A locost style track car, Car9

[962porsche]

marcus .
using the numbering system in my drawing who long are the runs of the tubings from the # 2 dash hoop to the #1 main hoop . that would be bars # 17 18 and 22 .
how much better and or stronger would the chassis be under pitch , roll and yaw along with side impact if you split that run in half and V braced it from tubing # 1 to tubing # 2 .

[horizenjob]

The front and rear hoops are about 32" apart, it's tilted now so varies a bit. Let me think about your question a bit. Not completly sure I understand. Do you mean putting an x brace between the hoops? Instead of the middle bar?

Here's what I found looking at the mount for the rear suspension trailing arms. I thought I would start with just using the roll hoop material, which is 1 3/8" x .083" tubing. The trailing arms can attach in different spots according to anti-squat / anti-lift and other reasons. Normal settings would be about 5" up or down from the chassis rails. There are pictures below showing the stress and also the humorous way the software thinks the frame would collapse. It doesn't actually have an opinion about that, it just tries to show how the elements move under load.

The load chosen was again 1000 lbs., as a braking load. This is producing one of the highest stresses I've seen in the model so far at 28,000 psi in bending moment next to the chassis rails. That would be well within reason for that tubing. So we are not making assumptions for safety factors or true maximum loads yet, but it's a reasonable ballpark for a sanity check. The drawing above with trailing arm bracket the full length of the tube span would be at least 8 times stiffer. I have to make a model for that shape tube so haven't got real numbers yet, but I feel pretty comfortable after this first look.

The deflection of the tube came to .038".

[horizenjob]

The software I'm using to perform FEA doesn't work with complex shapes, it is used to analyze trusses and beams. That is quite suitable for space frame cars. That means I can't model something like a section of roll cage material with a 1x2 rectangle welded to it along an edge. Using sections of 1x2 and 1x3 rectangular tubes seems like a close enough approximation to the roll cage material with brackets welded to the back side of it.

Here are the numbers and also a picture for the 1x3 tubing substituted for the section of roll cage tubing where the trailing arms connect. This should compare conservatively with the 1 3/8" x .083" roll cage material with 1x2 rectangle welded to it. It is interesting that the stress distribution shown in the picture here is different then the one above. It took me awhile to understand what was going on. In the picture above the highest stress is where the tube joins to the chassis rails, a 28 KSI bending moment. In the case of the rectangular tube the highest stress occurs in the center of the tube, an 11 KSI bending moment. I think what is happening is that the roll cage material by itself is more flexible so it bends enough in the middle to relieve the stress slightly. It can't bend where it's welded to the chassis rail though.

The reduction in stress brings it a value more common in the frame, despite it's weight of probably less then 150 lbs., it is not highly stressed and has a large factor of safety.

The defection at the trailing arm mount only reduced from .032" to .0136". This was a little disappointing and doesn't agree with the factor of 8 I mentioned above. So possibly math doesn't work in this peculiar region of numbers and no one has noticed before, or I could just be out of my depth here - a person better suited to using clubs to solve problems. I will have to look more into this whole club thing.

I laughed, I cried and then I had one of those Doh! moments. The frame is flexing other places under this load and not only that I can use this software to measure what that looks like anywhere else. Nearly half of the displacement is in the rest of the frame, the end of the tube element at the bottom rail is moving by .0057". SO the displacement just within the trailing arm mount is really only .0079". I think 8 thousandths is acceptable for the trailing arm and will not disturb the suspension geometry or cause undesired bump steer.

So that means the deflection has been reduced by nearly a factor of 4. I think the remaining loss in stiffness come from bending in addition to stretching in the frame.

Here's a picture.

