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Learning how to build Lotus Seven replicas...together!
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PostPosted: August 23, 2016, 4:42 pm 
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It's been long proven that mid-engined is preferable to rear from a performance standpoint, and the fact that nobody but Porsche still does it should be evidence enough of that fact. A couple related musings:

-One of the great virtues of Locosts is that they are phenomenally forgiving and easy to drive hard. For us amateur drivers that's huge. I remember driving at a marque-sponsored track day where none of the fastest cars belonged to that marque. In fact lightly prepared Miatas were regularly passing many of them. The reason? Most of the cars from that marque were middies, and I believe that most of the middie drivers were fundamentally afraid of driving their cars at anywhere close to 10/10ths.

-Funny anecdote. When Mario Colucci asked Carlo Abarth to design the first mid-engined Abarth, Abarth refused, feeling that rear-engined cars were inherently superior. At last Colucci got over with the argument that while rear-engined cars were superior, not enough potential buyers were talented enough to extract that potential. So the iconic SP 1000 was "dumbed down" so to speak. Although I think that ego-stoking Abarth also played a big part in it.


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PostPosted: August 24, 2016, 5:05 am 
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FastG wrote:
So my 7 is a mid engined car.


Yep. Though some people use the term "front-mid" to differentiate such layouts from "behind the seats mid."

Something to keep in mind is; as wheelbase goes up, the location of the engine matters less. And once the engine is light enough, it doesn't matter where you put it.


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PostPosted: August 24, 2016, 10:00 am 
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[quote="kreb"]It's been long proven that mid-engined is preferable to rear from a performance standpoint, and the fact that nobody but Porsche still does it should be evidence enough of that fact.

You are right about Porsche being the last rear engine supercar maker. Now Alpine Renault 2016 also moved to mid engine. However my feeling is that some of this has a lot to do with fashion. Porsche built the Cayman (mid engine), therefore Renault now feels that they have to move the Alpine s engine from rear to mid. IMHO both designs have their strong and weak points. as illustrated in the article I have quoted upstream in this thread. At my modest Locost (second time) builder level I have decided to give the rear engine a chance. My plan is to try and minimize the oversteering tendency of the design with a redundant air spring/compressor/computer system that if it works will tilt the car body opposite the centrifugal and acceleration forces. I will welcome opinions about this idea. :cheers:


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PostPosted: August 24, 2016, 10:31 am 
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Well good luck. Advanced suspension design is above my pay grade, but in general my thought is that the primary advantages to rear-mounted engines are better braking ability going into the turn and better traction getting out. offsetting that is dealing with the desire of the car to spin, and maximizing adhesion with an uneven load. In rough terms I use 60r/40f as the point at which the liabilities can begin to overwhelm the benefits. Please don't construe anything that I say as an attempt to be discouraging. Unless what someone does is suicidal, I am a firm believer in doing what excites you and finding out what happens.


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PostPosted: August 24, 2016, 11:02 am 
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I thought that Wilkey Bug (Urban Assault) was a pretty spectacular performer for an extreme rear engine.


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PostPosted: August 24, 2016, 11:33 am 
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Food for thought: There are some key physical changes to the Porsche 911 over time that extrapolate out to correlate with the extremely rear weight biased Deltawing racecar design. This is not merely a coincidence.

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PostPosted: August 24, 2016, 1:56 pm 
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Driven5 wrote:
Food for thought: There are some key physical changes to the Porsche 911 over time that extrapolate out to correlate with the extremely rear weight biased Deltawing racecar design. This is not merely a coincidence.



Rear weight biased is a lot different than all that weight hanging out behind the axle. We are talking apples and bananas here in your comparison.

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PostPosted: August 24, 2016, 2:32 pm 
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I think we should take the 911 or vw aircooled out of the equation because the engines are so light, if they were flipped on their trans to go midi there would probably be more weight hanging rear of the axle centerline, I think a porsche g50 trans must be heavier than the engine, I can manhandle a flat six onto the workbench on my own. An aircooled six or four pot is light compared to anything with a water jacket and all the heavy stuff like a flywheel or clutch is very close to the axle centerline, most of the wieght of these units is also very low . Now if we were to hang some mericun iron out the back we would have a unhealthy wieght bias :)

Bob

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PostPosted: August 24, 2016, 2:59 pm 
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carguy123 wrote:
Rear weight biased is a lot different than all that weight hanging out behind the axle. We are talking apples and bananas here in your comparison.
...And it's a purpose designed racecar vs a street car, blah, blah, blah, yadda, yadda, yadda.

