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Learning how to build Lotus Seven replicas...together!
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 Post subject: Re: Power to Weight
PostPosted: October 9, 2018, 11:39 pm 
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Location: White Rock, BC, Canada
geek49203 wrote:
There are lies, damned lies and HP numbers.

Having said that... I wonder if the weight thing doesn't "matter more" than the HP thing, especially at different points in the curve? For example... adding 500 hp to a locost that already has 200 would produce a wow number of roughly 2 hp / pound. Not sure that the last 500 is at all usable tho, right? Burning off back tires while pushing thru hopeless Seven aero?

Now, if you take a 1400 pound 200 hp locost, cut 400 pounds off of the thing, you'd have 5 pounds per hp, but it would all be usable. So at that point in the graph, it would mean more than the prior example?

And as wiser voices have said, the driving differences would be vast.


I wonder if there is a magic gradient of power/weight/acceleration. Meaning if you want a 10% increase in acceleration you need a X% increase in weight (for stronger parts, more tire etc), and Y% increase in HP (with all other factors staying the same). You look at the top engineered "street" cars currently and they seem to float around 2800lbs and 900-1100hp with gigantic heavy tires.


TurboBird, when you say "with a torquey V6 and it will break the rear end loose in the first 3 gears, even with sticky tires (Toyo R888's)" Do you mean straight line or in corners? I've got a bit more power/torque than you and in a straight line I can be full throttle 1/2way through 2nd. I do have some anti-squat though. Maybe you're geared lower?

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 Post subject: Re: Power to Weight
PostPosted: October 10, 2018, 7:51 am 
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Joined: July 17, 2008, 9:11 am
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Location: West Chicago,IL
geek49203 wrote:
There are lies, damned lies and HP numbers.

Having said that... I wonder if the weight thing doesn't "matter more" than the HP thing, especially at different points in the curve? For example... adding 500 hp to a locost that already has 200 would produce a wow number of roughly 2 hp / pound. Not sure that the last 500 is at all usable tho, right? Burning off back tires while pushing thru hopeless Seven aero?

Now, if you take a 1400 pound 200 hp locost, cut 400 pounds off of the thing, you'd have 5 pounds per hp, but it would all be usable. So at that point in the graph, it would mean more than the prior example?

And as wiser voices have said, the driving differences would be vast.


(very) Minor point, you would have 0.5 HP/ pound. Oft we talk about "power to weight" when we really mean "weight to power" You got it right in the next paragraph though. :cheers:

In a straight line, as long as the tires are not slipping, all HP (actually torque, but I won't go there since they are mathematically related) is usable.

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 Post subject: Re: Power to Weight
PostPosted: October 10, 2018, 10:05 am 
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Joined: October 24, 2008, 2:13 pm
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Location: Carlsbad, California, USA
Jeimuzu wrote:
I think it would be interesting and possibly helpful in some situations. If it's not feasible, or people don't want to do it, or it doesn't matter, no big deal. It was just an idea I had.


What you originally proposed would be interesting for future builders. For most of us, we won't have the access to the analytical tools necessary to do an accurate job. I'm thinking of tools like a dyno, accurate scales, etc., etc.

There are some seat-of-the-pants kinds of things that can be used to formulate an opinion of what you'd like your personal build to be like. It's non-scientific, but if you start looking at YouTube videos of 7-like cars (including the modern Caterhams) you'll see that manageable performance of a street version of a sevenesque car requires some restraint with respect to horsepower and torque.

After watching as many road test and driving/track videos as I could find, I concluded that about 300 HP was about the maximum you could have and still have a manageable/drive-able street car. You can have considerably less than that and still have a thrill ride, in my opinion. That will be true if your Locost weighs 1300 pounds or 1500 pounds, again, in my opinion.

Horsepower is a thing you can add later through engine modifications should you decide you need to do so.

Cheers,

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 Post subject: Re: Power to Weight
PostPosted: October 10, 2018, 11:39 am 
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Power to weight of other people's cars only means anything if you have additional metrics that correlate to it. So even if there were a list of Locost power to weight ratios, it wouldn't tell you anything without knowing how each of those cars drives/performs. For other people's Locosts I think hp is one of the last stats I'd care to know, but the idea of a list of completed car stats/measurements, including wet weight (preferably with weight distribution/corner weights) with and without driver, would be something that I would see as rather useful for others to extrapolate design and decision making information from.

That is not to say I don't find value in power to weight ratios for the design and decision making process. It just needs to have some other known attributes to correlate to, which is most easily found in production cars. I have two different power to weight ratio spreadsheets.

One took a series of stock production cars of varying (crank) power to weight ratios and 1/4 mile times, and plotted them against each other. An exponential line of best fit (R^2=.9999) with equation (1/4 mile = 6.4967 * [lb/hp]^0.3016) was run and it seems to give a reasonable (read:surprisingly accurate) expectation of what ballpark 1/4 mile times might be achievable with different weight and power levels.

The other used the NASA Super Touring 'adjusted power to weight ratio' performance equivalence formula, that has been developed and refined to try to equalize the track performance of cars with wildly varying characteristics. I then plugged in a bunch of stock production sports cars, and can compare whatever I might want to build (production based or not) against that, to get some reasonable ballpark estimate of what types of cars it might generically be competitive with on a road course.

Both factor in an assumed 200lb driver weight. Obviously neither is by any stretch of the imagination going to provide perfectly accurate results, as there are far too many variables and assumptions involved, but at least provide some type of baseline to compare against for what the power to weight ratio might actually mean in whatever I might be considering.

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 Post subject: Re: Power to Weight
PostPosted: October 10, 2018, 2:30 pm 
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Joined: June 24, 2011, 4:42 pm
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Location: Boise, ID
That's pretty much what I was thinking. Not just how much power and how much weight, but the "database" could even have rear suspension style, front suspension style, anything really. There could be a column for the owner's to comment on how it drives.

Maybe it's just me, and I definitely don't mind, but having to go through ALL the different build threads to pick apart information for different systems is a huge undertaking. Especially when someone finds out something won't work 15 pages later. A post-build compilation of specs and different approaches I think would be super helpful to people still in the planning stages.

For example, there's two ways I think I'd like to go about this.
1) A turbo Miata based car
2) Foxbody Mustang based car

In either case I plan to make one of those a semi-track toy for a while, and once the frame is built, transfer everything I can. Questions like "How does a 250whp Miata based car that weighs 1200lbs compare to a 350whp Mustang car that weighs 1700lbs?" are what gave me the idea for a database in the first place. Yes, it won't be scientifically accurate to lab standards, but even seat-of-the-pants info would still be useful and interesting.


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 Post subject: Re: Power to Weight
PostPosted: October 15, 2018, 10:57 am 
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Joined: July 17, 2006, 3:09 pm
Posts: 273
Location: Holly, MI
You mention "track"in your response. I'm going to assume you are asking how these different types of builds perform in a sporting environment. Here's what I can tell you:

1. The driver means more than anything else when trying to compare cars. If you want to compare cars, you need the same driver. And, a competent driver at that because it takes a while to get comfortable with a new car.

2. Low weight makes for a great car, but a heavier car with more power and tire makes for a faster lap. I've been on track with a number of lower HP cars that weigh a couple hundred pounds less than mine and they simply can't hang on the long straights.

3. Higher HP cars cost more. Engines, clutches, tires, transmissions, and brakes are all more expensive and need to be replaced more often.

Ken


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