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Learning how to build Lotus Seven replicas...together!
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PostPosted: November 29, 2013, 11:24 pm 
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Not exactly.

Think about the Porsche 911 and how long it took them to tame that beast!

Centering the mass rather than having either end be heavy makes for a more agile car.

Read up on the At-om. Every last magazine, video or review talks about how hard it is to drive it consistently fast. It's always trying to bite you. That's with a transverse mid design. In that case the engine is barely in front of the axle.

Now think of a FWD car where the front is super heavy, by comparison to other cars. They are hard to drive fast. It can be done, but it's hard.

Now find a place between those 2 extremes such as a front middie or a rear middie and the car is inherently easier to control since you aren't trying to control a pendulum at either end. That's not to say you can't get it wrong in your set up, but there's a bigger margin of error. It's easier to make it turn or stop turning.

Given enough resources, time and maybe just plain ol' dumb luck, you can make almost anything work (think 911 again), but what are the odds you'll get it right quickly enough that you don't kill yourself or that you won't get tired of the car.

The traditional Locost is a front middie. What you are suggesting is a rear middie. Both are inherently more in the sweet spot AND the traditional Locost has a ton of resources and decades of development time so you are more likely to get it right the first time.

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PostPosted: November 30, 2013, 12:04 am 
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johnnybusa wrote:
...it sounds like the further back you put the weight the better off you are if you're a good enough driver, is that about it?

Exactly.

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PostPosted: November 30, 2013, 1:12 am 
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seems like single seater rear mid boxer is the way to go based on some really quick scribbles. get your feet up in between the front wheels to help get some weight kind of towards the front, ultra short, ultra light, still a bit of traction hopefully. what are yalls opinions?


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PostPosted: November 30, 2013, 1:19 am 
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I agree with carguy123.

That said, (here comes another bad "almosty" explanation) when the engine, transmission, driver, and differential are all arranged in one long thin line like a front mid-engined car, the PMI is spread out along that length; When the driver, engine, gearbox (and included differential) are crushed down into a shorter length like a typical rear mid-engined car, you have a shorter PMI, and the car becomes more reactive. These faster reactions aren't really a problem until the required reaction times exceed the driver's ability (which is a learned skill) to provide those inputs quickly enough to keep the car under control at the limit.

A bit more: Almost every modern car regardless of driveline or engine configuration comes from the factory with a comfortable helping of understeer included. That understeer is there because it's easy for most drivers to understand intuitively, and it's typically safer for most drivers than a car that is balanced towards neutral or oversteer characteristics. Usually it's up to the owner to tune out the understeer if they want a neutrally balanced or oversteering car. And here comes another crude over-generalization: One part of the driving dynamic that is pretty much unique to RWD and some AWD cars is that the OEM understeer can typically be overcome and turned into balanced cornering or oversteer with added throttle, assuming you are close enough to the car's absolute traction limit. I have tracked one OEM FWD car that I could put into oversteer with just additional throttle, but that was (in my experience) an exception, and probably the result of poor rear suspension geometry that allowed the rear traction to give up too early. Most tend to just increase the amount of understeer they provide.

Thinking of my weight-bar example above, because the car's behavior is limited by the plane of the road, the bar's PMI would need to be similarly limited by having both weights on the ground. The distance between those weights along the bar would approximate different possible PMI configurations and prove a very loose idea of how quickly the bar+weight assembly is willing to swap ends once tire traction has been exceeded.

Lifting the bar off the ground was also mentioned above. In this case that's not an applicable comparison unless we're looking to see how the car behaves when completely airborne .. not an ideal situation for a car seeking maximum traction.

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PostPosted: November 30, 2013, 1:30 am 
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johnnybusa wrote:
seems like single seater rear mid boxer is the way to go based on some really quick scribbles. get your feet up in between the front wheels to help get some weight kind of towards the front, ultra short, ultra light, still a bit of traction hopefully. what are yalls opinions?

That would probably make for a really twitchy handful anywhere near the limit. Most big power supercars have a long wheelbase. This length gives the designers room to spread out the weight (and thus the PMI) and also cut down on the choppiness associated with very short wheelbases. Going middy is fine, but don't be afraid to make it a bit longer, wider and lower. All will provide handling benefits that outweigh the added weight over a go-cart sized chassis.

Also: Using one's toes as a crumple zone doesn't sound like a great idea. Somewhere in one of NASA's (the racing guys, not the rocket guys) rulebooks is a rule stating a requirement to have all of a car's occupant's feet completely behind the centerline of the front wheels. I'm not sure if it's in the CCR or a specific set of class rules, but it's a great rule for safety.

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PostPosted: November 30, 2013, 2:09 am 
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so what if I made it longer but still just put the pulleys in the back of the seat and made a narrow chassis, with long arms going to the wheels like they did before the aerodynamic bodies? http://farm5.staticflickr.com/4095/4818 ... 8c18_o.jpg

I don't know what's going on with the suspension of this thing I was thinking simple short long arm but this seems like it would work well with a boxer and subie transaxle


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PostPosted: November 30, 2013, 2:37 am 
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As you're getting closer to your goals your questions are becoming harder to answer in a "This is the best way to do that" manner. The problem is every suspension design (in fact every car design) is a series of trade-offs based on the goals set for the car in question. If you were looking to build a traditional locost, there would lots of help and experience available to keep you moving forward. As you get further from that design and more into "one of one" territory, then eventually the pool of prior experience starts to dry up.

