[Kartbuilding] "real" Gravity car racing, no motors, but lots of brakes!

[Stephen Burke]

Hi Dino,

Thanks for your informative and interesting email. Its always good to
hear from a fellow karting enthusiast. It sounds like you have a great
opportunity to build, test and race different karts and modifications to
these karts.

I would love to see some photos of the hills you use for racing the
karts! To reach speeds of 50mph must take some steel hills. The karts
would want to be built really really well to achieve such speeds and
also to handle corners at these speeds. From the hills I seen in San
Francisco, I can imagine the speeds you would reach!

I personally think that security can add to the performance of the kart.
As you example, having brakes on all four wheels means you can brake
more evenly and prevent the rear axle from locking up. I wonder how
heavy the drum brakes are however? Would it not be better to use disc
brakes off a small moped/scooter?
The roll bar and 5 point safety harness etc. sounds like you have
everything well covered. I myself have a thick pair of overalls which
take a good lot of the impact if I come off the kart.


As fr the high dollar cars, I can only imagine all the money that would
be thrown at these karts, from special tyres to precision bearings etc.
It would be good to get to study some of these karts, find out how much
they weigh and their wheelbase, ground clearance etc.
I myself don't have too much experience with professional karts,
especially down-hill/gravity powered karts. The professional karts I
know typically have high powered engines and this is where they have the
edge.

It sounds like ye have done a lot of testing with the wheels and tires.
So ye have decided on 8" diameter wheels with 15" diameter tires. Would
it not be better if you could have 12" diameter wheels, and 14"-15"
diameter tires? The tire pressure would have a LOT to play also.

As for the two car's that you have. Car #1 and Car #2. With wheelbase
shape, as far as I know, having a square shaped wheelbase will improve
stability, but reduce cornering capability, and having a long
rectangular wheelbase will have reduced stability but increased corering
capability (I would need to check this). Typically in racing kart
regulations the width is 2/3 the length of the wheelbase. I know that
having the front wheels wider apart can improve cornering.
A few questions: how stiff/flexible is your chassis? It is normal for a
kart chassis to be able to twist and flex a small amount.
The steering setup. The camber and castor angles can make a mega
difference to cornering. At full lock, the outside wheel is not
90degrees to the road, instead it is at a slight angle, with the top of
the wheel angled in a little.

As for the modified shifter karts - I had to laugh - people cutting the
rear axle, so both rear wheels would spin independantly. I think that
the rear axle should NOT be live. I think that the front and rear axles
should be fixed, and to have the precision bearings in the wheels. Now I
know this makes things awful difficult for the braking. Brakes on the
front wheels might suffice. For the front wheels, you might have to
attach a disc brake to the front wheel/hub, and to attach the calliper
to the King Pin in some fashion.


Using a 3D Cad system can greatly improve design, weight and center of
gravity. It is even possible to do a FloWorks Simulation of the kart,
putting the kart in a wind tunnel. There is Cosmos/Simulation FloWorks
within SolidWorks which can do these types of tests. See:
http://www.youtube.com/watch?v=xy8-0FlQeu4
The rolling resistance and mass of the kart and wheels makes a huge
difference. I would go for as light as possible.

As for making the chassis from a complete piece of sheet metal, I'm not
sure how well this will work. It would work ok as long as the weight was
kept down. Personally I would weld up a frame of 1" steel tube and then
fabricate sheet panels to improve aerodynamics.

I took a quick look for "Laydown Enduro karts". I came across a few
pictures ( http://www.ckra.org/newkarter/ckra_whatisit.shtml ). They
look to have a fairly wide wheelbase, however I can't tell the sizes.
What I can tell is that even the wheels themselves are covered to
improve aerodynamics! Every piece of the kart, front end etc. is all
covered.

Well I hope you have got some ideas. I think its a case of trial and
error and refining the kart.
If you have any images of your kart, feel free to forward them on and I
can put them on the kartbuilding website to help any fellow karting
enthusiasts.

