JJLudemann's Design Thread

[James Michael]

I have seen rod ends fail. In particular there was a FM (I believe) at Texas Motor Speedway who failed his rod end on his front upright under hard braking. It trashed his car quite badly and I looked at the wreckage. After some close examination, I could see his that his rod end failed in the treads from fatigue loading. You could see the fatigue striations before the final static failure occurred. It all seemed to make sense. Everytime he braked (especially hard), the rod end was loaded and the fatigue failure started at the fwd facing end of the rod end which is most highly loaded under braking. The driver did say his car had been handling at bit strangely so this might indicate the fatigue crack growing.
A static analysis of the rod end will not suffice for sizing. You have a high Kt stress in the threads under bending loading and you can induce a fatigue crack and subsequent failure under repeated loads. I did not know what his rod ends were made of (possibly the lower grade type) but in any case this was a classic example of a rod end failure from fatigue. I am a stress/fatigue engineer and rod ends are checked for this type of failure in the aerospace industry.
Sizing is up to you but given it is difficult to come up with an accurate spectrum of loading, I would air on the cautious side for sizing and be sure to use the higher grade rod ends especially for the suspension.
James

[R. Pare]

Does anybody know of a catastrophic failure of one of these rod ends (of any size) in actual use? I sure would like to know before I get too far along.

Thanks,

-Jim

Two things to think about:

1 - ANY misalignment of the rod end off of the pushrod centroid will induce a bending moment.

2 - Friction at the rod end ball will cause a torque moment.

We had a pushrod adjuster break (bend) in the trailer on the way to Florida once when it wasn't aligned perfectly. A pretty dramatic demonstration on how critical the alignment is! it ain't the compression loads that you have to worry about the most.

[James Michael]

Also, just normal operation induces bending loads and axial loads on the threads of the rod ends. When you brake the car wants to keep moving fwd but the tires are stopping the car which loads the rod end in bending. When you go thru a corner the car want s to go off track but the tires prevent it, loading the rod ends axially. The axial loads and bending loads but a high stress in the root of the rod ends threads. This is typically why rod end threads are rolled not cut which tends to put a compression stress at the surface which helps to prevent fatigue cracks (similar to shot peening). A rolled thread also has a lower Kt stress.

[Billy Wight]

At your suggestion I added in the pushrod and ran a bunch of additional load cases on the model. The bending is just because it's a quick & dirty model, and I assumed the bending stiffness is minimal compared with the compression/tensile stiffness.

Hi Jim, Did you apply the appropriate end releases on the beam elements? You're looking to create a mechanism (your suspension) that is only constrained in compression by the pushrod. You should not have any bending present, any bending in the model is force that is not being reacted by the rod end, so you're not seeing the correct result. I've uploaded a video to youtube that shows how the constraints can be tested. You should replace your rigid "shock" element with a spring to test the linkage is created correctly (as shown in the video), then switch it back to rigid to run the analysis. http://www.youtube.com/watch?v=a87hzVSr3H8

[Mike Devins]

are you using rod ends for the upper and lower ball joints?

[JJLudemann]

Also, just normal operation induces bending loads and axial loads on the threads of the rod ends. When you brake the car wants to keep moving fwd but the tires are stopping the car which loads the rod end in bending. When you go thru a corner the car want s to go off track but the tires prevent it, loading the rod ends axially. The axial loads and bending loads but a high stress in the root of the rod ends threads. This is typically why rod end threads are rolled not cut which tends to put a compression stress at the surface which helps to prevent fatigue cracks (similar to shot peening). A rolled thread also has a lower Kt stress.

You're talking about the notorious "rod end in bending", right? The Formula Student judges have railed against this so strongly that I doubt it will ever be used again in any new design. It's even earned its own acronym: REIB. See for example:

http://www.formulastudent.de/academy...ds-in-bending/

My design uses spherical joints at the outboard ends of the A-arms, and rod ends at only the inboard joints, so I don't think this failure mode applies.

Keep 'em coming, thanks,

-Jim

[JJLudemann]

Two things to think about:

1 - ANY misalignment of the rod end off of the pushrod centroid will induce a bending moment.

2 - Friction at the rod end ball will cause a torque moment.

We had a pushrod adjuster break (bend) in the trailer on the way to Florida once when it wasn't aligned perfectly. A pretty dramatic demonstration on how critical the alignment is! it ain't the compression loads that you have to worry about the most.

Hmmmm, #2. Speaking of which, do people use metal on metal, metal on teflon, or metal on teflon + kevlar rod ends? I guess #2 is a pretty good argument to keep your rod ends lubricated...

