A locost style track car, Car9

[horizenjob]

This winter I decided to stop day dreaming and try and get my ideas for a Super Seven / Locost type track car out of my head and on to paper. I have some questions and could use some (maybe a lot of) advice.

For this type of car to see much track use it seems reasonable for it to have a roll cage. With that as a given and the fact it is a tube frame car to start with, it then seemed reasonable to start with an SCCA spec roll cage and attach the rest of the car to that. In a normal Seven / Locost ( and most Lotus's ) much of the chassis stiffness and strength comes from the transmission tunnel or backbone. I wanted to avoid this because it doesn not contribute to driver safety. It sort of puts the driver between a rock and a hard place . In Car9 the weight and strength was put into the cage around the driver instead.

This frame looks different then the traditional ones, but it is similar in size and would basically fit under a Locost's bodywork. Raising the upper tubes under the hood and connecting them to the rear roll bar braces adds stiffness lost by the minimal driveshaft tunnel.

Here is a picture of the rear of the frame. My first questions are about the legality of the roll cage. The upper rear braces for the roll bar are at the required 30 degree angle backwards, but I can't tell if it's allowed for the two braces to meet in the center of the back. The other question is wether it's OK to tilt the main roll hoop a few degrees. Even 5 degrees would help me a great deal. I don't mean extra or out of plane bends, just the whole hoop tilted 5 degrees.

I have worked hard to design the rear of the frame to not have tubing around the IRS that would intrude into the passenger compartment in an accident. I was surprised during the modeling how important the tubes behind the driver were for stiffness, The two little tubes under the middle of the rear hoop contribute 30% to the stiffness of the whole car.

I'll get a picture of the front of the car soon, just like in real life the drawing is taken apart right now for more work

[brucesallen]

The driver protection bars along side are good and bad. Great for protection but with the very narrow cockpit I wonder if you can fit in. I wonder if you will need to bow the bar out some.

[horizenjob]

Here's another picture with a driver. I realize I haven't really given any background or even dimensions yet.

I am drawing the project in Google SketchUp. I don't know if that is the best choice, but it is free and available for both MAC and PC. I keep a copy that I update in the Google 3D Warehouse, so anyone can download it and play with it and even improve it and upload. SketchUp supports "layers", a way to stack or enable/disable different parts of the drawing. There are 15-20 layers so far and they include different parts for the suspension, a Ford Duratech and a V8, Subaru and Ford Thunderbird IRS, driver, some bodywork etc. I'm either indecisive or perhaps this will help other folks who want to draw their project,

I originally drew the car as 40" wide, but a friend wants to build a V8 version so I made it 44" wide. The nose is about a foot longer so I think the proportions are not too different. Wheelbase is 96" now, but can get 2-3 inches shorter if I tilt the roll hoop.

It's not drawn in, but the cage would need appropriate padding around the passengers. I drew the passenger as a lesson in fractions for my son. I'm not sure if he's scared of my driving or embarrassed to be seen in public with me!

Just like a real shop you can see the mess cumulating in the back corner.

[paultg]

How do you get in/out with the angled side bars up past your head?

Very cool project!

[horizenjob]

How do you get in/out with the angled side bars up past your head?

I think you can just step over them. I don't know if you would step onto the seat or the transmission tunnel. On my FF you use them to lower yourself into the seat, but that won't work here. It's a good practical question and it's hard to tell from a drawing. I wouldn't mind if it had a step on the back of the roll bar or marking like on an airplane wing that should where to step on the trunk. Not sure that helps though.

I wonder if you will need to bow the bar out some.

I meant to answer this with the post above, it seems the driver fits but I am finding it's hard to tell some things from drawings and pictures. Last time I was at the track, Les let me sit in his car but it's still hard for me to picture. Maybe I need to mock up the cockpit a bit with some scrap wood.

The thought is to design the car for the track without compromise. It will still basically be as useful a street car as a Super Seven / Locost. Then if someone builds this and decides on some compromises for street use, that's possible. You could leave those forward braces off or add curved braces outboard of the current frame.