None of which specifically invalidates the correlating design principles and underlying physics.

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PostPosted: August 25, 2016, 10:55 am 
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I agree with Driven5, that the Deltawing car is a good example of how pretty extreme rear weight bias, similar to what you can have with a rear engine layout, can work. Yes, the moment of inertia will be lower for a rear-mid design like the Deltawing, but it's still a lot of rear weight bias.

There are other advantages with a rear engine, such as the possibility of easier engine removal and replacement, with only a floor jack instead of a cherry picker/hoist. The shifter will also be much easier to make "Miata-like" than a typical rear-mid design with their complicated linkages or friction adding push-pull cable shifters. The bigger rear tires (and smaller fronts) you will be driven to will look good.

To go along with the "Not too short" wheelbase you may want with a Corvair engine in the rear, you could recline the driving position more than normal, lowering the CG and making the car look cool (yes, here come the "too low for the street arguments... Noted, but that's no fun. Just drive like no-one can see you, as much as possible, and maybe mount a wing to the top of a full roll cage to act as a sun shade and "see me" device). One or two wedge-shaped gas tank(s) between the seats and engine compartment, on one or both sides of the transaxle, will put the tank close the CG location. A battery way up front may make sense, but closer to the CG is good for a gas tank.

Regarding the suspension system you mentioned, how will opposing the normal tilting of the chassis while cornering, braking and accelerating, or even tilting the chassis opposite the normal roll or pitch angle, improve grip? I think simpler is better.

Your plans sound great so far. I'll be looking forward to seeing what you come up with. I'm curious to see where you run the axle tube of the DeDion rear suspension. Will it be above the transaxle, or will there be room to run it below? Maybe a frame of light members that are both above and below the transaxle like a Lotus 58?
Attachment:
Lotus 58 DeDion Rear 1.jpg
Lotus 58 DeDion Rear 1.jpg [ 90.38 KiB | Viewed 639 times ]

Attachment:
Lotus 58 DeDion Rear 2.jpg
Lotus 58 DeDion Rear 2.jpg [ 95.38 KiB | Viewed 639 times ]


Dean


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PostPosted: August 25, 2016, 2:23 pm 
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Keep in mind the Deltawing has an electronic diff that works kind of like a skid loader to help with the turning and to help with the oversteer.

There's "on paper it looks good" and then there's real life with real life drivers in wildly varying road, treaffic and weather conditions. It took Porsche 60 years to tame the rear engine configuration so how long will it take you?

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PostPosted: August 25, 2016, 3:13 pm 
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DeanD3W wrote:
I agree with Driven5, that the Deltawing car is a good example of how pretty extreme rear weight bias, similar to what you can have with a rear engine layout, can work. Yes, the moment of inertia will be lower for a rear-mid design like the Deltawing, but it's still a lot of rear weight bias.

There are other advantages with a rear engine, such as the possibility of easier engine removal and replacement, with only a floor jack instead of a cherry picker/hoist. The shifter will also be much easier to make "Miata-like" than a typical rear-mid design with their complicated linkages or friction adding push-pull cable shifters. The bigger rear tires (and smaller fronts) you will be driven to will look good.

To go along with the "Not too short" wheelbase you may want with a Corvair engine in the rear, you could recline the driving position more than normal, lowering the CG and making the car look cool (yes, here come the "too low for the street arguments... Noted, but that's no fun. Just drive like no-one can see you, as much as possible, and maybe mount a wing to the top of a full roll cage to act as a sun shade and "see me" device). One or two wedge-shaped gas tank(s) between the seats and engine compartment, on one or both sides of the transaxle, will put the tank close the CG location. A battery way up front may make sense, but closer to the CG is good for a gas tank.