A rod end bent while the car was on the trailer... Now you've got me thinking...

-Jim

[R. Pare]

Use only teflon lined rod ends, never metal on metal.

I would take issue with the premises outlined in that rod-ends-in-bending paper you linked to. While rod ends at the inboard ends of your a-arm legs may not be the best from an engineering standpoint, adequately sized rod ends will do the job admirably, and allow a small degree of freedom in adjustment that captivated spherical do not. To insist that only captivated sphericals are acceptable is rediculous.

[nulrich]

Agreed. Given you have two legs intersecting at an angle at the outboard end, it's impossible to avoid bending loads on a rod end used as the outer pivot. Best practice is to use a staked spherical in a housing designed for the wishbone loads there.

No such problem on the inboard end, where you can align the rod end shank precisely with the load path and get pure compression and tension loads.

It was convenient to use staked sphericals in machined housings on the upper wishbone inboard ends for our car, but we use rod ends on the lower wishbone inboard ends to allow adjustment.

Nathan

[R. Pare]

Even at the outboard end, "best practice" depends on the criteria.

In this case, the judges are guilty of not teaching the students that in some cases, sheer mass in a structure can more than make up for a lack of "best engineering" practice.

If cheap, fast to construct, and versatile is part of the main criteria, using a large rod end at the outboard end is perfectly acceptable.

If the main criteria is for the arm to be as light as possible and idiot-proof, and to heck with the fact that it may make construction XX times more expensive, then staked sphericals is the way to go.

For the author of that article to claim that high grip cars have not/did not use rod ends in bending only shows his lack of historical knowledge - even F1 cars used them up into the '80s, as well as Indy cars, endurance cars, Indy Lights, Trans Am cars, FA's, and so on.

Were they the lightest and best performance a-arm designs possible? Heck no, but they worked because the components were sized correctly to withstand the loads applied.

The judges in this case are guilty of not recognizing that "best engineering practice" is not always the best route to a specific outcome, and shortchanging the students education in that regard.

[Dick R.]

All four wishbones (A-Arms) on my 85 VD "DB-1 copy") look exactly like the last picture in the linked article. Also, best I can tell from some Ad pics the front A-Arms on the DB-1 are the same way.

Dick

[JJLudemann]

Just found a source for THK rod ends in my town in Thailand, so I checked out their website and found this paper regarding rod end safety factors:

https://tech.thk.com/en/products/pdf/en_a23_005.pdf#1

They're saying a rod end with a fluctuating load in a constant direction requires a safety factor of 3 to 5, and the safety factor must be further multiplied by the cube root of the RPM of the joint. So for example a joint seeing 8 RPM would need an additional factor of 2 for safety (8**(1/3)*(3 to 5) = 6 to 10. I'm not sure what the effective RPM of a suspension rod end would be, but we probably need to throw some small additional factor in there for it.

It's not absolutely clear what they mean by "constant direction". If that means the sign can't change, then you'd have to use the factor for load in varying directions, which is 5 to 8. Youch!

-Jim

[JJLudemann]

Having observed that the ease of welding depends heavily on access to the joint, I think I'll weld in the firewall before attaching the rear of the car. I really really really don't want a grenading engine to pierce the fuel cell, so what would be an appropriate thickness for the firewall? I'm thinking 0.049" (1.2mm) steel, same as one of the options for the chassis side intrusion protection, followed by a gap of maybe 1", followed by the fuel tank wall of 0.059" aluminum, maybe with the gap filled with foam-in-place foam? Or is it too hot to put foam in there? Also, kevlar, maybe? Who's had an engine explode, and how much damage did it do to the firewall?

Thanks in advance,

-Jim

[jaltaman]

and in my Phoenix, it did nothing to the firewall.

Dustin employed multiple layers of protection from the engine forward to the fuel cell: a shaped fiberglass plate that mounts close to the engine, a 3" gap to the false firewall (thin piece of steel), and about an inch later, the firewall itself.

I'm sure it depends upon the direction and force at which the shrapnel is ejected, but in my case everything launched forward was contained by the fiberglass plate. Because afterwards, when we got it into the pits for inspection, we found impact marks only on the fiberglass plate - the false firewall was unscathed.

Hope that helps (although something tells me you would figure it out - you have skills I can't even fathom).

John

[glenn cooper]

Having observed that the ease of welding depends heavily on access to the joint, I think I'll weld in the firewall before attaching the rear of the car. I really really really don't want a grenading engine to pierce the fuel cell, so what would be an appropriate thickness for the firewall? I'm thinking 0.049" (1.2mm) steel, same as one of the options for the chassis side intrusion protection, followed by a gap of maybe 1", followed by the fuel tank wall of 0.059" aluminum, maybe with the gap filled with foam-in-place foam? Or is it too hot to put foam in there? Also, kevlar, maybe? Who's had an engine explode, and how much damage did it do to the firewall?

