GT40 vs the Competition

[Ole Nylende]

Put it down like this; A car is a result of a bunch of compromises as a result of a few fixed design parameters.
For the gt40 the height was one of the parameters, for a modern car it could be safety or some comfort/safety measurement.
Alot of the modern performancecars (not mentioning any specific ones) are actually quite shitty when it comes to the suspension geometry.
The original gt40 suspension geometry is actually not that bad, i would say more modern cars are way worse and you have to ask yourself "what the he"#! where they thinking of?"

On the other hand and as mentioned by others alot has happened to brakes, shocks etc and cannot be compared to 50+ year old technology.
So i would say if you take a gt40 with its original suspension geometry, put on some larger diameter rims with some modern sticky rubber and then some modern performance brakes and really good proper setup dampers you should be able to make it a competitive car. It would of course require a bit of effort to get everything dialed in.

Quite a few also refers to a spaceframe replica as a gt40. When it comes to details like suspension geometry there is not much relation to the original car. Put it like this; If you put a ford granada or cortina suspension on a kit car, would you expect it to handle way better than the ford granada or cortina did?
I see you dont want the corvette suspension, well, the corvette is a performancecar so why not use something built with that in mind?

 

[Randy V]

Exactly my point Ole…
Chevrolet spent mega millions engineering the suspensions in the C4-5-6 etc Corvettes. The 1984 C4 Corvette with Z51 option on stock tires pulled 1g consistently on the skid pad. I believe it was the very first mass production automobile to have ever managed that feat.
If I were scratch building a GT today, you can bet I would start with the C5 Corvette suspension.

 

[Darin C.]

Exactly my point Ole…
Chevrolet spent mega millions engineering the suspensions in the C4-5-6 etc Corvettes. The 1984 C4 Corvette with Z51 option on stock tires pulled 1g consistently on the skid pad. I believe it was the very first mass production automobile to have ever managed that feat.
If I were scratch building a GT today, you can bet I would start with the C5 Corvette suspension.

Sorry, pass for me.

 

[Neil]

Sorry, pass for me.

Using software such as Suspension Analyzer will tell the tale. It can also save someone from creating an ill handling car.

 

[Ole Nylende]

Another car to look at would be a mazda mx5 "miata" which has been proven to be a really good design from factory.
However as a total "bang for the buck" package i would still suggest the corvette parts.

Yes there are alot of different analysis programs available, also a few commercial grade ones. However they all require some background knowledge and wont tell you exactly how it needs to be. You need to make decisions regarding what parameteres to focus your design around, how much travel etc. So as for most other stuff, shit in shit out.
There are also a few books available on the subject.
Adams Herb has written one quite good and easy to understand, another one is Göran Malmberg who also has put out alot of good info on his homepage: www.hemipanter.se I think he also used to post a bit on this forum some years ago.

One obstacle you quickly will run into is the size of a gt40. It is a really small envelope to design a good chassis and suspension within.
If the goal is to build a competitive car purely for racing with the gt40 dimensions i would suggest to make it a single seater with a central driving position.

If you want a quick and easy route to something usable, go the corvette route. All the pickup points for the suspension is available online and you will be doing detail design of your chassie within short. Or buy a AP chassis which is made for the corvette bits.
If you want to reinvent the wheel (not that it is something wrong doing so and I would admire if you did) and design your own chassie and suspension make sure you have adequate knowledge of physics, math and cad software plus a some hundred to thousand hours to spend on "engineering".

 

[Randy V]

One of the Gotchas in the creation of a GT40 suspension is the width of the tires to be used and the desired turning radius for the front end. You may have noticed how very short the upper wishbones are. That’s more than likely a compromise to allow a footbox wide enough for the 3 pedals and a little maneuvering room. When using the stock geometry of the Corvette C5, you’ll most likely need to give up some of that footbox room..
There’s always the Mustang-II front suspension - although I personally find the handling to be mediocre….

