Metal front bumper/tow hook mount

[greengoblin]

I would build the gussets by cutting a piece of the tube in half diagonally at the correct angle and length. It will make a much stronger gusset than a piece of flat stock cut out in a triangle.

Here is a picture of the gusset I made for my crossmember using tubing.

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[Calan]

I would build the gussets by cutting a piece of the tube in half diagonally at the correct angle and length. It will make a much stronger gusset than a piece of flat stock cut out in a triangle.

Yeah. I just tossed a simple gusset onto the model to prevent the plate from rolling, as I wanted to see what happened with the tube. With such a small tube and thin wall, it actually becomes more of a weak link than the plates.

 

[snowborder714]

I wonder how this is causing so many problems when Drew used 1.25" square tubing with the same wall size. He did say his bends, and that he got used to bending it back. Maybe we're caring more about the forces on it than he did?

I really need to figure out where to rent one of those gauges to get us some real world force data.

 

[Calan]

I wonder how this is causing so many problems when Drew used 1.25" square tubing with the same wall size. He did say his bends, and that he got used to bending it back.

Question asked, question answered.

I really need to figure out where to rent one of those gauges to get us some real world force data.

Yeah.

I'm still going to see what we can do here at work if things will settle down a bit.

 

[Calan]

Could you make something like this work?

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As you can see, stress is much lower, and max deflection is now about .36 inches. With a thicker wall or a horizontal gusset across the back of the main tube, you could gain even more.

One problem with it is it's starting to get heavier. This would weigh somewhere around 31 lbs if made out of something like A36 HR steel.

 

[turbosax2]

What size material are you using? The problem is, the tow hook has to come out exactly at the top of the bumper. This is a fixed distance from the tube in the frame rail. But then we also want it adjustable to accommodate a 2gb with a cut mustache, which is exactly 2" higher.

A full length gusset horizontal gusset isn't possible because of the FMIC. A partial length gusset may be.

 

[99gst_racer]

What size rectangle tubing is that, Craig?

 

[Calan]

What size material are you using? The problem is, the tow hook has to come out exactly at the top of the bumper. This is a fixed distance from the tube in the frame rail. But then we also want it adjustable to accommodate a 2gb with a cut mustache, which is exactly 2" higher.

I'm just guessing at tube sizes, and picked a standard size. The mounting tubes are 2" square x .188 wall, which assumes a flush fit groove weld for the bolt tube. Ideally these would be 1.75" square with a 1/8" external weld on the bolt tube, but 1.75" square tubing isn't easy to find.

The main beam is 1.5" x 3" x .188", which is a common size. You can also get it in a .25" wall, which would be quite a bit stronger yet (but heavier).

IMO, making the frame itself adjustable in height is going to kill you on strength. You'd be better off finding a clever way to lower the hook off the bottom of the frame I think.

 

[turbosax2]

Let me give you some specs to work with.

-The absolute biggest tube that will fit in the frame rail is 1.5" square.
-Per my current measurements, the absolute biggest width tube that will fit between the FMIC and the bumper is 1.5".
-The distance from the bottom of the frame rail tube to the top opening of the bumper cover is around 1.75".

These are all fixed, nothing I can do will change them.

That being said, the tow hook will need a spacer between it and the bottom of the tube to avoid notching a hole in the bumper cover. I'd guess 0.25" to 0.5" for that.

 

[greengoblin]

What about just being simple and welding a tow ring or even bolting one directly to one side of the frame rail and have it exit the bumper cover like on sierra sierra's car?

Does it have to be bolt on? I think once you start looking at mods like this bolt on solution aren't worth it most the time.

Also what do the rule sets say about load requirements?

 

[turbosax2]

I won't discount any solution at this point. But there's a high likelihood it will be a bolt-on affair.

Another thing I've wondered about is the affect of pulling the car only from one side rather than the middle.

 

[ed1380]

If you want to accommodate for the 2" mustache cut, then make the horizontal tube at a height where you can weld the hook at the top or bottom of the tube depending if you have your bumper cut.

 

[snowborder714]

Why would you want to weld the tow hook on top of the bumper? Then it would be sticking down into the opening.

 

[ed1380]

This is a side view. Arrow point to front. Red is 2 different tow hook locations

Drawn on my phone. Mspaint got nothing on me!!

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[Calan]

-The absolute biggest tube that will fit in the frame rail is 1.5" square.
-Per my current measurements, the absolute biggest width tube that will fit between the FMIC and the bumper is 1.5".
-The distance from the bottom of the frame rail tube to the top opening of the bumper cover is around 1.75".

Based on that ^, this is about the best I can come up with at the moment:

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This gets you in the ball park (if that rear large gusset will work), but it's still borderline at 6000 lbs of load. We're at roughly .38" deflection and 50k psi of stress at the center of the tube. That is using 1.5" x .188" sq tubes, and a 1.5" x 2" x .12" front tube. If you could get the front tube with a thicker wall it would help a lot.

With a common mild steel alloy this should weigh around 22 lbs.

 

[snowborder714]

So if I have the mustache, I would use the lower mounting option. All fine and dandy as the mustache will cover the bumper and the tow hook will stick out at the top of the opening, just below the mustache.

If I don't have the mustache, the bar will still be at the same location, however it won't be hidden anymore. It will be out in the opening of the bumper. But you're correct in that mounting it on top would get it fairly close to being at the top of the bumper opening. Just not sure how someone would like being able to see all of that, or if it would cause any functional issues.