Thanks in advance,

-Jim

None.
By firewall I believe you are refering to the fuel cell enclosure, which in the case of the VD RF99 is roughly .045 (?) aluminum.

Let's not forget that the crankcases are having to be breached, and the spinning bits are very close to the inner wall as well.

I believe most of the force is taken by the act of punching through the admitedly near tissue paper thin cast aluminum crankcase, BUT I've not seen any shrapnel do anything really stupid, but I seem to remember reading on here about someone who claimed to have.

I believe the sheet steel would be overkill as well as weight!

PS I was also running (at the time) a thin sheet of aluminum with self adhesive silver heat shield fabric (which burned off mostly) on the aft section of the frame, which was about 3" from the header tubes.
It was "burnt up some", but essentially unscathed...

[jaltaman]

I misspoke - the false firewall is aluminum, and also has the self adhesive silver heat shield Glenn refers to. Don't know why I said it was steel - guess it's too early for me to be making posts.

[S Lathrop]

In the Citations we have a .065 aluminum enclosure for the fuel cell. This is located about 12" forward of the engine. I recommend that the headers be enclosed in a heat container/box. This can be a single sheet of aluminum(.032) or stainless (.020) on both sides to the headers. The combination of all the separate components should provide adequate protection. You could add a insulating blanket on the back side of the fuel cell and a second sheet of thin aluminum. Doing a good job of protecting the fuel cell from the exhaust heat will result in sufficient ballistic protection for the cell as well.

[RobLav]

I had an R1 blow up that a piece of the conrod cap pierced a very small hole in the rear of my fuel cell enclosure. The enclosure was .063" aluminum. That rear piece is now changed. This time around, I made a sort of blanket out of .040 aluminum with appropriate rearward facing bends in it between the header and the engine case. On the inside of that aluminum is some carbon fiber and S-glass with yellow foam in between to absorb some of the potential shock. My header collector is also surrounded by another piece of thin sheet aluminum and attached to the "blanket".

[JJLudemann]

and in my Phoenix, it did nothing to the firewall.

Dustin employed multiple layers of protection from the engine forward to the fuel cell: a shaped fiberglass plate that mounts close to the engine, a 3" gap to the false firewall (thin piece of steel), and about an inch later, the firewall itself.

I'm sure it depends upon the direction and force at which the shrapnel is ejected, but in my case everything launched forward was contained by the fiberglass plate. Because afterwards, when we got it into the pits for inspection, we found impact marks only on the fiberglass plate - the false firewall was unscathed.

Hope that helps (although something tells me you would figure it out - you have skills I can't even fathom).

John

Thanks! Every data point is useful. Predicting how an engine explosion will pierce the firewall is beyond the capabilities of current computers, and will be so for many years. Is the fiberglass plate between the engine block and the headers, or in front of the headers?

-Jim

[jaltaman]

Between the engine block and the headers (as close to the block as possible).

[JJLudemann]

None.
By firewall I believe you are refering to the fuel cell enclosure, which in the case of the VD RF99 is roughly .045 (?) aluminum.

Let's not forget that the crankcases are having to be breached, and the spinning bits are very close to the inner wall as well.

I believe most of the force is taken by the act of punching through the admitedly near tissue paper thin cast aluminum crankcase, BUT I've not seen any shrapnel do anything really stupid, but I seem to remember reading on here about someone who claimed to have.

I believe the sheet steel would be overkill as well as weight!

PS I was also running (at the time) a thin sheet of aluminum with self adhesive silver heat shield fabric (which burned off mostly) on the aft section of the frame, which was about 3" from the header tubes.
It was "burnt up some", but essentially unscathed...

More useful information! Thanks. I guess I thought an engine explodes with more force than that.

-Jim

[JJLudemann]

In the Citations we have a .065 aluminum enclosure for the fuel cell. This is located about 12" forward of the engine. I recommend that the headers be enclosed in a heat container/box. This can be a single sheet of aluminum(.032) or stainless (.020) on both sides to the headers. The combination of all the separate components should provide adequate protection. You could add a insulating blanket on the back side of the fuel cell and a second sheet of thin aluminum. Doing a good job of protecting the fuel cell from the exhaust heat will result in sufficient ballistic protection for the cell as well.

