Chassis Weight

[Jac Mac]

Where do you draw the line.....?

Cheers

About one car length in front of you- its called a finish line!
Now get back to work and polish those chassis tube's, you might need to lose some more weight!! With all that extra space in the shop after your sweep up things should be flying together.

Jac Mac

 

[Scott Calabro]

Hi Brian,

I will absolutely participate!

Cheers,
S

 

[Doug Clark]

I don't get the point to worrying about this, ANYONE could build a chassis as light as they like.
Heck I'll build you one weighing 50lb if you like, it'll be a death trap though.

I'd propose a different measure:

Divide the vehicle torsional stiffness by the total mass of the car (no one ever drove a bare chassis round did they).

What this gives is a mass against stiffness measure, so for example using theoretical numbers:

Car mass: 1000kg (9810n)
Stiffness: 15kn / degree.

= 15000 / 9810

= 1.53

Whilst this number is meaningless in itself comparison between vehicles might yield something useful as it would appear to me that cars with a higher number are better designed - constructed.

Just my 5 cents, I'll be quiet now.

 

[Russ Noble]

Doug,

That's more or less the point I was making.

However the fascination about chassis weight is that this is the building base for the car. Everything flows on from that. Every ounce of weight has to be accelerated, braked, turned. You start off with a heavy chassis then it needs bigger wheels, bigger brakes, more powerful engine, bigger fuel tanks, just to have the same performance and range of the lighter version. Of course because you are running these heavier other components, attachment brackets and links will need to be heavier and stronger which is more weight still. The whole thing just becomes a compounding disaster!

On the road this means higher running costs for the same performance. On the track, and GT40's were originally primarily race cars, it means the heavier chassis is likely to be less competitive. You only have to look at F1 to see the big difference in lap times between full tanks and near empty tanks.

Your formula is an excellent concept and I agree that could be an interesting yardstick. Vehicle mass is easy enough to determine, but torsional stiffness which should be a standard measurement, is either not usually listed, or available, and when it is it is not often done using the same methodology from kit to kit. See this thread http://www.gt40s.com/forum/gt40-tec...-torsional-stiffness.html?highlight=torsional

So you come back to chassis weight and total weight and have to make a judgement as to the merits of the design and construction and materials used. It is probably to a large extent personal preference where one draws the line between excessive weight and excessive fragility. Horses for courses but it's nice to be able to compare.

It is also easy to build a heavy death trap so one must use ones own judgement about the engineering and design involved.

Cheers

 

[speed220mph]

This ought to blow all of your minds. The original MkIV tub as tested at the Ford testing facility showed a torsional rigidty of 7,500 ft-lb/degree. I don't remember what the bending figures were. Bare weight was 75 pounds. We used a honeycomb sheet as a main part of the structure, which was glued and cured over hours in a large oven, a very expensive operation.

 

[Russ Noble]

I don't think 7500 ft-lbs/ ° is particularly high in an absolute sense but given the 75lbs tub weight it is exceptional for a two seater. And it was obviously sufficient to do the job.

That is the point, to build something as light as possible, that will work as intended without going overkill on the design criteria. No doubt Ford could have built a tub much stiffer, and presumably heavier, but from a practical perspective they may have effectively achieved nothing from doing that, other than making a heavier, slower car.

This was one of the reservations I had about Iains beautifully built http://www.gt40s.com/forum/gt40-build-logs/22164-rf-117-a.html , there is a lot of steel/weight in his chassis/roll cage but he has explained that he is governed by a minimum weight and will have to add ballast anyway so in that case it makes sense to do what he has done. With no weight restrictions he may have built his structure completely differently.......

Cheers

 

[Julian]

Can't tell you the weight of my DRB chassis, but I can say that it's quite beefy in comparison to many other kit cars I've come across. Iaian is fortunate in that his car is a RF and can be log booked by CAMS as a 2B sports car. For some reason that neither CAMS nor DRB can seem to resolve, the DRB GT40 is not on the 2B sports car list, even thought the DRB Cobra is! I gave up in the end and log booked my car as a 2A open sports car. Problem solved!

Had my completed car on the weighbridge last week so I can get it registered. Came in at 1060 kg with not much fuel. Still need to fit other fuel tank, so final weight will go up maybe 15 kg, and also fit roll hoop, say another 10 kg. But alloy heads will hopefully compensate for the above additions. So it should be under 1100 kg when on the track with one full tank (40 L) of fuel, which should make for a competitive package. I hope...

 

[Doug Clark]

I had a quick read through the torsional stiffness thread. I don't want to stir that up again

Having read it though some of the general points, stiffness of the frame is not the same as stiffness of the car so apples do need to be compared with apples. Secondly it's easy to get high numbers if the test rig over constrains the movement in the component.