Here's a picture of the frame being modeled in FEA ( Finite Element Analysis ). This type of software can calculate the stress and strain of every tube in the frame in response to a load. I use this to see how much the frame twists and how stiff the suspension attachments are. This turned out to be a lot of fun and a good deal easier to learn then the drawing software. It was also very frustrating. One of the basic ideas was to get a stiff chassis by making the sides tall, so the forward braces of the roll bar connect into the upper chassis rail in a strong place and the upper chassis rail is taller. This is like a modern Formula Ford. It turned out though that the main roll hoop warped under stress. So the way the rear bracing for the hoop worked out was crucial. Effectively the rear braces come together in a pyramid shape at the back and they provide a huge boost in stiffness.

In the FEA software you can tell it to virtually clamp some locations of the frame and apply loads on other places. In this picture the front suspension mounts have been fixed and also the rear center of the frame is on a pivot. Then a vertical upwards load of 1000 lbs. is applied to the main roll bar on the driver side and a vertical downwards load of 1000 lbs. is applied on the passenger side. I picked the load because it's a nice round number and is also more then normal use. Currently the frame flexes by about 0.177" or about 3/16".

[962porsche]

from the top of your head to the top of the main hoop ( broomstick ) how many inches ?
the top of the main hoop looks very tall .
do the upper door bars drop lower in the back were they meet the main hoop ? or is it just an allusion ?
with in reason the closer you can have the upper side beams to the harness bar the better .
the dash bar has no support from folding over on it's self the same goes for the top of the main hoop .
when ever possable it's always best to weld bars coming into a bend area at center point on the bends . this would apply to things like the dash bar and tops of main hoops .
because your getting away from just a roll cage and more of a tube chasses email me dkbairbrushing@conversent.net the designs you have and i would be more than happy to go over some options .

[horizenjob]

Thanks for the detailed response, plenty to think about!

the top of the main hoop looks very tall .

It may be a bit tall now. The little dotted line on the drivers head is supposed to be 2 inches tall, but doesn't account for the helmet. I've moved the bar up and down a bit. I'm 5'8", some people who drive the car might be taller. I do expect the driver to sit low in the car.

do the upper door bars drop lower in the back were they meet the main hoop ? or is it just an allusion ?

They do, it's a nod to traditional Super Seven styling. They are about 16" above the bottom of the bottom rail at the main roll hoop.

the dash bar has no support from folding over on it's self the same goes for the top of the main hoop .

I don't think I understand. There is a diagonal from the passenger side of the dash hoop to above the clutch pedal on the firewall. I had a bar across the dash hoop just under the bends, but it didn't seem to carry any loads from the suspension, so I deleted it.

when ever possable it's always best to weld bars coming into a bend area at center point on the bends . this would apply to things like the dash bar and tops of main hoops .

Was worried about being able to do the fishmouthing of the tubing and also getting in the driver's way. I may need to build a mock up out of wood scrap to see if that would work.

The models of the traditional Locost chassis I was using weighed about 120 lbs. and this chassis comes in at 130lbs., but it's several times stiffer, I think. There could be mistakes in the models, but I don't think they are far off.

[John F]

Like a Factory Five challenge car without the skin

[horizenjob]

I'll try to get some pictures with skin soon. I don't have it showing now because I'm asking for advice on the chassis. I am trying for a look somewhere between a Lotus Super Seven and 60's Indy and F1 cars with longer noses.

How many cars does Factory Five make? What's a Challenge car? It looks cute.

The goal for this car is a very simple, strong and stiff frame. So it's pretty minimalistic. Just from a quick look at the challenge car, I think this car is a good bit stiffer. Tube frames are potentially so light, I'm not sure it makes a big difference so far as weight.

I still have to finish redrawing some things, but here it is with a V8.

[horizenjob]

I can give a quick explain for why this car is so stiff when you look at the picture above. The roll hoop has vertical legs which allow the forward braces to be straight. That basically doubles the height of the frame in the cockpit and when you double the height of a truss it becomes eight times stiffer. Then the rear braces meet together at the back and from a pyramid with the truss beneath them, tying the bottom rail and the top rails into a point.

