Gravity Racer, take III

#1
Yes, I am committing to another build. It is an addiction...

Let me start by saying that I was extremely happy with how the Mk V worked out this past year and I am not sure that anything else I would build will be able to out run it. It will be difficult to put aside a car that is running within a sneeze of the track record which was set on a MUCH more favorable (HOTTER) day. Despite this, I think I need to try. This is not the radical departure I was contemplating last year, but a fairly conventional trike layout, with one front wheel and two rear wheels. Why you ask? Read on.

At SEMA this fall, I chatted with someone who runs at Bonneville with a sidecar motorcycle and we had a conversation I just couldn't get out of my head. He asked me why I did a couple of things a certain way and then told me what he did and why and what the results were from their wind tunnel testing. Sometime during that ~10 minute discussion, I realized I was going to be building a new car for this year.

I dug up the go-kart front end and acquired an old junked bike to put together a prototype chassis, adding in the front wheel of a pocket bike. This minimizes the height of the running gear underneath the bodywork. I think there's a bit more rolling resistance with smaller, wider, and stickier the kart tires, but I am hoping the smaller size turns into an aero advantage once at speed (>20mph or so). In switching to a single steered front wheel, it also simplifies/reduces the components out in the airstream as well.

My main concern is stability through the Learning Curve. I was pulling almost 1g laterally through there last year and I really don't want to have it flop over on me. The track and overall width will be similar, although narrower, but I am also thinking I will be able to keep it even lower overall than the Mk V as well. Dynamically, I don't think the corner cares if I have one wheel in the lead or in the trail position, as long as the height of the CG is low enough to keep some weight on the inside wheel. I would be curious if anyone has any evidence to the contrary. There was a lot of discussion when the Delta wing came out. I am not afraid to give the concept my own shot.

So, without further adoo, I introduce the prototype for the Scuderia Kouba Mk VII:



It doesn't look like much right now but it will come around. The handlebars are on there just as placeholders, it will not be head first. I have the steering planned out regarding how to get it steered while laying down with my head at the stern. At this point the proof of concept indicates that it can be done, so I will be giving it a shot.

I have a new frame started which should be much more square and flat, and it will carry a wider track rear axle- it will be the go-kart front end with fixed tie rods, so that I have independently rotating rear wheels. I still need to figure out a few dimensions for it but it's definitely on its way. Watch this space for further updates.
 
#4
People are constantly (continuously?) re-inventing the three-wheeled vehicle. It has a fundamental problem- the end with one wheel has zero roll stiffness and, accordingly, the thing's handling is atrocious, Morgans included.
 
#5
Roll protection will absolutely be incorporated. It may not be SCCA spec but I don't want to scratch my skid lid.

I admit, I am skeptical regarding its cornering ability. The single thing making me go forward with the project is the rest of the fleet I have built is 3-wheeled and corners just fine. I am well aware of the Robin film and its roll-over talents. Looking at its layout, it's quite obvious as to why it turns turtle at the slightest hint of a corner.

Looking at the Morgan, the motor is down between the wheels and the cockpit is both narrower and lower than the Robin, making it respond differently to directional changes. Looking at the Mog, would it be any more or less stable if its driving direction was turned around? Physics would still apply the same force to its CG, introducing the same moment about its roll center, right? I've seen the Hamster flog one around pretty liberally (I think he has a thing for them) and it seemed to be under control.

Also, the Mk V is 22" tall at its tallest point. It was that tall because they are 20" tires and there's ~1.5" of ground clearance. The Mk VII has 10" tall tires, so it will only be as tall as I need to see over my own feet. The rear end I will be using is a bit wider than the one shown in the pic above- maybe not quite twice as wide, but close. I think the geometry can be made to work.

So yeah, there's no lateral weight transfer on the 1 end but there is massive roll stiffness on the 2 end. Based on the mechanics working out very well on the other three entries which have been down the hill successfully (albeit in reverse), I am willing to give it a try.

I am also willing to admit that I may be wrong, but I'll never know if I don't try.
 

Doug S.

I may be 70, but still trying to age disgracefully
Lifetime Supporter
#6
Glad to hear you're going to do this again, Chris! Last year's race was a great "write-up". I'll be anxious to see what comes of your creativity and driving this coming season.

Cheers!...and good luck!

Doug
 
#7
Thanks Doug!

Worked on the new car a bit this morning, getting the rear axle ready to attach to the new frame. Have it all laid out and aligned on the shop floor, just need to commit to a method of attachment. The rear axle is quite a bit wider than the live axle one from the prototype:



Frame members are just tacked together and the front fork is aligned and tacked in with two little scraps to hold it while I lock it in. Rear axle is just placed over the aft end of the frame. I need to figure out where the top rails of the chassis will be positioned and then start tacking them in as well, and then determine the wheelbase and weld in the rear axle.

From there I will do the steering system build out and then finish the brakes. After that it'll be time to make a body, so yeah, it's like I'm almost done already.
 
#10
Rear steer is unattractive to me, as the primary method of directing the front end into a corner is to direct the rear end out. That feels like a recipe for disaster. I was privy to discussions about rear steer in a Formula SAE car where they tried to incorporate it but they found it a bit unstable in cornering. That's enough for me to walk away for a vehicle that will be used once a year and needs to be built for less than $500.

I've been thinking a bit about leaning but haven't quite gotten a manufacturable solution.

In the 80's, GM (I think) came out with what they called the Lean Machine. I was thinking about that as I started this prototype but have no quick and dirty way to make it happen. Maybe it's something to think about for next year, as long as the 1F2R concept plays out.
 
#12
Rear steer is not a good choice. I asked Andy Green how he ever became comfortable with the rear steering of Thrust SSC and he replied "I never did really".
 
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