 

[turbosax2]

I missed your question earlier, Kevin. Here are rules:

RTA

2-9.3 A tow hook or strap must be installed on both ends of the vehicle. If identification of the tow hook or strap is not obvious to safety personnel it must be clearly marked with a sticker or tape. OEM screw in hooks are permitted.


HPDE NASA
11.4.20 Tow Eyes
It is STRONGLY recommended that all vehicles have at least two (2) easily accessible (and usable) tow eyes; one (1) in front and one (1) in back. They must not protrude dangerously from the car, and they must be accessible without manipulation of the bodywork and/or panels. They should be strong enough to support the weight of the car. If no tow eyes are available, the towing crew will hook onto other things that may cause damage to the driver‟s car. The towing crew will not be held liable for that damage. Again, tow eyes are STRONGLY recommended.
The tow crew will attempt to avoid damaging the participant‟s vehicle. However, should damage occur in the course of towing, or preparing to tow, NASA and/ or the tow crew will not be held responsible for any damages.



NASA Required Safety Equipment
15.12 Tow Eyes
It is required that all race vehicles, except open-wheel (formula) cars, have at least two (2) easily accessible (and usable) tow eyes, or tow points; one (1) in front and one (1) in back. They must not protrude dangerously from the car or require manipulation of the bodywork and/or panels to access the tow eyes. They must be strong enough to support the weight of the car. If tow eyes or tow points are not available the towing crew will hook onto other things that may cause damage to the driver‟s car. The tow crew and NASA will not be held liable for any damage. Note- "spec built" cars must comply with this rule (e.g. FFR)




SCCA Prepared
F. All cars may have towing eyes, hooks, or straps, which do not dangerously
protrude from the bodywork.

 

[turbosax2]

I'm not opposed to spending money on better materials, if they exist (provided the cost isn't outrageous). That main tube is only going to be ~3ft. long.

We use this stuff at work, it may be worth looking into:
Profiles - Bosch Rexroth Corp.

They even have a deflection calculator:
Security Violation - 03

With 40mmx80mm tube, it only deflects 11% with 6000lbs. of force and being clamped at both ends. That's 0.18".

 

[greengoblin]

I wonder how this is causing so many problems when Drew used 1.25" square tubing with the same wall size. He did say his bends, and that he got used to bending it back. Maybe we're caring more about the forces on it than he did?

I really need to figure out where to rent one of those gauges to get us some real world force data.

My guess is Drew's car was never dragged through the sand sideways like Craig is simulating.

 

[knochgoon24]

I'd think that most cars would be ok with 1/2" deflection when towing it out if it keeps the truck from pulling it out by the radiator support, lower control arms, or whatever else is sticking out.

How much deflection are we talking before permanent deformation?

 

[Calan]

-The distance from the bottom of the frame rail tube to the top opening of the bumper cover is around 1.75".

Can the front tube be a little taller, so that part of could stick down into the opening slightly? Or does it have to be flush with the top of the opening?

If you could use 1-1/2" x 2-1/2" tubing for the front beam, it opens up a lot of options for wall thickness since that is a common size.

 

[turbosax2]

That's kind of what I was thinking too. I'm going to try to calculate a reasonable force number at some point. I'll be absolutely dumbfounded if it's anywhere near 6000 lbs.

 

[Calan]

Just found this on a forum. Supposedly it is taken from an SCCA rule book.

2.10.8: TOWING APPARATUS – All cars must have one (1) front and one (1) rear permanently installed towing eyes/straps/cables, with a minimum hole diameter of 50.4 mm (2"). The apparatus shall be strong enough to pull the vehicle out of a gravel trap. This means that the towing apparatus must be able to withstand the weight of the car and the gravel that gets picked up, approximately 5000 lbs. total. If the towing apparatus is located more than 305 mm (12") above the ground, it shall not be rigid enough, in the area between the structural part of the chassis and the bodywork, to cause any damage, other than
superficial, to another car. Towing eyes that stick out of the bodywork shall either be hinged to create a blunt surface, or thin enough that it will bend if it comes in contact with metal bodywork of another car.

2.10.8.1: The towing apparatus must be positioned in such a way that:
-They are easily accessible should the car be stopped in a gravel bed.
-They do not protrude beyond the perimeter of the bodywork greater than 1" as viewed from above.
-They are easily accessible without removal, or manipulation, of bodywork.

2.10.8.2: The towing apparatus must be clearly visible with the loop painted in, or the strap material woven in, a strongly contrasting color. There shall be an arrow that contrasts strongly with the vehicle paint scheme, pointing to each tow eye/strap/cable.

We may go hook up a strain gauge to my car this afternoon and try to drag it across the pavement and see what we get...if time allows.

 

[snowborder714]

Those are old rules I believe Craig. I pulled all the rules Eric posted from the '11 rule books for RTA, NASA, and SCCA. I was surprised when I read the new rules and thought I was missing something as I had remembered the 2" ID rule for the hook, but it's not there any more.

You should make time allow for that test

 

[Calan]

Strange. I would think that if anything, race sanctioning bodies would want a standardized hook ID to ensure that the tow vehicle just drops and goes.

FWIW - Most of the hooks I'm finding seem to be rated for 10k.

That's kind of what I was thinking too. I'm going to try to calculate a reasonable force number at some point. I'll be absolutely dumbfounded if it's anywhere near 6000 lbs.

I would think that a tow truck driver yanking on a car stuck in sand (think one or more flat tires, seized calipers, etc) could easily generate forces in excess of 6000 lbs.

But... the question isn't so much about how much force it takes to pull the car out of various situations; it's how much do you allow for before breakage, and where does it break.