How much better is lighter?

[DocDirk]

Well I have satisfied my urge to build and drive a heavy West Coast cobra-ish cruisemobile. It's a lot of fun and I'll keep tweaking away to make it even moreso. BUT as my OCD and curiosity kick in again I'm wondering how it would be to build an extremely light, perhaps more nimble cobra replica.

I'm wondering what the obvious and not so obvious tradeoffs, rewards and/or sacrifices are involved in pursuing a light car project? I'm not a racer but I wouldn't rule that out. At any point does the car lose too much comfort to be fun on the street?
Can it be a lot of fun off the track at any weight? Or is there a point where lighter just makes no sense? Are those 1800lb cobras I've heard of like driving a rocket powered go kart?

I beg your forgiveness for perhaps asking questions to which any cobra replica owner worth his salt would already know the answers.

Thanks!

Dirk

[CobraEd]

Drill a million holes all over the frame kinda like the Eiffel Tower was made.

.

[Ron61]

The lighter the car, the more power to weight ratio you have making it faster normally. However, the handling will be different and so will the safety factor if you drive it on the street much. A builder here built a body for a guy that was so thin that it cracked when he started putting the stress of hard turns and twists on it. But he wanted as light of a car as he could have. They also drilled 4" holes all along the frame to remove metal, so much so that i would have been afraid the frame would collapse after a few hard jolts of coming over a steep hill and the front wheels coming off the ground a little. I think he finally just junked the car and used the drive train to build one that was a little heavier and stronger.

Ron

[DocDirk]

Thanks for your replies, I'm still on the learning curve. I was hoping to see answers along the line of:

"Lighter is great until you reach a HP/# ratio of xxxx then you'll have more handling issues than you would like"

or then "......your comfort factor below 1900# is severely compromised for the performance trade-off"

or "I've driven one at xxxx# and it was something I wouldn't want to do from SF to LA"

or "It's a hoot and the lighter the better for all applications!!"

or "Build a superlight only if you intend to track it primarily"

or "You need more money than you'd think to actually pull off a very light cobra build - I've done it"

etc.

Well, maybe building a superlight cobra isn't done as much as I'd thought it might. I just like extremes and would consider it a fun project if I could get an idea of what I would be getting into.

Regards!

Dirk

[RICK LAKE]

DocDirk Dirk The Smith brothers built a carbon fiber body on one of the FFR cars and raced it at Run&Gun. The car was fast until a 360 on the track slowed it down a little. 20 pounds of 200 mph tape was added to put the car for repairs and the car won class back in 99 or 2000. I believe the body weight was like 100 pounds, silly lite. John Cheasnut is building cobra with a mono cock frame in aluminum. With a SBC the weight is about 1,900 lbs with ZO6 drivetrain and brakes. Handling is great and power to weight ratio is high with a 400 hp motor. Any Aluminum body car is going to be about 100-200 lbs lighter that a fiberglass body car. If you are thinking track car, lighter is better. If this is a street car, and the driver in your area don't know the law or run stop signs all the time. I WANT THE TANK!!!!

[Ibr8k4vetts]

I believe a 100lb is = to a 10th of a second in the 1/4 mile.

[Tommy]

Dirk,
As I read your initial post, your question boils down to this - What are the advantages and disadvantages of building and operating an ultralight car? Here's my answer in list form.
Advantages
1. Experience the satisfaction of taking on and completing a very challenging task (i.e., conceiving, engineering and building such a car).
2. Experience the pleasure of sharing your building experience with other car people after the job is complete.
3. Enjoy the exhilarating and humbling experience of trying to master such a car on the track. (I don't see street driving being much different than in any other powerful Cobra replica. Most of them can scare me silly, ultralight or regular.)

Disadvantages
1. High costs for one-of-a-kind engineering, material and parts.
2. Potential safety compromise if you don't properly engineer and construct the car to achieve the ultralight weight.

If it were me, I would see lowering the weight of the car as a means rather than an end. I wouldn't invest the time, money and effort to build an ultralight car unless I had in mind a specific end use that would justify all that effort.

[COBRANIP]

DocDirk, I have the same curosity and am working (?) on a similar project but am not far enough along to shed any light. I am starting with a Herb/Matt Adams aluminum frame (200 lbs) and a FFR carbon fiber body (probably under 100 lbs). I am looking at it primarily as a track car. There is no doubt a heavy car will have a more comfortable ride (assuming a relatively heavy sprung weight to a relatively light unsprung weight...that is simple physics).
One other benefit is for any given HP/WT ratio the lighter car will require less HP... which I think can translate into a "more reliable" and less extreme engine.
The biggest problem I anticipate is traction. As you get lighter and lighter how do you get the power to the ground?
I really enjoy the engineering challenge which is my motivation.
I would think the biggest advantage would be in something like an Autocross course.