 

[Neil]

One of the Gotchas in the creation of a GT40 suspension is the width of the tires to be used and the desired turning radius for the front end. You may have noticed how very short the upper wishbones are. That’s more than likely a compromise to allow a footbox wide enough for the 3 pedals and a little maneuvering room. When using the stock geometry of the Corvette C5, you’ll most likely need to give up some of that footbox room..
There’s always the Mustang-II front suspension - although I personally find the handling to be mediocre….

The original Ford Mustang II front suspension was designed by Chuck Weiss and it has very good geometry. Some aftermarket suppliers have not used the exact geometry (Arm lengths, pickup points on the chassis, etc) and those changes compromise what Chuck designed. Before he passed away a few years ago, I had the opportunity to talk to him about his suspension work at Ford. We have had some interesting guests at out "car guy" lunches here in Tucson.

 

[Randy V]

Neil - I am aware of a few different renditions of the MII front end. The one I am speaking of has the single mount point LCA and a strut rod. I tried to help get a street rod with this system to work properly and it just had too many quirky issues given his ride height and the castor and bump steer changes as the suspension went through all the gyrations….
My own street rod uses the Heidts MII front end and it’s _okay_ but still has bump steer issues that I need to address..

 

[Ole Nylende]

As pointed out above there is limited space in the nose section of the car where you need to keep your feet.
The upper wishbones are relatively short with the gt40 upright geometry.
With the mustang ii uprights using standard hubs etc the upper wishbones need to be even shorter if one is going to fit the car with normal backspace rims.

The mustang uprights will generate quite a bit of scrub radius. For example in sweden this amount will give you a struggle passing initial registration.
Scrub radius is a subject with many opinions. Some like to keep it down to a minimum, others dont care to much for it and most probably dont even know what it is. It is the distance between the point in which the upper/lower control joint axis meets the ground and the centre of the tire. It is basically the axis in which the tire will turn about. If you have a car with close to zero scrub it will be easier to steer but you will also loose most of the feedback you get through the steeringwheel.

 

[Neil]

Neil - I am aware of a few different renditions of the MII front end. The one I am speaking of has the single mount point LCA and a strut rod. I tried to help get a street rod with this system to work properly and it just had too many quirky issues given his ride height and the castor and bump steer changes as the suspension went through all the gyrations….
My own street rod uses the Heidts MII front end and it’s _okay_ but still has bump steer issues that I need to address.. View attachment 133049
View attachment 133048

"The one I am speaking of has the single mount point LCA and a strut rod." Yes, that is the original Weiss design. I used that on my upgrade of the Mirage prototype "MANTA 2" when I finished its front suspension conversion. I used original Ford a-arms but modified then by boxing the lower arm for increased stiffness and adapting it for a coil-over. I had then zinc plated with a gold passivated finish just for looks. It is necessary to first decide on your desired front ride height- everything else depends on this. By moving chassis attachment points around in Suspension Analyzer V2.4, you can see the effects of camber gain, bump steer, caster change, etc as well as the effect of anti-dive. I think the most common mistake is to bolt on a Mustang II front suspension without any consideration of how far off their ride height is from Ford's original design. Somewhere on one of my computers is the graphical data plot of this suspension in bump & droop. If I can find it, I will post it. It has the X-Y-Z coordinates of the a-arm pivot points as well.

 

[Neil]

As pointed out above there is limited space in the nose section of the car where you need to keep your feet.
The upper wishbones are relatively short with the gt40 upright geometry.
With the mustang ii uprights using standard hubs etc the upper wishbones need to be even shorter if one is going to fit the car with normal backspace rims.

The mustang uprights will generate quite a bit of scrub radius. For example in sweden this amount will give you a struggle passing initial registration.
Scrub radius is a subject with many opinions. Some like to keep it down to a minimum, others dont care to much for it and most probably dont even know what it is. It is the distance between the point in which the upper/lower control joint axis meets the ground and the centre of the tire. It is basically the axis in which the tire will turn about. If you have a car with close to zero scrub it will be easier to steer but you will also loose most of the feedback you get through the steeringwheel.

True, but easily accounted for by adjusting the front wheel offset.

