Gravity Racer, part IV

[Doc Watson]

'lowest drag body I can fit over myself ' .... try this?!?!

 

[Chris Kouba]

Sorry Doc, need a palate cleanser....

Sticking to topic though... it has been tried:

Was never exceptionally fast and with the real soapbox wheels, it made a racket of a roar when rolling down the hill at speed. The human body never evolved for aero considerations, so we're better off building something around us to slice the air.

 

[DaveC]

it has been tried:

He had everything backwards! Only weenies go feet first!

 

[Jac Mac]

I wonder if a better ( project specific ) solution might be to move the front/steering wheels on a positive/negative camber arc, this would have the effct of widening the front axle to the outboard tire when under corner loads ( just like a bike when you lean into the corner. )

 

[Chris Kouba]

Another long-time competitor (Charles) has it down though:

This may look like it's supposed to be a fast layout, but he struggles to break below 78 seconds.

There's also a rule about driver's head needing to be at least 2' behind the front bumper of the car, so that would stop a true skeleton sled type of entry from being run.

 

[Randy V]

I wonder if a better ( project specific ) solution might be to move the front/steering wheels on a positive/negative camber arc, this would have the effct of widening the front axle to the outboard tire when under corner loads ( just like a bike when you lean into the corner. )

So, turning Left, the RF wheel would be in Negative Camber and the LF wheel in Positive Camber?
If so, it would seem the track width would not necessarily change - rather it would shift to the right of chassis centerline.
Ackerman must be in play here somehow and I can’t wrap my head around that this morning..

 

[DaveC]

So, turning Left, the RF wheel would be in Negative Camber and the LF wheel in Positive Camber?
If so, it would seem the track width would not necessarily change - rather it would shift to the right of chassis centerline.
Ackerman must be in play here somehow and I can’t wrap my head around that this morning..

I'm guessing Ackerman could probably be ignored given the narrow tires and lack of low radius turns?

 

[Randy V]

I'm guessing Ackerman could probably be ignored given the narrow tires and lack of low radius turns?

Possibly - but I have to wonder if it is wise to ignore Ackerman since the primary use of Ackerman is to increase traction in turns.
Like you - I suspect, perhaps, the turns that these Gravity Racers take would have relatively wide radius's and maybe Ackerman is no big deal to them.

 

[Jac Mac]

So, turning Left, the RF wheel would be in Negative Camber and the LF wheel in Positive Camber?
If so, it would seem the track width would not necessarily change - rather it would shift to the right of chassis centerline.
Ackerman must be in play here somehow and I can’t wrap my head around that this morning..

Correct on all counts... weight and any induced downforce ( really its centrapental? force, might have to dig out your old principles of flight ). Since the inside wheel has less induced load that is transfered to the now wider-shifted outboard RH tire contact patch the RF wheel thinks it is on a wider axle. Ackerman is still very important as Too much or Too little will induce more scrub/drag and slow the racer down. A simple way to test the theory might be to place a prone board on a ten speed bike & head down hill. In the video's the rear wheel looks to be ready to snap into oversteer at a moments notice so similar mods to the rear might help as well.

 

[Chris Kouba]

Figured out the rear suspension today using an old front unit off the GT, meanwhile the front end is being held off the stops by bungy cords (for now). I did roll it out for the first time to see it beyond the constraints of the shop. I think it's off to a good start:

Triangulation is coming, just wanted to get through the layout first. Also a roll hoop, brakes and steering.

On the geometry discussion, I don't lose sleep over Ackerman due to the minimal amount of lock we use during the event. With the camber I have built in, the outer wheel does tip inward as more lock is applied, but not by a massive amount.

Jac- What video are you referring to? One of the derby cars?

 

[Chris Kouba]

I'm guessing Ackerman could probably be ignored given the narrow tires and lack of low radius turns?

You are spot on Dave.

 

[Jac Mac]

Figured out the rear suspension today using an old front unit off the GT, meanwhile the front end is being held off the stops by bungy cords (for now). I did roll it out for the first time to see it beyond the constraints of the shop. I think it's off to a good start:

Triangulation is coming, just wanted to get through the layout first. Also a roll hoop, brakes and steering.

On the geometry discussion, I don't lose sleep over Ackerman due to the minimal amount of lock we use during the event. With the camber I have built in, the outer wheel does tip inward as more lock is applied, but not by a massive amount.

Jac- What video are you referring to? One of the derby cars?

Post #13 on this thread. Have you ever tried a run down one of the courses on a Bike to see how much it has to lean to negotiate the course?? No cheating, dont use the brake!

 

[Chris Kouba]

Those moments in the video are induced by hitting potholes or a sewer. They do look pretty scary but it's a consequence of no suspension and hitting something. When the pavement is smooth, they drift through the big turn quite well. The latest winner was probably the most pronounced drifter, and I assume that to be a based on its weight distribution. I don't remember the 2018 car drifting much, and it had more weight on the front wheels in total, plus it had TWO wheels (and tires) in the back.

I have taken a bike down the course and fortunately, it just about levels out on the actual run into the Learning Curve and even gains a small amount of elevation through the apex (1-2' according to GPS). Coasting through there on a bike had me maybe in the 20's and wasn't a challenge at all. The bike racers I've talked to only talk about getting to around 40 mph when they've had races there.

 

[Chris Kouba]

Now with steering!!

Yeah, the plastic strip is a little wider than needed but it's a good help with the visualization. The lumber scraps are also for visualization only. Whatever goes back there needs to be strong enough for the tow-back duties on the hill.

Still needs a bit of alignment and real springs for the front end. The good news is that it does spontaneously roll across the shop floor. That is excellent news!

 

[DaveC]

What vehicle is used for towing? Does the participant tow, or does the event organizer?

 

[Chris Kouba]

Tow hooks front and rear are required and we are daisy chained up behind volunteers' cars. It's a fairly massive event which relies upon many, many volunteers, and they just use their personal vehicles to tow back up. Each team is required to furnish at least one volunteer, and we have had at least one person on the tow back squad each year.

Found a pic from 2019:

 

[Chris Kouba]

Brakes installed. Successful first fully functional pass down the small hill in front of the house is in the bag. Steering is comparatively slow, but I think it's appropriate. All the others were super sensitive. I need more spring rate in the front and can probably drop the rear a little bit- that will come with real springs instead of bungies though. The real breakthrough... the BRAKES!!!!

They just plain work!

They are awesome. I built a foot pedal and with a moderate push, the car just stops. It's fantastic. It's never happened before. They are phenomenal.

The friend who drove the Mk V to 3rd place last time, Andreas, was over tonight and he made a pass in it, commenting on how smoooooooth it was with the suspension. And regarding the comparison to the go kart hardware, there is no comparison. The 16" chassis rolled up and rounded the corner whereas the 10" chassis barely made the bottom of the saddle. Not going to bother with any further development on that chassis...

With Andreas on board:

With a little more development at the house, it should be ready to head out for a real test day.

Good times!!!!

 

[Tom]

if this buggy turns into a real rocket, I wonder if fiddle brakes on the front might come in handy to help initiate turns, assuming scrub radius on front is minimal.

 

[Chris Kouba]

Not familiar with fiddle brakes.

Are they the same as "turning brakes"- pulling on one side or the other? I'd pass on that concept. The brakes are not used on course, or if they are, something has gone wrong and you have lost.

 

[Doc Watson]

...but where is the black latex suit?.... ;-) looking good Chris.