Metal front bumper/tow hook mount

[Calan]

Wes... you're an engineering type. Any thoughts on what we've done so far on the design and getting it to fit in the 2G shell?

Other ideas?

 

[knochgoon24]

Wes... you're an engineering type. Any thoughts on what we've done so far on the design and getting it to fit in the 2G shell?

Other ideas?

I've been poking my head in from time to time, but I just started a new job. 30 hrs/wk + classes filled my schedule pretty quickly.

Anyway, what price do you want to put on the weight? Is shedding 25% (say 15lb vs 20 lbs) of the weight triples the price, is it worth it? This is going to need answered soon. You should probably mention a target construction price.



I don't have any FEA software on my home computer, but what if we start looking it at this in a KISS light?


First, I would forget the idea of using aluminum. If you want to see why, see what size of a solid bar it would take to keep deflections under control. From this, it makes it easy to calculate the necessary area moment of inertia. Then you can mess with different cross sections within your size bounds to see what you can come up with. If the size of the solid bar ends up being bigger than the space you have, you can drop the idea right there. I have a sneaky suspicion that it's going to be too complicated to get a design the at works without the price ballooning.


How thick of a wall would you have to run to get the deflection under control if you used a 1.5" x 3" chromoly box tube? Would the part end up as a reasonable weight? And the big thing, where could you get it bent at to get that initial bend in it?
This:
-Simplifies the materials needed. Bulk purchase + less waste = less cost.
-Reduces the mount of welding needed = less cost.
-Possibly increases the cost of tooling (bending) = more cost.
-Possibly requires a material that's not commonly or available at all. = more cost.
-May be a tad heavier than desired.

So maybe that doesn't work out so well. Now look at 2 stacked 1.5" square tubes. Spot or weld them completely down the length if need be. Now that you have some more material (that extra double wall in the middle) how thick would these need to be? And now that there's 2 pieces, bending them gets easier.
-Still keeps the the materials list short.
-Slightly more welding needed, but it's simple.
-More parts to bend, but easier.
-Material should be more common.
-May be a tad heavier than desired, since it's still chromoly.

For the tow hook, I would make the inserts threaded. Then maybe weld a tab out of .25" steel so that it places the bolts in double shear. It won't block airflow but should make it so you can get the bolts out after it's been towed.



Also, what if the hook was placed towards the left or right side of the opening some? The main problem we've been having has been with bending.



And I think you really should look at designing these for public sale. A lot of people remove their crash bumper to fit larger FMICs, but then they have saggy bumpers and nothing to protect their intercoolers in the event of a fender bender. While wouldn't protect much in a crash, if it's designed to hold 5000lbs in pulling, it would hold more than that in compression. You're talking about a pretty solid thump to the front end. If the price is right, they'd sell.

 

[Calan]

Anyway, what price do you want to put on the weight? Is shedding 25% (say 15lb vs 20 lbs) of the weight triples the price, is it worth it?

Eric wants it to weigh less than a pound and have only one weld.

I think he should start talking to some of the guys at JPL for design ideas.

First, I would forget the idea of using aluminum. I have a sneaky suspicion that it's going to be too complicated to get a design the at works without the price ballooning.

Eric has stated that weight is more important than cost, so some type of design made from aluminum seems like the best alternative, considering the weights that I keep estimating. The real question is how low is he willing to drop the load requirements; if it's decided that it only needs to hold up to 2000 lbs of force, it's not a big deal as far as strength goes.

But you are 100% correct; for a given load, you hit a wall at some point where there simply isn't enough cross-sectional area available to prevent bending without using a material with a higher yield strength.

Now look at 2 stacked 1.5" square tubes. Spot or weld them completely down the length if need be. Now that you have some more material (that extra double wall in the middle) how thick would these need to be? And now that there's 2 pieces, bending them gets easier.

That's basically what I was shooting for with the latest iteration, while keeping at least part of Eric's requirement for the beam to be curved away from the IC. The gusseting transfers a large portion of the stress back into the frame rails through the straight beam, while minimizing the stress at the corners as much as possible.

With a curved beam, the wall thickness has to be pretty beefy since you've essentially already pre-stressed the beam and there is no room for a backing gusset...although two stacked curved beams would help considerably I think.

 

[knochgoon24]

I kept editing that post pretty hardcore over the last 15 min. Sorry Craig!

Weight is more important that cost, but there has to be a limit. If he's willing to pay a out the wazoo, I'm sure I could make a titanium and carbon fiber piece that weights less than 10 lbs while being able to drag an elephant home from his afternoon safari.