Regarding the suspension system you mentioned, how will opposing the normal tilting of the chassis while cornering, braking and accelerating, or even tilting the chassis opposite the normal roll or pitch angle, improve grip? I think simpler is better.

Your plans sound great so far. I'll be looking forward to seeing what you come up with. I'm curious to see where you run the axle tube of the DeDion rear suspension. Will it be above the transaxle, or will there be room to run it below? Maybe a frame of light members that are both above and below the transaxle like a Lotus 58?
Attachment:
Lotus 58 DeDion Rear 1.jpg

Attachment:
Lotus 58 DeDion Rear 2.jpg


Dean


Thanks Dean for your points. As you can tell, I still have quite a bit of soul searching to do. I agree with most of what you are saying.
At this point (but it may change) I am looking at amending The Locost (Book) chassis to fit my corvair engine possibly with round tubes. I will make it at least 12" longer and make the back seat engine bay with stronger tubes . The DeDion tube may be in front of the transmission (in between the transmission and the back seat bulkhead if it makes any sense). I plan a Watt lateral control arm system. The cockpit and seats , steering wheel will be moved forward to the front by possibly 8 to 12 ". The seats will be about 1 1/2 to2 " lower than in my first seven. I plan to fit two sizeable gas tanks near the front wheels. There I will also fit two batteries. One for the car, one for the tilting suspension air compressor and software. On the subject: tilting the vehicle to counter centrifugal forces, I expect will transfer weight in the direction opposite to the turn, hence (I think) help minimize the oversteering attitude of the design. If I am wrong I will can this system. Back to the front: I will also set up the spare wheel there and a side oil reservoir with cooler and pump. And a small trunk.
For the body I will stick to what I know. Keep it frugal, like the original seven and as much as possible reminiscent of the Seven. ....Wider back wheels,... yes what a good idea. :cheers:

Thanks again for your time. This is all very useful to me. :cheers:


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PostPosted: August 25, 2016, 3:28 pm 
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carguy123 wrote:
Keep in mind the Deltawing has an electronic diff that works kind of like a skid loader to help with the turning and to help with the oversteer.

There's "on paper it looks good" and then there's real life with real life drivers in wildly varying road, treaffic and weather conditions. It took Porsche 60 years to tame the rear engine configuration so how long will it take you?


I am approaching the subject with a fair deal of humility. Chances are that I will not succeed. I may be wrong or simplistic in my understanding of suspensions. All this is not a good enough reason for not trying. The air/hydraulic suspension was developed by Citroen (DS-19) in the early sixties. My family owned one and I drove it. All I recall is that the car was a lot better at cornering than its predecessors, albeit all front wheel drives and having little to do with my current project.
All I can say is this: I will try it, buy the hardware, etc and if it works I will report about it here in a couple of years. The proof will be in the pudding :)


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PostPosted: August 25, 2016, 4:00 pm 
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TRX wrote:
FastG wrote:
So my 7 is a mid engined car.


Yep. Though some people use the term "front-mid" to differentiate such layouts from "behind the seats mid."

Something to keep in mind is; as wheelbase goes up, the location of the engine matters less. And once the engine is light enough, it doesn't matter where you put it.

Warning: rant ahead:
I think the whole "front-mid" expression is more for marketing than anything else. For example, if the furthest forward point on your Locost engine are the heads of the water pump pulley bolts, and the front face of these bolts stick past the front axle centerline by 1mm, do you really think that you can say you've converted your car to a mid engine car (or "front-mid" if you prefer) just by shaving 2mm off the end faces of the bolt heads? I don't think so.

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PostPosted: August 25, 2016, 4:00 pm 
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Hi Phil,
The weight transfer will occur based on the track width and CG height in a given corner. It might feel reassuring to ride in a vehicle with active tilting ("anti-tilting"?), like a tilting 3-wheeler or a boat, but I don't think you will find any benefit in cornering speed or the balance of the car.

Especially with a well designed DeDion and wider rear tires, I don't think the handling will to too evil. Won't the axle tube need to go either above or below the transaxle, since to reach around the front will be too great a distance?

Dean


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