 

[Randy V]

Nice work Neil! I like your modifications.
I think I have the same Suspension Analyzer - runs on DOS as I recall. I’ve not seen it in years, but recall it being in a Red folder. I might just have to go digging and enter the configuration parameters for my street rod. The last time I used it, I think I was working out some Mcpherson strut issues on my A/Sedan Camaro - back in 98-99 ??
Ackerman on the MII was another potential issue - had you addressed or measured?

 

[Neil]

Nice work Neil! I like your modifications.
I think I have the same Suspension Analyzer - runs on DOS as I recall. I’ve not seen it in years, but recall it being in a Red folder. I might just have to go digging and enter the configuration parameters for my street rod. The last time I used it, I think I was working out some Mcpherson strut issues on my A/Sedan Camaro - back in 98-99 ??
Ackerman on the MII was another potential issue - had you addressed or measured?

The original software was DOS but the version I used was capable of running on Windows 7. This version is capable of strut analysis as well as a-arms , etc.

I did not care about Ackerman at all; it is what it is. Ackermam isn't really important except for minimizing tire scrub in tight turns around a parking lot.

 

[Howard Jones]

Here's the reality of the situation if you buy any of the current manufacturer's kit car GT40. Then you want to run the car against other current very high-performance sports cars like a GT3RS Porsche or an R8 Corvette at open track days and be competitive on lap times, the first thing you need to do is make a decision on tires and wheels. If you want to run the fastest group of all of the open track vendors that I am familiar with at the top tracks in the country then you will need to use DOT radial slicks such as Hoosiers R7's.

The parts they send you for suspension parts with your chosen kit are fine for this purpose. Keep the a-arms, uprights, trailing links, and lateral links. If they offer a bushings or rod ends choice then run rod ends. Make sure you buy shocks that have the damping range to accommodate the much higher spring rates that will be necessary for running slicks on a track car. Talk to the car (kit) manufacturer and ask about spring rate suggestions. Buy the optional antiroll bars if available.

Next brakes. 15-inch wheels will force you to use marginal-size brakes. My advice is to run 18-10 on the front and 18-12 wheels on the rear. You can fit 14-inch rotors in the front and 12-inch rotors in the rear. Use two-piece aluminum hat rotors at least 1 1/4 inch thick. There are several vendors that make good track-worthy brakes. Again ask your kit manufacturer for recommendations.

Why 18s? ALL the current top race series have gone to 18-inch tires. Imsa used sizes will work very well on a GT40 and there are lots of them. When purchased used they will save you huge money. All the other sizes are becoming dinosaurs. Again I am talking about slicks for track use.

If available buy an SCCA, FIA, or NASA-approved roll bar cage with the kit. Install a full containment seat and a 6-point harness if you can find one that will fit.

Fuel cells. Fuel cells cost a lot of money.....sorry. And they need to be time-changed every few years but maybe you only need one on one side. 30-minute sessions typical at open track days will use about 6-7 gals every 30 minutes in a 500hp 2500-pound car. You could install a fuel cell in the passenger side pod and leave the aluminum tank type on the driver side. Then run the driver's side empty when on track. Ask the kit manufacturer if they could accommodate you with a discount for the unused fuel tank.

Thats pretty much all the big stuff

Otherwise, If you are building your own chassis then you surely are capable of designing an effective suspension. THAT discussion is far to lengthy for this thread IMHO.

 

[Darin C.]

Was hoping this would spark some footage examples.

 

[Randy V]

Was hoping this would spark some footage examples.

There are examples to be found here:

Video and Pictures Buffet

A place for threads about your car!

www.gt40s.com

 

[Darin C.]

 

[Darin C.]

 

[Cliff Beer]

All I can say is the C8 Z06 I had left the GT3 I had in dust....and the GT3 left behind any GT40 replica so far in the dust that it wasn't clear what lap they were on.....

Yeah, you can highly modify a GT40 replica to go a lot faster with bigger brakes, bigger engine, changed suspension, modern transaxle, slicks, etc...but then it isn't really a GT40 anymore.....