Plus, a little more weight over the front of a FWD should help traction.

I really think moving the mounting point towards a side is going to be the best bet if he wants to keep this aluminum.

 

[turbosax2]

Anyway, what price do you want to put on the weight? Is shedding 25% (say 15lb vs 20 lbs) of the weight triples the price, is it worth it? This is going to need answered soon. You should probably mention a target construction price.

I can't answer that without a specific scenario. If the entire assembly weighed 20 lbs. and cost $50, sure, I'd shed 25% weight for triple the price. If the cost were $200, then no, I wouldn't triple that price to shed the weight.

And trust me, I've been looking for lighter materials. Anyone know where to find titanium in square/rectangle tube?

Also, what if the hook was placed towards the left or right side of the opening some? The main problem we've been having has been with bending.

Might be worth modeling to see what happens. I can tell you for sure my taste for symmetry wouldn't approve, though

And I think you really should look at designing these for public sale. A lot of people remove their crash bumper to fit larger FMICs, but then they have saggy bumpers and nothing to protect their intercoolers in the event of a fender bender. While wouldn't protect much in a crash, if it's designed to hold 5000lbs in pulling, it would hold more than that in compression. You're talking about a pretty solid thump to the front end. If the price is right, they'd sell.

I've been trying to keep mass production in mind, but I'd rather make me happy than everyone else. How much would people pay for these? $100, $200, $500? I have no idea, to be quite honest.


And to address the question about bending this stuff, I don't know. I don't know if I can do it, if I have to outsource, where to outsource it, etc. Once we get a design that we think will work, I'll get ahold of some material, figure out how much it needs bent, and see what I can do.

I like to put all the options on the table, then find the right balance for me amongst the midst. This is how I treat any purchase and/or project.

 

[Calan]

And trust me, I've been looking for lighter materials. Anyone know where to find titanium in square/rectangle tube?

The problem isn't finding (or maybe even being able to afford) lighter materials; the problem lies in being able to machine and weld them.

For a piece like this, it really needs to be made out of steel or aluminum IMO. Once you get outside of that, cost skyrockets and practicality drops like a rock.

Let's say you get it welded up and it needs a few tweaks here and there to fit, or you want to add another brace. With AL or steel it's a non-issue, but with anything else it may not even be possible, depending on the material properties and fabrication processes.

 

[turbosax2]

OK then, how about 5000 series aluminum?

 

[knochgoon24]

I can't answer that without a specific scenario. If the entire assembly weighed 20 lbs. and cost $50, sure, I'd shed 25% weight for triple the price. If the cost were $200, then no, I wouldn't triple that price to shed the weight.

Fair enough. But I think you're looking closer to the $200 mark than the $50 mark.

And trust me, I've been looking for lighter materials. Anyone know where to find titanium in square/rectangle tube?

I'll tell you now, you can't afford it.
http://www.onlinemetals.com/merchant.cfm?id=1255&step=2&top_cat=1353

The FSAE team I'm a part of used $60k worth of Ti on the car last year, and that only made control arms and the rear sub frame for the car.

Might be worth modeling to see what happens. I can tell you for sure my taste for symmetry wouldn't approve, though

We can just put 2 hooks on your car, one on each side. Sanity preserved.

I've been trying to keep mass production in mind, but I'd rather make me happy than everyone else. How much would people pay for these? $100, $200, $500? I have no idea, to be quite honest.

I'd try and shoot for the $200 mark. But I'm a bit of a Jew.

And to address the question about bending this stuff, I don't know. I don't know if I can do it, if I have to outsource, where to outsource it, etc. Once we get a design that we think will work, I'll get ahold of some material, figure out how much it needs bent, and see what I can do.

I like to put all the options on the table, then find the right balance for me amongst the midst. This is how I treat any purchase and/or project.

Fair enough. Seems you've come to the right place to brainstorm. 7 pages in 4 days and how knows how many brain-hours of though.

Craig may need a 10,000 hrs tune up after this.

 

[turbosax2]

I'm thinking the likely scenario may be closer to shedding 33% of the weight (between steel and aluminum) for an additional 50% cost. Depending on the total price, I may be willing to do that.

Ya, I knew I couldn't afford the titanium But I still like having all the options on the table.

One thing we haven't mentioned much yet is using round tubing. Is it stronger at all? Originally, I didn't want to use it because of the difficulties it presents bolting a flat tow hook to it. But we're well past that concern with the new designs.

 

[Calan]

I've thought about round tubing. In some cases, it is stronger due to the way stresses are transferred around the surface. But it still depends on the cross sectional area. Also, it will absolutely require a mandrel bend for best strength. Any crush in it will impact the strength considerably.