Before I worked out the rear pyramid the frame flexed by warping the main roll hoop. The bottom of the roll hoop leg moved fore and and aft 40 thousandths under severe load. David (962Porsche) sent me a PM and said it wasn't legal to join those rear roll hoop braces together in a V. They will have to come back straight ( or nearly straight? ) and maybe I can adjust the truss underneath them to make up for that. The details around this are probably 30% of the stiffness of the car.

[horizenjob]

BTW, I may not have made this clear, but I consider this an open design. That's why I upload the drawings to SketchUp 3D warehouse and also publish the FEA models. There is a thread on LocostUSA.com about this. I've come here to get advice from people with more track experience. So it's just now that the project has made enough progress to get interesting. I'm getting some help with the numbers on the suspension design so that will start showing up soon.

So if folks have comments about what they would like to drive or what's attractive or unattractive about this project please say something. This should be a genuinely fast car even with small or low powered motors. I don't really want a car any slower then my Formula Ford and I don't want to lose the feeling of driving a quick nimble car that you don't really think about driving. With this thing you should just feel like a pair of eyeballs flying down the track!

I thought bodywork might look like this. I'll just be up front and say my drawing skills are not all that great.

[962porsche]

when ever i build a chasses or cage the things i look at are .
1st . what is the cage being built for ? type of racing , class , wheel to wheel or time attack ?
2nd . safe and strong . you always must thing of the worst case scenario in a crash .
3rd . as many straight run with the tubing as possable .
4th . as light as it can be weight is the key .

i tend to try and build cages to fit a driver of 6' 2" tall thats when ever i can .
when it comes to fishmouth you will just have to grind out untill the tubing meats flush . i use a 4 1/2 " flapper wheel after i use a tubing knocher.
i like to mig weld my cages not tig weld them . a tig weld may look pretty but it's a softer weld and it will build allot of heat . i tend to use .035 or .040 wire in the mig .


some things i'm looking at and would change are .
were the 4 link picks up on the front legs of the hoop . now i did not see were you stated what tubing size your going to use ? but if your going to use say 1 3/4 tubing with a wall thickness of .140 your asking allot out of it to carry suspension loads with out any bracing to the suspensions pick up points . i would remove the cross brace there and then X-brace from the suspension pick up points to the tubing on the fire wall's tubing .
because there is no bracing on the dash bar . i would run some tubing from the front of the dash bar again to the tubing of the fire wall . as it is now flip the car on it's roof and skid it along the road and that bar could just fold over on it's self . think of the worse case !
the same hold true for the top of the main hoop were there is no support .
i would run the back stays ( rear legs ) bars from the top OR as high is you can go on the main hoop straight down to the upper rear hoop . and remove the V bars there . you could add in a bar that goes from the upper left to the lower right backstay . but that will add unwanted weight . i would do it to a car that gets wheel ed to wheel ed raced but not a time attack car .
by doing the rear backstays that way you will need to redesign the bracing from the upper to the lower hoops . if you just turn what you have up side down the point of the triangle should come in contack with the pick up of the back stays on the upper rear hoop .
the harness bar needs to be moved up to sholder hight . i still don't like the fact that there is no other bracing in that area fron the main hoop / harness bar to any other tubing . your trying to fallow the line of the side panel but i would try and some kind of triangled bracing there .
i got your email of the chasses i will draw out the changes i would do and send it back to you . because i only have a rendering of the chasses and i'm not there in front of the car i do not know just were the skin of the body sits .

[horizenjob]

The tubing I would like to use for the cage is the minimum in SCCA's specs. I should check NASA too. So that's looking like 1 3/8" x .083" or .095" for a car that weighs less then 1700 lbs. (with driver?). That's what you see in the drawing for the required parts of the roll cage. The smaller round tubing is 1" and there is also 1" square and 1"x2" or 1"x3" rectangle tubing. Those tubes are all .065".

I agree the trailing arms were not strongly located. I haven't done the math on those mounts yet. What I have been thinking was using a piece of rectangular tube welded to the back of that roll bar leg to make the mounts. You would have to remove one of the narrow sides of the rectangular tube. I think that will make the leg enough stronger and not interfere with the bracing that's already there. I put in a picture below.

