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

Stephen Burke sburke at burkesys.com
Mon Mar 9 22:29:37 GMT 2009


Hi Dino,

Sorry too for my slow reply.

Its definitely clear that downhill racing is more complicated than 
ordinary mechanically propelled karts!

There are some nice parts on that www.azusaeng.com website. You could end 
up spending quite a lot on a kart! Do you find that supplier good? Does 
delivery cost much? People do be always emailing me asking about suppliers 
of kart parts.

Interesting to hear about the narrow tires providing unstable handling at 
high speeds. In relation to ground clearance, what clearance do you have? 
It sounds bad that you came off the kart a few times. How does the width 
of the kart affect the handling and overturning? 18" at the front and 30" 
wide at the rear, and 77" long.


The engineer at Litton Industries who made the kart which leans over when 
he corners. Its kinda like a side car on a motorbike. Interesting idea 
though.

Adding suspension to the kart will be interesting. I had thought of doing 
it on my karts. It would be easy enough to fit the front fork suspension & 
damper off a moped on the rear axle. Doing this for the front stub axles 
would be difficult however. I would still be interested to see if a more 
flexible chassis would help instead. Going down the suspension route adds 
a lot of complexity.

You have a nice list of features planned for your new car. They are all 
good. I can't fault any of the setup specs you listed. Its a case of try 
it and see, in my opinion.

The very best of luck with the build. If you take any photos, feel free to 
forward on a few.

-steve


On Tue, 17 Feb 2009, Dino Dinaso wrote:

> Hey Steve,
> Sorry for the long wait for my reply.
>
> I'll pick out some of your questions and try to answer them.
>
> Q-I wonder how heavy the drum brakes are however?
>
> A-The drum brakes I use are not too heavy at all, remember weight is your
> friend in gravity powered cars, heavier equals higher average and maximum
> speeds.
>
> Q-Would it not be better to use disc brakes off a small moped/scooter?
>
> A- Disc brakes have more stopping power but can drag if not mounted
> correctly, I like to use wheels and brakes that are a known quantity, so I
> can replace a wheel in a moments notice, I use AZUSA brand wheels and brakes
> http://www.azusaeng.com/
>
> Q- 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.
>
> A- you are correct, taller thinner wheels will give the maximum coasting
> performance, most of world record in coasting cars and human powered cars
> were set with 20 inch wheel sets.
> We have done extensive testing on wheel types that are best for our type of
> racing, taller wheel in general are better, but most important is the ratio
> of weigh distribution across the wheel and tire to minimize the rotating
> mass. The best arrangement is to have the heaviest part of the wheel nearest
> to the center and have tires that are as light as possible; this lowers the
> moment of inertia.
> Unfortunately, if we follow the example set forth by the bicycle racing
> crowd and the other gravity racers and use tall, slim wheels and tires, our
> high speed handling suffers dramatically.
>
> I spent $600. buying five 16" Sachs/ Motobecane alloy moped wheels and
> having them machined to accepted bearings to support 5/8" cantilevered
> axles. We fitted them with super light weight 20" BMX tires and guess what?
> They didn't roll any better, but handled much worse than our 8" aluminum
> wheels with 15" radial tires! (They are for sale, great for the front of an
> electric car!)
> The verdict is that tall, narrow tires provide twitchy unstable handling at
> high speeds and are terrible for cornering.
>
> There is a distinct trade off between performance versus handling with
> gravity car wheel/tire combinations, you want them to be short and fat for
> handling but you also want them to be tall and thin for performance!
>
> For example, last Sunday for my first race, I was testing some new light
> weight 6" nylon wheels with 3.5" wide slick tires that had a diameter of 12"
> on my car.
> These wheels weighed 6.2lbs which is considerably lighter than the 15"
> diameter, 4" wide radial tires and 8" aluminum wheels at 7.8 lbs. (only
> about 2" of the tread of the radial tires makes up the contact patch)
> (The same brake drums were mounted on both sets)
>
> Guess which set up rolled markedly better?
>
> Surprise! The taller and heavier wheels and tires performed much better in
> rolling performance!
> This is because the tires were much lighter at the periphery of their
> diameter!
>
> I'll be getting another set of wheels that are the same diameter but about a
> pound lighter and will use similar light weight tires on my new car.
>
>
> A-As far as we can tell, our group (and the others that we have inspired)
> are the only ones that do real road racing in gravity powered cars.
> We all race at once down mountain roads that have lots of tight turns.
> All other gravity racing is either one car on a closed course with mild
> turns or down a ramp just to see whose car can coast the farthest.
>
>
> Q-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.
>
> A- Our frames typically flex very little
>
> Q- 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.
>
> A- This is exactly the type of set up that I use, one large 3/4 inch bolt
> holds on the wheels which have precision bearings pressed right into them.
> The brake drums are held on with the same bolts that hold the two halves of
> the wheels together.
> I use drum brakes that float slightly, there is a forked section formed into
> the backing plate for the brake and a fixed pin welded to the car rides
> between the forks to allow the brake assembly to float just a little.
>
> Some of our guys have experimented with differential braking like the FOUR
> CROSS racers use, I think that it would be effective on a gravity car is
> some cornering situations but am still thinking about the idea.
> We have one guy that is an engineer at Litton Industries and he has created
> an incredibly complex and literary stunning piece of coasting machinery.
> This mans entire car leans over when he turns.
> It looks really cool, but it does not seem to perform very well and
> ultimately looks unstable when he is turning. It is really an impressive
> marvel to behold though.
>
> There are a few decommissioned Shifter Karts that have been fitted with
> rudimentary fairings that roll and handle well, I can almost stay up with
> them and neither of my current cars has a fairing.
>
> Having good aerodynamics is VERY important for maximizing the performance of
> a gravity car.  (my new car will be fully faired, including the bottom)
>
> After last weekend's hard racing, wherein I rolled my car not once, but on
> three separate occasions, I have decided to add some suspension to the new
> car that I am building.
>
> A few of the other guys have suspension on their cars.
> It seems counter intuitive to put suspension on a gravity powered car
> because one of the main the purposes of suspension is to absorb energy.
> The up and down contours of the road need to be read and exploited while
> driving, it is important to connect as many "falling" sections of the road
> as possible to maximize their positive affect on imparting energy to the
> cars momentum.
> But suspension also imparts positive affects on handling, particularly in
> cornering. I now believe that a well tuned, albeit stiff suspension system
> will be an advantage.
> Anytime a tire hits anything abruptly the suspension well minimize the
> affect.
> I've bought an unbelievable seat ($100.) that belongs in a real race car. It
> is 39" tall and has adjustable reclining position and sliders to locate it
> forward and back.
>
> I've got the seat reclined back at about 75 degrees which puts the top of
> the seat below 30"
>
> Wheel base as measured from the outside edges of the tires is 42"
>
> Wheel base as measured from the center of contact patch/ center of axle
> front to rear is 50"
>
> Tires are 15" in diameter and 4" wide
>
> Chassis is 18" wide in the front and 30" wide at the rear.
> Chassis is approximately 14" high in the front and 30" high in the rear.
> Over length is 77" (I'm a tall person 6' 5" 225lbs)
>
> Right now, I am pretty sure that I will have zero camber.
> I will have about 20 degrees of king pin angle
> I will have about 10 degrees of caster
> Standard Ackerman set up
> Very Slight toe in
> Rack and pinion steering
> Adjustable seat position
> Adjustable steering wheel position
> Basic independent suspension on all wheels, adjustable in rear
> 4.5" drum brakes / cable actuated on all four wheels
> Five point seat belt system
> GPS for speedometer and Odometer
>
>
> Please speak up if you think that I'm heading in the wrong direction on any
> of my design ideas for my new car!
>
> Cheers,
> Dino
>
>
>
>
>
>
>
>
>
> 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
>>
>>
>> www.practicalqualitysystems.com
>>
>>
>>
>>
>>
>
>


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