[Snake2998]

The simple answer is that on the track a well engineered light car will be faster in every way, except maybe acceleration, than a similar well engineered heavy car given the same power to weight ratios.

On the road it doesn't matter nearly as much because to overcome weight you can just add power - that might mean adding more weight and cost but then adding lightness is also normally very expensive in it's own right - especially if properly designed and engineered.

[Excaliber]

One of the problems with a light weight glass body would be durability issues. Spider lines in the paint from stress for example. Shakes, vibration, twisting issues would be more evident, cracking easier on light impact.

Recently had a chance to compare the thickness of a Classic Roadster to an FFR, whew, it's night and day when it comes to structural integrity. Of course the penalty for the C.R. is a weight gain. There is no free lunch.

[Buzz]

Originally Posted by COBRANIP

The biggest problem I anticipate is traction. As you get lighter and lighter how do you get the power to the ground?

Weight over the drive axle comes in handy traction-wise when a vehicle is stuck in snow or sand. For acceleration and applying power when exiting a turn, a lighter car has less mass to heave into motion and is less hamstrung by inertia. In other words, a lighter car with the same power will accelerate away from a heavier one off the line with less wheelspin simply because the tires have less dead weight to struggle against.

[Double Venom]

Mike Z is pretty close on-
For drag racing 100 lbs does equate to .1 to .3 sec's. on a 1/4 mile the strip. (Results equate to cornering -auto cross to).

It does come down to traction and handeling though. Just drilling 'holes' in the frame without a true engineered design can, and usually does result in ugly results! Ergonomics don't usely come into play unless you start with seats, firewalls, engine tunnels and the likes.
DV

Only built one car, - DV II - where we had to add weight to make a SCCA class! It had to come in at 3,300 lbs! With the driver we made it by about 45 lbs!

[Buzz]

To elaborate further on the weight/traction topic, there are other important factors that influence a car's ability to "stick" during acceleration - both linear and lateral(cornering) as well as negative(braking). Weight distribution - location of the center of gravity - and chassis dynamics influence how a car will load/unload its tires and thus behave during changes of speed or direction.

A good example is a pickup truck with an empty bed. Owners need to add sandbags over the rear axle for additional traction on icy roads. An apparently logical conclusion would be that more weight is better for putting the power to the ground but that would be incorrect. The traction problem does not occur because the truck doesn't have enough weight (it already has too much); it occurs because of how the weight is distributed.

Ditto for lateral acceleration. A heavy car initially resists acceleration due to inertia which is an object's natural tendency to resist changes in speed or motion. Once it is moving, the heavier car builds more momentum making it more resistant to changes in speed and direction, all of which place additional demands on tires, brakes and suspension. Basically, more weight means more resistance to change in direction and state of motion.

Factors like the height and static location of the center of gravity have a huge impact as well - a CG located high in the chassis will exaggerate the effects of weight transfer, increasing the tendency of a car to unload the front tires during acceleration, the rear tires during braking and the inside tires when cornering. The less actual weight, the car has, the smaller the relative impact of weight transfer on vehicle dynamics.

A heavier Cobra will ride more comfortably on bumpy roads assuming the suspension is capable of absorbing the vertical acceleration which the chassis resists when driving over a bump. A lighter Cobra (with all else being equal) will drastically and noticeably outperform it in all other performance categories like acceleration, braking and cornering - which are the things that people generally buy Cobras for in the first place.

One other point when it comes to ride quality in heavier performance cars. They generally tend to end up being more stiffly sprung as owners try to compensate for their handling characteristics. Lighter cars, when properly set up, can be more lightly sprung, which can close the gap quite a bit on the ride comfort issue.

Up to the point where the car becomes lighter than air and starts to float off the track or efforts at removing weight destroy structural integrity - for performance, lighter is almost always WAY better.

EDIT - after rereading,I couldn't resist:

Basically, more weight means more resistance to change in direction and state of motion.

Is this why larger and heavier folks are more likely to be Repub than Demo? Herin may lie the REal reason Obama is launching an attack on soft drinks and fast foods!! They're coming for your burgers!!

[Double Venom]

Is the above the same BUZZ that I know as a generous and absolute Artist? Wow, impressive to say the least!

One shouldn't be that talented! What line was I in when they passed out talent?

DV

[Buzz]

You're way too kind DV! Especially considering what you yourself have accomplished. It's good to see you posting regularly again, my friend.

[DocDirk]

Thanks to everyone for your helpful answers! I won't be building another cobra right away but you've given me much food for thought. I think I'll buy a least expensive used cobra and make it a fun 10 year project.

Regards,

Dirk