Best of luck,
-steve




Dino Dinaso wrote:
> Hello Stephan,
>
>  
>
> Thank you for your web site!
>
>  
>
> I have built and raced two gravity cars here in southern California for the
> last 2 ½” years.
>
>  
>
> I race with a couple of groups, one being the San Fernando Valley Illegal
> Soap Box Federation; we race on steep city streets and mountain roads.
>
> There are about 35-40 cars in the typical SFVISBF race and the courses are
> from 6 to 10+ overall percent grades that are from 1 to 3 miles long with
> top speeds to over 50Mph.
>
> I also race a smaller group (the 805 Crew) that runs the more extreme
> mountain race courses that have from 7-12+ overall percent grades that are
> from 2 to 6 miles long with many more tight turns and attain top speeds to
> over 55 Mph.
>
>  
>
> There are basically three types of drivers/cars; the largest group that
> makes up about 80%, are guys that just want to participate, they are not
> interested in maximizing performance or having good safety systems.
>
> The remaining 20 percent of drivers/cars are interested in maximizing
> performance, but for some unknown most are still are not necessarily
> interested in using good safety systems.
>
>  
>
> We have evaluated all of the great experiments that the big boys have tried
> in what they call “extreme gravity car racing”.
>
> We’ve seen the exotic high dollar cars built by all of the major car
> companies, the super aerodynamic cars with skinny (wimpy wheels), the
> leaning cars, the cars with active mass distribution, the cars with four
> wheel steering and adjustable suspension.
>
> They are all very interesting and all very, very expensive.
>
> But none of those extreme gravity cars races in a group and they don’t race
> down long, winding mountain roads. 
>
>  
>
> I am working on building a new car for the upcoming regular season which
> starts in March (we race all year round as long as the roads are more or
> less dry.
>
>  
>
> We have tried 8 different types of wheel and tire combinations on our two
> cars.
>
> We have settled on using Azusa engineering 8” Tri-Star wheels with 5/8” w/
> precision bearings and lightweight round ribbed tires with an OD of 15” to
> 16”
>
> We also use the 6” Azusalite Nylon wheels w/ precision bearings and 410/350
> slicks that are about 12.5” OD 
>
> We use 4 ½” drum brakes on all four wheels with a 5/16” cable system that is
> actuated by a foot peddle.
>
> We use this wheel/tire/brake system mostly because it is available, light
> weight, permits easy wheel / tire changes, is inexpensive and works well.
>
>  
>
> We have built Ackerman type steering into both cars with a bit of toe in
> (approx. ¼”); we use a small dune buggy type rack and pinion unit with helm
> joints for the control arms and 5/8”-36 splined U joints for the steering
> column linkage.
>
> Neither of our cars currently has any camber or caster built into the
> spindles.
>
>  
>
> We have a swing arm type suspension between the seat and the chassis because
> bumpy roads can be really rough on the bones.
>
> I am the only driver that runs four wheel brakes and have found that it
> gives me an extra margin of safety
>
> I also am the only guy that uses a good roll bar, a five point harnesses,
> body amour, a neck donut, gloves, motorcycle boots and a full face helmet
> with goggles. (I have crashed hard at speed several times and received only
> a few bruises)
>
>  
>
> I have learned a lot about driving and building little cars, but I would
> like your opinions on some design points
>
>  
>
> Car # 1 has a wheel base of 33“W X 52 “L  (and is about 2” high) I have
> found that this car is often unstable when cornering hard at speeds of 30-35
> Mph and has rolled several times because of this.
>
>  
>
> Car #  2 has a front wheel base of 40 “W X 47 “L  (and is about 3” high)
> (rear wheels are 36” apart, an experiment)  I have found that this car
> corners much better at speeds of 35-40 Mph and has only rolled once in a
> corner at about 42-45 mph in a bad reducing radius corner.
>
>  
>
> There are always trade offs, the fastest and best handling cars on very
> steep hills are decommissioned shifter karts that have had the rear axle
> parted to create two separate stub axles with a disc brake mounted on each
> axle.
>
> But these same cars suffer from massive rolling resistance by virtue of
> their wide tires when the slope decreases; here the cars with the taller,
> narrower wheels have a rolling advantage.
>
> My #2 car can out roll the bastardized shifter karts, but because they have
> better handling, I must slow down more in the really tight and fast corners.
>
>  
>
>  
>
> By my understanding, in regards to high performance gravity car racing, the
> most important elements are rotating wheel mass, rolling resistance,
> aerodynamic drag and car weight.  
>
> My current plan is to use our Solidworks CAD software to make a chassis out
> of a piece of heavy sheet metal (like a slot car) and have the bottom, sides
> and rear formed together form this single sheet.
>
> (We have a manufacturing company and some great sheet metal vendors that can
> do this work very easily) 
>
>  
>
> The goal with this new car is to design it with minimal frontal area and to
> have a fully faired body to decrease aerodynamic drag.
>
>  
>
> But, I also want to build in maximum handling and durability, as we race
> mostly on steep mountain roads that have lots turns.
>
>  
>
> Right now, we are planning to build spindles that are set to 10 degrees
> camber and 20 degrees caster.
>
> The car will be 2”-3.5” off the ground depending on which wheels/tires we
> are running and of course we’ll incorporate Ackerman type steering and a bit
> of toe in.
>
>  
>
>  
>
> Given the type of car we are making and the type of racing we will be doing,
> can you suggest any ideas that we might include that could improve our basic
> design?
>
>  
>
> Do you know the guidelines for establishing the optimum wheel base ratio of
> width to length? 
>
>  
>
> The design is starting to resemble a “Laydown Enduro kart”, would know the
> wheel base for these sorts of cars?
>
>  
>
> Thanks very much,
>
> Dino DiNaso
>
> Office voice / fax: (818) 993-1022
>
> Mobile: (818) 458-1367
>
> E-mail:  practicalquality at earthlink.net
>
> www.practicalqualitysystems.com
>
>  
>
>
>