OK, now that I think about it, I'm planning on running a dual exhaust all the way to dual outlets. This lets me get the headers out of the engine compartment as fast as possible, and I can run them so that they never go near the engine block, but near the head only. Shrapnel and oil only comes out through the block, right? I can then put an oil shield below the headers, but above where shrapnel might fly. You may be getting the idea that I'm really afraid of fire...

-Jim

[Dick R.]

There might be something useful you can learn from transmission safety blankets.

http://www.jegs.com/c/Safety-Seats_T...11877/10002/-1

Dick

[Christopher Crowe]

The notion of running "dualies." Have you figured these for exhaust gas scavenging, etc? If so, which two pipes exit the engine bay together and, what are their lengths and where do they join, etc.? Then, do the pairs of twos ever merge into a common collector (of four) again?

This is an intriguing notion from the packaging point of view... but I really wonder about the employment of dualies from a horse-power-making point of view.

Any of your thoughts in this area would be terrific!

Thanks,

Chris

[S Lathrop]

Jim;

The transmission safety blanket might be an good idea.

I think you are giving up significant performance on 2 levels: Engine performance because of a less than optimal exhaust system, and aerodynamic by increasing the frontal area of the car in the space above the wheels. If you are running the exhaust in the area behind your shoulders, you are routing near the injectors, and other electronic equipment. Exhaust systems that pass near the shocks are a performance issue as well.

But by routing exhaust on both sides of the engine you do have the opportunity to blow the diffuser. It is certainly worth doing if you can. The Lola T97 had a blown diffuser and doing a corner flat vs partial throttle was important.

CG is not a big issue but when you add mufflers, the exhaust system does have significant weight.

[JJLudemann]

Building the Cockpit

Enjoy!

-Jim

[stephanedu14]

Hello JJLudeman..
It's been two days that I look at the assembly of your frame, a very fine achievement.
I will follow closely the result of the construction.
Good luck.

Bonjour JJ.
Cela fait 2 jours que je regarde le montage de ton chassis,une trés belle réalisation.
Je vais suivre avec attention la suite de la construction.
Bonne continuation.

[JJLudemann]

Latest news from the Caffeine Zone: the basic frame is finished! I know I should feel most triumphant, but all I can think about is the boatload of brackets that need to be fabricated and welded on... Then there's the small matter of the body...

Lots of excruciating detail is available here: Click a blue link. (:-)

I understand a giant shipment of race car stuff is on the way from the US to Thailand, arriving Monday. We'll see how long it takes to get through customs. I'm soooo happy that the US considers a halon fire extinguisher to be hazardous material. It's so dangerous that when I get into the worst trouble I ever expect to be in in my life, I intend to aim the entire thing at my face and fire it . When I told my customs broker that the "hazardous material" was a Halon fire extinguisher, she kind of went "Harumpf" and said she'd call me back later.

Attached should be a photo of the frame in almost-finished state. I forgot to take a finished photo. Needz moar caffeine.

-Jim

[Christopher Crowe]

What's she weigh in at, Jim?


Chris

[RobLav]

Where are supports for front and rear rollhoops? It sure looks like attaching and extracting the engine will be a bit of a chore.

[JJLudemann]

There might be something useful you can learn from transmission safety blankets.

http://www.jegs.com/c/Safety-Seats_T...11877/10002/-1

Dick

Poking around the net I can find both kevlar and carbon fiber transmission and engine scatter blankets. I've got both materials on order. Still thinking...

-Jim

[JJLudemann]

The notion of running "dualies." Have you figured these for exhaust gas scavenging, etc? If so, which two pipes exit the engine bay together and, what are their lengths and where do they join, etc.? Then, do the pairs of twos ever merge into a common collector (of four) again?

This is an intriguing notion from the packaging point of view... but I really wonder about the employment of dualies from a horse-power-making point of view.

Any of your thoughts in this area would be terrific!

Thanks,

Chris

Jim;

The transmission safety blanket might be an good idea.

I think you are giving up significant performance on 2 levels: Engine performance because of a less than optimal exhaust system, and aerodynamic by increasing the frontal area of the car in the space above the wheels. If you are running the exhaust in the area behind your shoulders, you are routing near the injectors, and other electronic equipment. Exhaust systems that pass near the shocks are a performance issue as well.

But by routing exhaust on both sides of the engine you do have the opportunity to blow the diffuser. It is certainly worth doing if you can. The Lola T97 had a blown diffuser and doing a corner flat vs partial throttle was important.

CG is not a big issue but when you add mufflers, the exhaust system does have significant weight.