I would also say that that there's no real point in manufacturers publishing these numbers as it just turn into another spec sheet bragging match just like weight and BHP are today.

Another thing is that people consistently underestimate loads induced in vehicles, the math of a wheel going over a kerb is a lot different to what actually happens to a real car, this is why volume manufactures beat the heck out of their vehicles at the proving grounds and designers are constantly being surprised by parts that break which 'shouldn't'.

Looking at RF117 thread the words strong, stiff and reliable come up a lot and light doesn't.

 

[Iain]

I am a strong beleiver in the quote from Colin Chapman
"The three most important things in building a race car are weight, weight and weight."
From my perspective with RF 117 we are oblidged to start with the standard car as produced by Roaring Forties, we are then allowed to alter that original package according to the rules governing our class. We have removed absoultley everything we are allowed to and have used the lightest components that are safe and legal when adding components, the addition bar work to the roll cage has all been done in chrome moly, we have been very selective in the guage of aluminium we use for panels, we have gone to enourmous lengths to get the really heavy bits,(Engine & Gearbox) as low as is possible in the car and when the time comes to get it up to the minimum weight we can place lead where we want it.
The bottom of the dry sump, the bellhousing and the lowest point of the gear box all sit 165mm from ground level, this involved a custom made flywheel & bell housing, so beleive me when I tell you we have and will continue to think very hard about weight !

Iain

 

[Russ Noble]

The bottom of the dry sump, the bellhousing and the lowest point of the gear box all sit 165mm from ground level,
Iain

Sorry Iain, I don't believe that for one minute! To quote from another forum 'the spreading of misinformation is a cardinal sin':lol:

I bet that's meant to be 65mm, the same as mine.

All the same, I agree with the way you have modified your chassis for the regs that you are running under. The stiffer the better, particularly when you can come in under the the minimum weight. Well done!

Best wishes,

 

[Iain]

Russ, I was thinking about the 165mm on the train on the way home, you are right the ride height is actually 75mm, the 165mm is the dimension from centre of crank to underside of sump / bellhouse / gearbox.

Iain

 

[Iain]

Julian,

As Ross Nicol will confirm the way to get a car onto the magic 2b list is to submit the certificate of description to CAMS with proof that DRB (or the car owners) have manufactured and registered for road use 10 cars in a 24 month period, (used to be 10 in a 12 month period, but thats when men were men !) I went through this process for my Cobra and I beleive Ross went thru the same process for RF, thats why I chose it over any other GT40...... Its on the magic list..... I am sure you / DRB would comply, it does give you a bigger choice when entering events, please feel free to contact me if you need any further advice on the process

Iain

 

[Julian]

Iain,

Thanks for your advice. I've been in touch with CAMS and DRB about this and neither of them seem to be able to sort it out, despite DRB claiming to have submitted all the paperwork over a year ago. I just gave up on the idea and went 2A, which allows me to do whatever the hell I want to the car! To be honest most of the competition I do is Hillclimbs/Sprints, so it don't really matter as I think I'll get thrown into the 'outright' category anyways. The other thing, is in WA for race events it's an all in combined affair for 2B, 2A, sports sedans etc.

I like the extra work you've done the RF chassis. Will be interesting to see what a maximum effort GT40 will do in competition. There's precious little info out there on DRB or RF, so I'll be starting from scratch in getting mine competitive, though not to the same level of effort as yourself! Still, I think it has all the ingredients required to be competitive: Low CoG, wide tracks, adjustable suspension geometry, big rubber & brakes, low weight, good weight distribution for traction out of corners. Just a matter of getting it all to work together in harmony, easy to say, could be hard to do! Looking forward to the challenge though. Spent 10 years or so competing in my old RX-7 prior to the GT40, so was overdue for a change I suppose! I just hope I can get the GT40 quicker than my old car, which is still out there with new owner being campaigned hard and often...

 

[Russ Noble]

Mine weighs 273 lb, but I 'know' of another thats lighter still !

Jac Mac

Can't be mine, that's just come in at 275 lbs including full roll cage See here http://www.gt40s.com/forum/gt40-build-logs/16138-kiwi-scratchbuilt.html

 

[Ian Clark]

Hi Guys,

The CAV GT monocoque bare, with integral tubular steel roll hoop, side impact and foot box reinforcement comes in at 380lbs. That is essentially the complete one piece weldment of the tub. The transxale bracket, rad support and rear body subframes weigh 45lbs. Hmmmm were's my calculator, oh that's 425lbs.

This is all stainless steel by the way and in as much as we hope no one gets hurt in any car, there have been a few CAVs take real hard hits and the passenger compartment remained intack. Ugly but intact.

There is certainly something to be said about putting the weight in the right places, protecting occupants and providing a ridig platform so the suspension can do it's work. I'm sure all manufacturers strive to hit those marks.

Cheers