[962porsche]

i tend to still do things the old fashion way . i'm building a model useing modling tubing . i like to be able to have a model right in front of me . the size tubing i'm using would scale to 1 1/4 " so far i'm not happy with quight a few things . there is to much chassis yaw flex . most of the yaw flex is in the nose area as i'm tipping this the glue is seting with many changes i made to the sides and floor . the 3 biggest changes i made were splitting the runs of the side tubing as in the last email i sent you . and also splitting the run from the fire wall to the front suspension . these changes also made me change the diagonal tubings all the way from the front to the back of the car . but now left and right sides are built with equilateral trianglation .

[horizenjob]

i tend to still do things the old fashion way . i'm building a model useing modling tubing . i like to be able to have a model right in front of me .

I understand. There is a value to being able to handle and see the model in front of you. The previous time I started in on this project I made real models from welding rod. Brake tubing could also make a nice space frame. Scaling can be difficult though. For example if you use 1/4 size tubing for a 1/4 size model, the tubing will be quarter strength in tension and compression but only 1 / 256 as stiff in bending and twisting. Your model may be stiffer for yaw in real life then you think.

It was easier to learn the stress modeling software then the drawing stuff. Much easier actually. I'd be happy to give you a hand to get up to speed on "Grape" which is a PC program I use. You just type in the coordinates of the nodes, describe the sizes of the tubing you're using and then you connect the dots on the screen and say what size tubes go where. You can tell it where to apply loads and it will tell you how many pounds any tube in the frame is carrying, how many PSI that comes out to and the amount any node moves. It also gives you the weight of the tubes and the whole model.

Once you get this far it starts to get very easy to make changes and try some different things. Considering you can save the changes to new files and go back to earlier versions etc. - it starts to make real models look more and more difficult and time consuming. The big thing is it's almost certain you will find a surprise or two lurking in your frame somewhere. For instance in my frame here's the surprise - the highest loads by far are in the little piece of vertical tubing above the braces in the roll hoops. They are torsion loads...

You could likely start getting some results in a day or so of effort and could be looking at your current model after a couple of days. The advantage is that it lets you see things too! When the tubes light up with different colors you see more about your model.

Then when you change a tube at the very back of the car and see the diagonal on the very front bulkhead suddenly light up you start having a deeper learning about what is going on in your frame. That's what happens when I fix things behind the main roll hoop in Car9.

[horizenjob]

David asked another question earlier and mailed a scan which I'll include below. It's about the changes to the passenger door area. I'm not sure what color that is. It's definitely "not red" which is one of my two main color categories. It's different then the other color he used, perhaps it's the "other blue"...

First, I'll say there is nothing really simple about these questions. For scale there is about 32 inches between the main hoop and the dash hoop and about 16 inches between the bottom rail and the upper chassis rail ( not hoop brace ). So that V shape would fit in there and divide evenly etc.. I had to refer back to the question so here it is:

how much better and or stronger would the chassis be under pitch , roll and yaw along with side impact if you split that run in half and V braced it from tubing # 1 to tubing # 2 .

For the most part so far I only test the chassis for torsional stiffness and stress / strain( deflection ) of the suspension attach points. I think these are the areas that take the most attention. WHenever I look at bridges and highway sign supports, I wonder about the things you're asking. it's everywhere in the world around us. Charles Dickens had a character in one of his books that spend his days trying to save ounces in railroad bridges for some steel Scrooge and his nights drinking himself to death. Anyway...

I've spent an hour on this reply already and think I'll have to actually run it thru the model. I actually have nodes in the bottom frame rails for this, but mostly used them for working out some bracing in the floor. give me a day or two on the actual runs.

I don't think it's a big change except for more weight. This bay of the frame already has a diagonal, it's only 1" x .065 and carries a significant, but not high load. Since this bay already has a pure diagonal, it's hard to make it better. The suggested frame has advantages with the angles etc., but it also adds up to longer lengths of tubing and tubing stretches under load.