Assuming a normal firing order of 1-3-2-4 or 1-4-2-3, it happens to work out that the two left cylinders share a pipe, and the two right cylinders share a pipe, giving a good mechanical layout. According to my limited understanding of the reflections and standing waves in the exhaust system, a 4-into-2 provides equally good scavenging as a 4-into-1 system (with less back pressure). I believe the best would be 4-2-1, as you could tune the pressure waves from the secondary cylinders to arrive at the right time, too, but with a 4-1 there's no way to get the secondary cylinder pressure waves to be useful. Am I completely full of hot air here? As to why the motorcycle mfrs didn't do 4-2-1, maybe they can't fit it on a motorcycle? I'm sure George Dean knows all about this stuff.

My side pods are bigger than some cars out there, but they just cover the exhaust pipes where they will stick out the side of the car. Then... blown diffuser... mmmmm... chocolaaaaate.....

-Jim

[JJLudemann]

What's she weigh in at, Jim?


Chris

OK, now I'll have to go buy some scales. I finally put the Solidworks model back together with the correct wall thicknesses, and it says 109 lbs. That's with untrimmed tubes and no allowance for the weight of the welds, so I'm just going to make a WAG and say the weight of the tube trims equals the weight of the weld fillets. Easy!

-Jim

[JJLudemann]

Where are supports for front and rear rollhoops? It sure looks like attaching and extracting the engine will be a bit of a chore.

Oops, you're right. I ran out of the thicker tubing, so I'm still missing the forward-facing main roll hoop braces. Dangitall, it's not really done after all . The thicker tubes that connect those braces down to the floorpan are already in.

I'm going to cut the top rails at the front and rear of the engine compartment and make those sections removable so the engine will come out the top. That also allows me to install the headers before putting the engine in the car. The dampers will push against a fabricated crossmember at the top rear of the engine compartment, and I can make this crossmember removable if necessary. I'm not exactly sure how small a hole I can lift the engine through. The engine and several hundred pounds of other stuff are arriving in Thailand today, and when it gets through customs will be my first chance to actually see one. Up to now I've been depending on Lee Stohr's CAD model.

-Jim

[JRMarchand]

Jim

I'm happy you change your mind about removing the engine out the bottom like on the 911 you used to love so much. On the latest picture it appears that you are going to leave the engine bay bottom open for those who want to hustle the engine out the p-car way Would it make for a better structure to have X brace under the engine and one behind the fuel cell.

The roll hoop doesn't look like much, why is that. Most roll hoops look stronger. Not second guessing your work. Kind regards


Juan Marchand

[JJLudemann]

What's she weigh in at, Jim?


Chris

OK, now I'll have to go buy some scales. I finally put the Solidworks model back together with the correct wall thicknesses, and it says 109 lbs. That's with untrimmed tubes and no allowance for the weight of the welds, so I'm just going to make a WAG and say the weight of the tube trims equals the weight of the weld fillets. Easy!

-Jim

86 pounds without the missing forward roll-hoop braces.
Yeah, I know. Me too.

-Jim

[JJLudemann]

Jim

I'm happy you change your mind about removing the engine out the bottom like on the 911 you used to love so much. On the latest picture it appears that you are going to leave the engine bay bottom open for those who want to hustle the engine out the p-car way Would it make for a better structure to have X brace under the engine and one behind the fuel cell.

The roll hoop doesn't look like much, why is that. Most roll hoops look stronger. Not second guessing your work. Kind regards


Juan Marchand

The floor under the engine will be a stressed-skin panel, eliminating the need for an x-brace. Same for the firewall panel in the main roll hoop, as allowed in the FB rules.

Regards,

-Jim

[JJLudemann]

Just posted an update to my blog detailing some of what I've been up to over the past month. Lots and lots of design. Before building the suspension attachment brackets I decided I'd better do an FEA on them, and one thing led to another until I'd redesigned just about every part all the way to the lug nuts.

Lots of pretty and informative pictures here:

Redesigning Suspension Components

[JJLudemann]

Fab update: I'm finally making chips! Also, getting caught up on my blog updates:

Contents: 1 Race Car Kit, some assy. req'd.

Modifying the Lathe Quick-Change Toolpost

Making Chips... Finally!

Fabricating Lower Suspension Clevis Spacers & Washers


The Thai government sure threw me a curve ball last month when they banned import of all used motorcycle parts. Yes, you read that correctly. Future cars will have to be built with brand-new engines, which, after adding the VAT and import duties, will run my around $10,000 each. So they quite possibly have killed an industry in its' infancy, one they didn't even know existed yet. The Law of Unintended Consequences, and all that. Sigh...

-Jim

[Nicholas Belling]

keep it up.. your are an inspiration to the entire community.

who did you aero dynamic work on your car ?
are you supportiong your entire rear wing structure from your lower diffuser assembly