 

[knochgoon24]

So funny thing, at one point, I thought of filling whatever pipe we use with concrete to keep it from crushing. It's about the same density of aluminum. I'm almost considering it again...

I just don't know how nicely it would play with aluminum as far as corrosion goes.


It's one of those things where you'd have to build one and break it before you'd know if it's a viable option or not.

 

[Calan]

I just don't know how nicely it would play with aluminum as far as corrosion goes.

It wouldn't be pretty I don't think.

 

[turbosax2]

And I forget if you already answered this, but say we could get away with just a straight bar. Is this stronger than a curved bar?

 

[Calan]

And I forget if you already answered this, but say we could get away with just a straight bar. Is this stronger than a curved bar?

Somewhat...especially if we can add some gussets to the front side.

***

Since you want all of the options on the table, I'll throw this out there...

If I was to design something like this without any pre-existing conditions (and based on what I know now), I would start off with the requirement that the bumper cover needs to be cut. (I know...but hear me out.

By locating the hook on the same level as the frame rails, you eliminate virtually all of the strength and weight problems right out of the box, since you no longer have to account for an upward bending moment, other than the difference in height between the tow vehicle and the car. And since the hook is higher, even this would be less of an issue. You also no longer have to worry about flipping the bar and using a spacer, etc.

Weight could be considerably less for a given load capacity...probably by quite a bit. In addition, fabrication becomes much simpler, and cost drops because of less material needed and less welding and fabrication. Since you would be gaining strength, using a single curved beam should work just fine, eliminating any concerns about it sitting against the IC. And you could probably bolt the front beam to the frame rail mounting tubes, similar to the way the factory crash beam mounts. In fact, you may be able to use the factory frame tubes, eliminating even more cost and complexity.

You could use a straight hook that threads directly into the strongest part of the beam, again increasing strength while at the same time simplifying installation and removal.

Last but not least, you could spend a larger portion of your design budget on coming up with a very slick molded housing and cover that would mount directly to the factory bumper cover. I can envision a nice molded inset that includes a way to mount a cover to hide the hole and protect the threads, that would probably add to the "pro race" look of the car, rather than detract from it. (Think of some really nice hood pins). Even without that, we're only talking about maybe a 1" diameter hole at the most, which could be plugged pretty cleanly with an off-the-shelf frame plug.

IMO (and being a designer), all of that ^ vastly outweighs the requirement to not drill a hole in the bumper cover.

Just my .02

 

[turbosax2]

Gussets where? I was thinking something like this, where the ends would be trimmed down and boxed up if necessary for clearance to the bumper cover.

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We can do little gussets on the insides where the tubes connect if that would help.

 

[Calan]

The hook is going to stick quite a ways out from the beam to clear the cover, which will create a lot of upwards bending and twist in the beam. You also need to reinforce the beam to keep it from being pulled straight outwards, unless it has a really thick wall and/or a center web (two beams stacked and welded).

You don't gain a whole lot by just making the same beam straight vs. curved. A little, but not a lot.

 

[turbosax2]

Somewhat...especially if we can add some gussets to the front side.

***

Since you want all of the options on the table, I'll throw this out there...

If I was to design something like this without any pre-existing conditions (and based on what I know now), I would start off with the requirement that the bumper cover needs to be cut. (I know...but hear me out.

By locating the hook on the same level as the frame rails, you eliminate virtually all of the strength and weight problems right out of the box, since you no longer have to account for an upward bending moment, other than the difference in height between the tow vehicle and the car. And since the hook is higher, even this would be less of an issue. You also no longer have to worry about flipping the bar and using a spacer, etc.

Weight could be considerably less for a given load capacity...probably by quite a bit. In addition, fabrication becomes much simpler, and cost drops because of less material needed and less welding and fabrication. Since you would be gaining strength, using a single curved beam should work just fine, eliminating any concerns about it sitting against the IC. And you could probably bolt the front beam to the frame rail mounting tubes, similar to the way the factory crash beam mounts. In fact, you may be able to use the factory frame tubes, eliminating even more cost and complexity.

You could use a straight hook that threads directly into the strongest part of the beam, again increasing strength while at the same time simplifying installation and removal.

Last but not least, you could spend a larger portion of your design budget on coming up with a very slick molded housing and cover that would mount directly to the factory bumper cover. I can envision a nice molded inset that includes a way to mount a cover to hide the hole and protect the threads, that would probably add to the "pro race" look of the car, rather than detract from it. (Think of some really nice hood pins). Even without that, we're only talking about maybe a 1" diameter hole at the most, which could be plugged pretty cleanly with an off-the-shelf frame plug.