I am working on the drawing of the roll bar rear brace. I hope it can be allowed to angle in a few degrees. The bracing under this is difficult to arrange without getting in the suspension's way. I tried hard to arrange for parallel links, but in the end had to opt for a reverse wishbone. In the drawing below I have the new rear brace at a 35 degree angle going straight back. I'd like to redraw the rearwards facing hoop and have the rear brace connect in the middle of the bend as you recommend. I don't think I can re-arrange the truss under there, but can put in a vertical tube.

[horizenjob]

Here's a new picture following some suggestions I received. It's been a little slow because my computer dies, had to borrow a machine, restore backups reload software and learn how to use Windows etc. I'll have the old machine running again later today or tomorrow and be able to pick up the pace again.

In this picture I have been able to make the rear braces face much more backwards and they do not share a node anymore. I hope some tilt inwards will be allowed because there is no car to attach them to if they go straight back. The main roll hoop and suspension bulkhead have been tilted backwards by 8 degrees. This has let me move the rear suspension forwards by 2-3 inches so the wheelbase is getting down to 92"-93" and we're getting some more weight on the rear wheels. I think we can use as much weight on the rear as we can get.

I also drew in some more realistic mounts for the forward end of the rear suspension trailing arms. The front hoops lower legs are reinforced with some 1x2 rectangular tube in this area.

I haven't done the FEA modeling yet but will work on that next.

This picture is still using the rear suspension from a Subaru - R160 diff, halfshafts, spindles and brakes. I think that's a good choice for moderate I4 engines. Things like Ford Duratech are only something like $500 in junkyards and are good for 140-180 HP without having to open them up.

I'm going to start using some Ford IRS parts in the drawings now to see how they work out. They are also very common and available new from the Ford Motorsports catalog. The variety of diff parts (gears and LSD), halfshafts and brakes seems attractive. The problem may be getting small enough parts. The diff weighs only a few pounds more.

David, 962Porsche, provided a diagram and some comments. I'll put them up later when I get my regular machine going and can re-size the drawing etc.

Thanks for the help folks!

[horizenjob]

Here's the sketch that David sent me in an email. He said it was OK to post the picture, so I'll make a small leap and include a little of his text.

Davids_frame.jpg (49.43 KiB) Viewed 12073 times

with the pick up points for the 4 link bars 14 and 15 I have 3 other ways that could be done . the way it is in my drawing would be the lightest and strongest . I move the cross bar # 3 up to were bars 17 ,18 , 19 and 20 meat . this I did because its seams to be at a better angle and it's now picking up were there are 6 triangles coming together . in the design you sent I can not see how all the bars meat at the base of bar # 1main hoop .

Thanks for taking the effort to send me this. I am basically doing what you suggested for the rear hoop braces. I didn't flip the support tubes in the middle back, just because it looks like they give a little more protection from the rear this way. In your picture your way looks better! I think I can flip them the way you have them anyway because there will likely be another tube as part of the diff mounting - so good protection in the back.

I see your thoughts on preventing the top part of the hoops folding. There is a variation in the drawing that has a high front hoop. I haven't shown the picture yet because it makes the car look like the one in the "Munsters" TV show. I have been putting a lot of effort into making the car stiff and the way the roll hoop braces connect and make a bridge from the back to the front of the car is a big part of that. It has been very hard for me to get those forward hoop braces to carry a load and not be dead weight. So I am reluctant to make those changes in your diagram. The braces for the front hoop top don't have to be the same material, 1" 16 gauge would be enough to add to their strength. These can likely brace the engine compartment as well, I'll try to draw that up soon.

For the same stiffness reasons, I think we have to leave the diagonal that is between the dash hoop legs and the firewall. That doesn't prevent something like your x-brace going in too, and to take those compression/tension loads from the trailing arms would not take large or heavy tubing at all. In my next post I'll show what the model is saying about the concerns on those trailing arm mounts, I've been wondering about them all along.