Ultima LWB

[XLR8]

Thought I'd share this for anyone who might be interested.
Been working on this project for a few weeks and nearly ready to start making chassis parts.
Inspiration: https://www.neovintage-rc.com/projects/project-kyosho-ultima

Chassis inspired by '91 Stealth build.
273mm wheelbase
25 degree kickup
optional battery placement
Ball bearing steering rack from RC10 worlds car.

[eugine]

Very interresting

[Dadio]

Cool , good luck !

[mikem65d]

Definitely locked into this thread.
very cool

[XLR8]

Thanks for the encouragement everyone!
The traditional "pile of parts" photo.

Hardware order from Fastenal should arrive Friday and a decal set from MCI is expected soon. Not going total box art this time - I'm trying something a little different (which, for me, usually means it's gonna be UGLY).

Anyway, enough shelfers, this one's gonna be a runner.
It will get dirty, scratched, beat up, and maybe even broken but it's definitely not going on the shelf.

[mikea96]

This would be great for a re-re ultima

[mikem65d]

Thanks for the encouragement everyone!
The traditional "pile of parts" photo.

Pile o parts.JPG



Anyway, enough shelfers, this one's gonna be a runner.
It will get dirty, scratched, beat up, and maybe even broken but it's definitely not going on the shelf.

LOL....mmmmkay

I ran out of shelves.
Can't wait to see this tho

[XLR8]

This would be great for a re-re ultima

I hadn’t thought of that Mike. If anyone is interested, I’ll post a set of plans for the chassis pieces but first I’d like to have a look at the re-re Ultima to see if any of the bulkhead or tranny screw locations have changed. Hopefully, in about week, Kyosho will release the new car and I can get one on order.

One design aspect that I tend to ignore when building a car is steering geometry; I just assume that the stock steering rack will yield the correct Ackermann for the car in which it is installed.

So this time, since I wanted to replace the stock steering rack that rotates directly on the aluminum pivot posts with a ball bearing rack (smoother operation, more precise, etc.), I’ve spent some time experimenting with different manufacturers arrangements, analyzing different pivot locations for the bellcranks and tie bar lengths until I hit upon something that, although not perfect, seems to be reasonably close.

In both examples shown below, the cars are at the same ride height (front arms at neutral plane), the initial toe-in is set to zero, then the left bellcrank has been rotated 30 degrees. This is probably a little extreme; with 30 degrees bellcrank rotation, the resulting turning radius is only about 2 feet. I could be wrong but I don’t think my other cars will turn in a 4 foot circle. Anyway….

Here is the stock Ultima (260mm wheelbase). In this example, the front wheels are actually a little toed-in for a car with this wheelbase. If toe-in had remained at zero, the axes for both front wheels would intersect the rear wheel axis at the same location; they intersect behind the rear axis so the front wheels are toed in.

And here is the LWB car (273mm wheelbase) with an RC10 WC rack, bellcranks spaced 27mm apart and a 25mm tie bar. Since the front wheel axes cross ahead of the rear wheel axis, the front wheels are a little toed out; certainly not ideal. I’ve tried several iterations, moving the pivot points to various locations, and this is the best I can do. Actually, to achieve perfect zero toe using these bellcranks, they would need to be positioned so close that the linkage attachment boss on the left bellcrank would interfere with the right bellcrank. As it is, they’re very close but they do clear. So, it’s a compromise.

Does any of this really matter? I’m not sure. Since the car will run on loose dirt I’m not sure that it does matter all that much if Ackermann isn’t perfect. Maybe if the car ran on high traction carpet or clay, it might matter more.

[Dadio]

Those pictures gave me a flashback 30 years to my college days studying motor vehicle engineering .

[Coelacanth]

Steering is one of those things that doesn't follow the "more is better" principle. Wheelbase, front & rear arm length, kickup, front:rear weight distribution, camber and the steering linkage system all play a role in the overall steering function. While this all can get very complex, what it all boils down to is where the sweet spot is between understeer and oversteer. That sweet spot varies also with a driver's skill and track conditions. Decreasing a turning radius, for example, is what you want if you find your car understeering, on a particular surface. If the car is either already steering well, or is too twitchy (oversteering), decreasing the turning radius will negatively affect steering or worsen the oversteer.

The Optima is known for having understeer under acceleration, for example. Simply widening the front track might reduce the understeer, or adding weight up front. Or getting more steering by changing the geometry, or shortening the wheelbase, all else being equal. One manufacturer's or car's design might only be optimal for THAT car's steering sweet spot, and not necessarily make things better for a different car, without taking all the other variables into consideration.

The point? You might be over-thinking this, until you test-drive your modded chassis first.

EDIT: I like the idea of going with a ball-raced bellcrank, it's super-smooth and sturdy, and if the linkage is done right, will have zero slop. I went that route when I built my RWD conversion of the Turbo Optima Mid SE.



It's just some aftermarket aluminum bellcrank, might've been an upgrade for a Tamiya, if i recall correctly...but it works fantastic.

[XLR8]

Those pictures gave me a flashback 30 years to my college days studying motor vehicle engineering .

This is what we drove when I was in college..

I don't think that vehicle required Ackermann steering.

[XLR8]

Steering is one of those things that doesn't follow the "more is better" principle. Wheelbase, front & rear arm length, kickup, front:rear weight distribution, camber and the steering linkage system all play a role in the overall steering function. While this all can get very complex, what it all boils down to is where the sweet spot is between understeer and oversteer. That sweet spot varies also with a driver's skill and track conditions. Decreasing a turning radius, for example, is what you want if you find your car understeering, on a particular surface. If the car is either already steering well, or is too twitchy (oversteering), decreasing the turning radius will negatively affect steering or worsen the oversteer.

The Optima is known for having understeer under acceleration, for example. Simply widening the front track might reduce the understeer, or adding weight up front. Or getting more steering by changing the geometry, or shortening the wheelbase, all else being equal. One manufacturer's or car's design might only be optimal for THAT car's steering sweet spot, and not necessarily make things better for a different car, without taking all the other variables into consideration.

The point? You might be over-thinking this, until you test-drive your modded chassis first.

EDIT: I like the idea of going with a ball-raced bellcrank, it's super-smooth and sturdy, and if the linkage is done right, will have zero slop. I went that route when I built my RWD conversion of the Turbo Optima Mid SE.



It's just some aftermarket aluminum bellcrank, might've been an upgrade for a Tamiya, if i recall correctly...but it works fantastic.

Excellent comment with so many good points -- thank you for posting!!
Am I over-thinking this? Of course I am. It's what I do, it's my craft, my passion, my purpose. I over-think, therefore I am.
I guess for me, keeping the front wheels pointed in the same direction is probably more about reducing rolling resistance through the curve than affecting steering response. I'm trying to reduce tire scrub. Again, it probably doesn't matter for a loose dirt track.
Your comments about the Optima are interesting. I guess I've always blamed the very high rear weight bias for the Optima's understeer. That heavy motor hanging off the back tends to make the front end very light - especially under acceleration.
My reflexes have slowed over the years and I run on a low traction loose dirt so I tend to favor some understeer - it makes the car easier to drive - until it pushes out and I clip a tree or a corner post on the deck.

[emerilnut]

This looks like this is going to be cool.

[SteveMax]

I will definitely be following this build

[XLR8]

The order from Fastenal arrived today. Also, I've gotten both chassis plates roughed-out and drilled today. I'm using G10; 1/16" for upper plate, 3/32" for bottom plate. I'll post pics when they're finished.
No fancy tools here; just a hacksaw, coping saw, files, sand paper, a drill press and some numbered drill bits.