So far as impacts, that's hard to really model. The big deal for that though is how this V connects to the rest of the car. If it only connects to the upper and lower rails it will change little except help cover the hole there. If the bottom of the V connects to a tube which crosses the car to the other side, then you have to bend the other side's bottom rail too in an impact. If the bottom of the V connect to diagonals in the floor which go to the hoop legs on the other side, now you have to bend the entire car to push in the bottom of that side. It would help if the vertical tube in the middle of the V connect to a top rail which has a sister member forming a shallow V around the top of the cockpit. This is the normal configuration of formula cars years ago. Look at Lotus F1 in the 60's...

I think the better thing is to leave the light diagonal alone, it is perfect for what it is doing and that makes it strong and light. Door bars that are curved outwards can provide the protection by doing just that one job. They should be curved along their entire length though and connect to nodes with reinforcement. For instance the main hoop should have cross bars where these connect. Door bars that have a flat center section would be weak on impact. The thing to do here is stand on an empty beer can and reach down and tap the side. Plenty of food for thought there!

[962porsche]

real modeling is just one more tool in the arsenal of building and design . with the changes i made there is now no deflectuin in the chassis under yaw along with pitch and roll . my design softwere i'm sure is like your it has it's limitations .

[horizenjob]

David, I put in your suggested change to the would be door area and it looks to be a good improvement. Here are the pictures and numbers. The weight of the chassis goes up by 1.1 lbs, but the stiffness increases by about 7%. I don't usually calculate out the stiffness in lbs.-feet per degree, but simply examine the deflection at the main roll hoop behind the driver's shoulder. In the old frame this deflection is .179" and with David's tube arrangement it is .167". This is with these members made from 1" x .065 round tube.

The vertical tube didn't make any contribution to the stiffness and in fact only had a load of about 12 lbs., so I deleted it. It's still in the picture below though.

Before:

Car9Gstab24doorbraceB.jpg (110.78 KiB) Viewed 6354 times

After:

Car9Gstab24doorbraceA.jpg (113.34 KiB) Viewed 6354 times

[horizenjob]

I color coded the tubes to help see what's going on. DOes this seem to help or is it an insult to the eyes?
There have been a couple of small changes. The wheel offset is moving to 6.5" on an 8" rim, the bracing suggested by 962Porsche has been adopted, the bottom rail tube is now large round tube all the way to the front suspension box, the V8 and Ford IRS parts are being shown by default.

I have some more comments and questions but things slowed down for a couple of reasons this week including a hard drive failure...

[962porsche]

the way it's colored makes it easier to see were the tubing runs to and from .
the exhaust were is it going to run under the car or down the sides ?
could you post a pic of the car with out it's wheels ?

[horizenjob]

Here's a picture from about the same angle without the wheels. If you want other angles or closeups, let me know. You can also download this model from the Google 3D Warehouse. I update that version once or twice a month, and I can push the latest whenever you want.

Right now the Ford IRS rear end needs more work. I think I will need to build out the bulkhead at the roll hoop a few inches, maybe 4", dont' know yet exactly. The diff and the uprights are different shapes then the Subaru, obviously.

I did get a chance to do some measuring of the Ford parts, so I will also check the model for proper proportions. The nice folks at "Forte's Parts Connection" are letting me measure and photograph the rear end, transmission and engine pieces. It's a big help. http://www.fortesparts.com/

I also want to look at putting a "V" in the front along the engine bay like what you suggested for the door area. I did go back and look a little at my stress model, because I was surprised how much difference that change made. It turns out I originally drew the car with those diagonals ( door and engine bay ) with roll cage size material. Using that larger tubing there is much less difference in stiffness with the 2 shapes. It's so much stronger that it moves the frame flex to other places. So I should check the weight too, it might be worth the pound or two to go back to the bigger tube and single diagonal. I found another problem in that model though, so I am going to do it over from scratch and it will take a day or two because I don't remember how to start a new one.

[horizenjob]

Sorry for the slow updates. I'm still working hard on this but recently it seems to have been hard to draw a line somewhere and give some status. Too many balls up in the air it seems.