IMO (and being a designer), all of that ^ vastly outweighs the requirement to not drill a hole in the bumper cover.

Just my .02

At first, cutting the bumper for it was totally out of the question. You have me rethinking though.

Bolting to the stock frame rail tubes is probably tough considering how they don't stick out very far. And they weight 2 lbs. each, so if we can use aluminum and shed some weight there it might be beneficial with hopefully not too much extra cost. But the stock ones may work.

So we're now talking about a thread-in hook rather than a bolt-on one, right? Where is it threading into? A threaded tube welded parallel to the ground in the middle of the crash bar?

You completely lost me on the "molded housing and cover" part.

If we keep the hook on the same level as the frame rails, what kind of material do you think we can get away with? With all of this testing you've done, I didn't pick up that having the hook off plane was the issue; I thought it was the fact that the hook was so far from each frame rail.

 

[turbosax2]

Since you mentioned thread-in tow hooks, I quickly started looking to see what's available. Rennline has some nice ones, and then I came across this:
Rennline GoPro Camera Tow Hook Mount-Rennline, Inc.

If that isn't a selling point, I don't know what is

 

[1983gsp]

Just a thought being thrown out here, dirt trackers and other race cars have lifting hooks protruding through the hood. Look at this Hummer. File:Hummer H1.jpg - Wikipedia, the free encyclopedia the hooks could bolt to the strut tower and 2 slots cut into the hood on either side.

In the case of a frontal collision, the tow bar in front would be useless. Whereas the lifting/tow hooks on the strut tower would still be unscathed for towing, lifting purposes. But, just getting caught in a sand trap I'm not sure if I would want chains scratching the hood.

Ill just bow out here, Im thinking I'm in over my head.

 

[97gst_spyder]

Just a thought being thrown out here, dirt trackers and other race cars have lifting hooks protruding through the hood. Look at this Hummer. File:Hummer H1.jpg - Wikipedia, the free encyclopedia the hooks could bolt to the strut tower and 2 slots cut into the hood on either side.

In the case of a frontal collision, the tow bar in front would be useless. Whereas the lifting/tow hooks on the strut tower would still be unscathed for towing, lifting purposes. But, just getting caught in a sand trap I'm not sure if I would want chains scratching the hood.

Ill just bow out here, Im thinking I'm in over my head.

Thats a great point!

 

[turbosax2]

It's' been done. Not my cup 'o tea.

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[1983gsp]

Granted, it is gaudy, but it is removable. Function over form.

 

[turbosax2]

I agree function over form, unless I can get my way and have both at the same time. I'm not too concerned with "gaudy" - this hook will probably be staying on my car all the time as it's not a daily driver. Presumably that hook would have to mount to the top of the shock tower, like this:
Nagisa Auto GT Style Tow Hook S13 240SX - More Japan :: Since 2003

I'm not sure how Beau mounted his, though. As for the argument that this would be beneficial in a front-end collision, my retort to that is that I plan to make a rear tow hook as well that could be utilized in that instance.

If I used a hook like that, I would retain the factory crash bumper for supporting the bumper cover, so no weight loss at all (although that piece wouldn't add much weight). I'd have to cut a slot in my hood. I'm not sure of the strength of the shock tower, either. Would it be up to the challenge of handling a 4-5000 lb. load? Maybe Kevin can chime in here as he's torn down a shock tower. I remember the metal there being quite thin.

And as you mentioned, I'm not too keen on my hood being scratched up.

 

[Flash]

For what reason? They exist, but I'm not convinced of their strength.
Retractable Tow Hook (14")-Rennline, Inc.

So it tucks neatly into the bumper and doesn't stick out and catch your legs every time you walk by the car.

 

[jtmcinder]

I'm waaay late to the party and only skimmed the thread, so feel free to tell me to shut up. With that said....

On rally and rallycross cars, the tow hooks are usually mounted as low as possible. The best place on an Evo X, for example, is as an extension of the main skid-plate. For a 2G, it would be off the lower front radiator support. Why? So that the force of towing not only pulls the car forward, but also lifts it, at least a bit. This helps to untrap the wheels from whatever you're stuck in, reducing the pulling force required, reducing the chance of ripping the tow-hook off the car.

Evo Xs have a mounting point for a front tow-hook built into the bumper, but we don't use it for the above reason. We put our tow-hooks on the skid-plate and have it extend out through the lower lip of the front fascia.