use the vehicle table to work out similar stats or sample stats or I could use the background tables to work out stats for new vehicles.

[Atgxtg]

So, an average horse (STR 28-29) would produce between 5 and 6 horse power! Ah, just seen the 1/.4 STR rule, so a STR 28 horse uses STR 7 over a period of time, relating to 1 horse power. Clearly, there are no flies on you.

One of the ironies about horsepower is that 1 horse actually produces less than 1 horsepower over the long term. Look up the history of the term and you will see just how "accurate" the term is.

It looks like a pretty good reference table. I like the simple way of converting between STR and HP when stopping vehicles.

How would you figure teams of animals? What STR would a team of four horses (STR 28 each) use?

The same way I'd add weights on the STR table. Up to around SIZ 88 or so x2 weight (or in this case hp/kW) is +8 STR, so for teams:

2 horses = +8 STR, for STR 36 (Peak: 12hp/9kW), STR 9 (Long term: 1.2hp/0.9kW)

4 horses = +16 STR, for STR 44 (24hp/18kW ), STR 11 (Long term: 2.4hp, 1.8kW)

8 horses = +24 STR for STR 52 (48hp/36kW), STR 13 (Long Term: 4.8hp/3.6kW)

Or you could just multiply the hp for the average STR of the horses by the number of horses to get the value. For example, if the average horse of the team is STR 28 (6hp/4.5kW peak output) and counts as STR 7 for long term (0.6hp/0.45kW) then you can just mutliply the output by the number of horses to get a total (6x4-24hp/18kW) and look up the STR on the table (44).

Another thing you could do is just add up the weight allowed due to the STR of the various animals and then look up to total on the STR table and use the power table to convert that into hp/kW. Since the are all interrelated, the numbers should come out the same no matter how you do it.

Looking at the two tables, it would be very useful to include STR and SIZ in the Vehicle Table. But, thinking about it, would you use STR or STR/4? Maybe using STR/4 as the base STR but STR for acceleration purposes, might be a good way of doing it.

One of the reasons why I worked on the SIZ table was to be able to do just that. The vehicle rules I'm working on will give you a SIZ score for your vehicle based on the weight/mass. I got a design spreadheet that does a lookup to give these values. When it is done, someone should be able to plug in some numbers for power, weight, and (optionally) coefficient of drag, and lateral acceleration and the sheet will spit out the game data.

For car STR I'm using full STR based on horsepower/kW. here are my reasons for doing it that way.

-A machine can maintain a higher constant output than an animal, for instance a car with a 120hp engine might not be using the full 120hp while crusing on the highway, but it is probably using 90hp (about 75% output).

-It it is much easier to push a machine (just step on the accelerator). than say a human.

-I should really be using torque rather than horsepower for STR, and torque is actually more constant than horsepower. It is just that hp is easier to find, and allows me to work out top speeds without knowing a car's gear ratios.

-Acceleration is also a function of torque, and would be more constant if cars didn't have to change gears.

Now what does change is momentum. A car at speed would be harder to stop because it has built up a lot of momentum. Technically that would be factored on SIZ and speed rather than STR+SIZ, but that is probably going too far for BRP. Realstically, MR. Muscles should be knocked away no matter how high his STR, since he can't really apply it. But for Superheroes, we ignore all that.

Certainly, what I'd like to see is a set of tables that relate Characteristics (STR/SIZ) to objective measures (Horse Power/Mass) and also a set of tables giving sample stats for vehicles with both Characteristics and Objective Measures. That way, I could use the vehicle table to work out similar stats or sample stats or I could use the background tables to work out stats for new vehicles.

That is exactly what I7m trying to do. By basing game stats on real world data, it should make it easier to handle interaction between characters and machines, as well as use the sample stats as benchmarks for writing up other vehicles and keeping it all consistent. That way people can port stuff over from one campaign to another without worrying about compatibility (in the past it was often problem when taking something from one RQ/BRP based RPG to another) .

The design rules should also help people to create new vehicles that do not exist in the real world, by plugging in the same types of numbers used for real vehicles. So if a PC designs a car with power, weight, Cd, and skidpad ratings similar to those of a Shelby Cobra, he will end up with game stats similar to a Cobra's.

Another neat things people can do with it are to modify vehicles to make their own custom vehicles. Since STR and hp now have a correlation, a character in a Supers setting could spend points to improve a car's STR score, and work out the increase to power and other stats.

One perk, to this project is that in order to test out my design and conversion rules, I needed to try it out on some vehicles. So when it is done, there should be a couple of hundred vehicles with game stats ready for use in BRP. The more vehicles that are written up, the greater the chance that a GM/player can find what they are looking for, or something close to use to "eyeball" stats for other vehicles. In other words, the better I make the design rules, the less often you'll need to use them. :eek:

Since I'm trying to cover all sorts of vehicles, including ground vehicles, aircraft, watercraft and spacecraft, and that I want to try to factor in for differences in technology and SF settings, it is an ambitious project.

The good news is- I think I'm getting really close to having the rules for cars and trucks ready. I should be able to put something up for people to try out soon.

[Atgxtg]

Here's a sampler of some cars for BRP. Most of the values work worked out by formulas in the spreadsheet with only some minor tweaking to account for technological progress (or lack thereof) and lack of data (corning ability for early 20th century autos). Cars are sorted into three eras, Edwardian, Vintage, and "Modern.Let me know how the stats look.

I've got over 300 cars and trucks in the spreadsheet and my goal is to have stats for 1000 cars for the vehicle rules. Both to test the accuracy of formulas and to give people a lot more options when selecting a vehicle for RPG purposes.