Since the picture above I have decided to try harder to move the motor backwards in the chassis to help get more weight on the rear wheels. I would rather see closer to %60 percent rear weight bias then %50 to help handle more power. The motor has been moved back 4" or 5", so the firewall and dash bulkheads have been redone. The firewall is less structural now and the dash bulkhead is now providing that strength and stiffness. The dash will be some type of a "Terry Hoop" as seen on early Lotus sports racers.

During this process I realized my FEA model contained a mistake because there was a diagonal across the firewall bulkhead which provided a great deal of stiffness. This was from an earlier iteration that used a Hewland transaxle in back. So I remade the FEA model from scratch to match the final dimensions of the V8 version of the car and put a lot of work into regaining the stiffness. Without the rear roll hoop braces meeting in the center I have not been able to use the rear of the frame to provide stiffness. That material is now just for driver protection.

Once can chase this stuff forever, it is fun and frustrating at the same time! So I have designed it to be reasonably stiff as a two seater, about 6 - 8 times stiffer then a normal Locost or Seven. With the addition of a 3' long diagonal from the dash to the passenger side of the roll hoop it becomes very stiff indeed. So this frame can cover both the case of a drive to the track car and a strictly track car that doesn't carry passengers.

Here is a picture of the stress and the numbers for three versions of the frame. As before in the picture the front suspension mounts are constrained - one of them in 3 axis ( 6 counting rotations ) and the other just vertically constrained. At the rear the frame can tilt on the middle of the bottom roll bar cross piece and 1000 pound loads are applied upwards on one side and downwards on the other. That is a torque of 3666 foot-pounds for this frame.

The stiffness would be: torque on the frame / ARCTAN( frame displacement / frame width ).

Frame as drawn in picture: 7816 ft.-lbs. / degree
Frame with passenger diagonal: 11,000 ft-lbs. / degree
Frame with upper rails at 14" like a Locost: 6060 ft.-lbs. / degree

[962porsche]

so i was wondering how you were comeing along with your project ?
the guy at our fab shop down in milford and i were talking about this the other day . they asked me what made you deside to build a front engine rear drive car as opposed to a mid engine rear drive car ?

[horizenjob]

what made you deside to build a front engine rear drive car as opposed to a mid engine rear drive car ?

The simple answer is that I liked Super Sevens back in the day and it seems the bodywork would be a lot easier. It also seemed it might be more approachable for other builders because people already build Locosts and this is very similar and can wear the same body work.

The project is at a couple of levels, in addition to what's here there is a library of parts being put together on Google SketchUp and the start of a couple of projects by other people. That part is going slow though. I did make a start on a midi version of this car and even a rear engined AWD version based on Subaru stuff. I think the midi with a Subaru or Porsche transaxle would be very fast indeed. The body work doesn't have to be really complicated, there is an example in the SketchUp library I maintain:
http://sketchup.google.com/3dwarehouse/ ... 2d46db6b9f

[horizenjob]

so i was wondering how you were coming along with your project ?

Most of the frame is settled now. It's a little heavier then I'd like at 175 lbs. Since the motor has moved back some, I may remove most of the front suspension box and flip the wishbones to face backwards. That won't change the wheelbase or actual suspension numbers, just there is more room for the wishbones to have trailing arms instead of leading ones for brake loads. I need to tie down the remaining details first so will leave that change till the end. Another advantage for this is that the tubing at the very front for radiator etc supports can be a little smaller which will make a better crush zone.

I started a little web site for this car to make it easy to coordinate work with other people interested in building one of these. It makes it easy to find up to date copies of the drawings, models and eventually blueprints being used. I am working on the suspension numbers now with a guy on the west coast who is planning to start building one this summer or fall. I put a pointer to the site in my signature.

It would be nice if the design could use either outboard or inboard suspension, but the jury is still out. So the next job is to finish the suspension details -pickup points, geometry, wishbones and coilover mounts. Mixed in that is rocker or push / pull rod choices etc.