At this point, spending some time and money on books would probably really pay off for you in helping you get what you want out of the finished product. Two of the books I've read are "Competition Car Suspension: A practical Handbook" by Allan Staniforth (http://www.amazon.com/Competition-Car-Suspension-Practical-Handbook/dp/1844253287/ref=la_B001JP3QZQ_1_1?s=books&ie=UTF8&qid=1385792675&sr=1-1) and "Tune to Win: The art and science of race car development and tuning" by Carroll Smith (http://www.amazon.com/Tune-Win-science-development-tuning/dp/0879380713). Both are good books, but of the two I think Tune to Win offers more long term value. While it has a less suspension theory then Staniforth's book, it has a much better section on diagnosing and correcting suspension dynamics problems. I do think both are well worth reading if you don't mind investing the time.

If it's going to be your car then you really should be the one choosing your compromises. To do that well you need enough background to really understand the costs associated with different design alternatives. More work here will give you a better car in the end.

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PostPosted: November 30, 2013, 3:23 am 
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Thanks the only book I have is chassis engineering, the more I draw the more it looks like an atim Diamond shaped from the top view instead of oval, maybe a good idea would be to cut up a model of a subaru and do the hot glue and tooth picks thing. Can you tell me why the real atim seems to be such a bear and how to avoid those mistakes? Seems like the guys on the forum make better atims than the company that makes the real atim, but thats just reading the opinions of others as ive never seen a real one or a clone


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PostPosted: November 30, 2013, 11:08 am 
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The reason it looks like the Atim is you are looking only at the chassis. You will modify the basic chassis shape with some sort of body.

The chassis is narrow at the front because there's no engine. It's wider at the rear because you need room for both you and the engine.

At some point in time all designs are just people, engines and wheels. Then you flesh them out to add all the creature comforts and give it a pleasing design to the eyes.

Erioshi is right about wanting to all pack the heavy components between the wheels, not outside the wheels, and the closer you can pack them together towards the middle of the car they less they will give unwanted inputs or impetus to the handling.

If you'd like to see this carried to the extreme, go visit dpcars.com. He had the engine beside the driver.

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PostPosted: November 30, 2013, 11:56 am 
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I waited 15 minutes so it was too late to amend my post above :(

I wanted to add that you might also want to look at an MR2 type of gas tank that fits between the seats instead of putting the gas tank way out front or way out back.

That also helps center your polar momentum (as I like to think of it).

I think of electricity as water and weight as the heavy end of a pendulum. That makes it all make sense in my head.

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PostPosted: November 30, 2013, 12:15 pm 
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To build on what carguy123 last said, keeping the gas tank in the center is pretty common in mid-engined racing cars. Many sports racers and formula cars actually locate the fuel cell between the engine and the driver (basically with two firewalls, one as the driver's back rest and another between the engine and fuel cell) to cut down on the impact fuel use has on weight distribution.

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PostPosted: December 2, 2013, 1:29 am 
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im watching a lot of documentary stuff on super cars and hyper cars to get inspiration on some of the stranger parts, one thing that keeps showing up that looks like a good idea is to make 3 chassis. the front that holds front suspension steering yada yada, a middle piece for you to sit in, and a rear piece for the engine and rear suspension. I don't know of any other reason to do this other than looking at it, it seems so much easier to build each part when you're not fighting the other parts. also if I wanted to change or scrap one section I could just unbolt that piece. obviously wires and hoses would still be a issue but it would still be easier than cutting out the section you don't like. any opinions, or has anyone done it?


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PostPosted: December 2, 2013, 8:50 am 
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I was at sebring a few month back the they had one of the Formula Mazda's (not sure or the correct name) and the car had literally snapped in half between the engine and the single seat. Now who knows if coming off the track caused it to break or if it breaking caused the car to come off the track. The car did not hit anything all the wings and splitters we intact. Either way I looked at it and decided racing a car that was held together with 4 bolts was not a good idea.

I think you are adding huge points of failure, most race cars a built that way so when the crash they car do repairs without starting the chassis from scratch. However you can still build it that way. I built the front section first, then worked on the rear. I had installed the rear coilovers before connecting the front section. However I would still fully weld the frame, if you want to change it you can just cut the pieces out and start again.

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PostPosted: December 2, 2013, 9:58 am 
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...looks like a good idea is to make 3 chassis. The front ....., a middle......, and a rear piece........., (If) I wanted to change or scrap one section I could just unbolt that piece.


The book chassis is just that, pretty much. Only instead of unbolting, you just take out your angle grinder with a cutoff wheel. Everything from behind the rear bulkhead is one section and what is in front of the footwells is another. The center section is the part that gets the roll bar etc making the center section the safe one. I can see no technical advantage of one design over the other. The bolted chassis would be slightly heavier as it would need the plates for the bolts to go into where the welded chassis does not.

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PostPosted: December 2, 2013, 6:26 pm 
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I understand it could be a weak link if built for extremely light weight but given enough bolts it would be just as strong. the main thing is it could constantly be improved as you get smarter at building and funds increase


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