Metal front bumper/tow hook mount

[snowborder714]

Here is a project that my brother and I have been working on recently. We’re both building our cars for autox/road racing, and one item that is highly recommended to have installed is a tow hook (preferably both front and rear). We’ve been throwing around ideas for a year or so on how/where to mount the front tow hook. After some discussion and looking at other purpose-built road race cars, we decided we should replace the front bumper with a metal unit and mount the tow hook to there. This will clean things up a bit, possibly lose some weight, give us an easily removable item, and hopefully not hack up any of the bumper cover.

Main purposes of this item are:
-tow hook mount
-bumper cover support

We are considering both mild steel and aluminum for the materials. Does anyone have any opinions on one material or the other (or possibly something else not mentioned)? Obviously this needs to be strong and sturdy, as it’s going to be possibly used to tow a car.

This is the design we have come up with so far. If anyone has comments or suggestions on it, please post up.

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[turbosax2]

We're using Brian's car to mock this up since he has an SSAC fmic. The goal is to make it clear that, provide adequate bumper cover support (so it doesn't sag), be a sturdy point for towing, be able to work with any DSM bumper (including 2gb w/ mustache cut). The model is made from 1.5" square tubing; it looks like that is the biggest that will fit.

 

[mattasi]

You guys are always doing cool stuff. Looks like solidworks 3d cad program? I am the head solidworks cad guy here at my work. Anyways, I would for sure not use alum. I would use A36 Steel, it's the same kind of metal the frame uses. Also if you get A36 steel be sure it is hot rolled, not cold rolled. If it is cold rolled it will be very brittle at the points you weld, and will snap/break.

Get hot rolled A36 steel.

I have been thinking about a front tow hook for over a year now... I don't autocross but I would like something simple for if I need to be towed.

 

[snowborder714]

You guys are always doing cool stuff.

Thanks

Looks like solidworks 3d cad program? I am the head solidworks cad guy here at my work.

Yup. It's a fun program to mess around with. I'm not sure how it'd be doing it all the time...probably fun at the moment since I'm currently editing a Word document for font and formatting

Anyways, I would for sure not use alum. I would use A36 Steel, it's the same kind of metal the frame uses. Also if you get A36 steel be sure it is hot rolled, not cold rolled. If it is cold rolled it will be very brittle at the points you weld, and will snap/break.

Get hot rolled A36 steel.

I have been thinking about a front tow hook for over a year now... I don't autocross but I would like something simple for if I need to be towed.

We were told by Drew (gixxerdrew) that aluminum could work as long as it were a stronger alloy. He mentioned/suggested 7075. Thoughts on that vs. the A36?

 

[Bud92gsx]

Suggestion..Tweaking your picture there..

Remove the plastic bumper support dealy...Weld two plates on each side where the bumper support bar will be mounted...Weld two tinier plates to the bar that will support the bumper..Drill and tap those 4 "tabs" so that the bar can be unbolted from the plates you welded on earlier...

Then you should have the support you want, that can be unbolted if need be..Weld your tow support to the bar..Reinfornce the areas that have been drilled and tapped..And that's all she wrote..

I'm sure that idea could use some tweaking..But it sounds pretty food and easy to me..

 

[snowborder714]

We plan on removing the fiberglass bumper, along with the styrofoam piece (obviously). This unit will replace that, as well as replace the two mounting brackets (that the bumper bolts to) that slide into the frame rail.

We're trying to avoid welding a lot as we don't have a welder or the skills to do so. Some welding may be required (or preferred in some areas), but we're trying to make it more bolt-in. Plus, heaven forbid something gets bent/broken/deformed during use, you'd have a lot more work to replace it if things were welded in. Then if someone ever asks for one to be made, the price wouldn't skyrocket because of outsourcing welding. But that's a distant thought and I don't want to steer this thread that way right now.

We'd also like to keep everything as removable and non-permanent as possible. Hence the minimal welding.

 

[mattasi]

Yea, 7075 Alum literally costs 10x more than regular alum. Regular alum will cost more than the A36 steel. So 7075 Alum will cost A LOT more and be weaker than A36 hot rolled steel. According to my data sheets 7075 will lose properties when it is welded, on a part like this it will fatigue and become weak over time.

 

[snowborder714]

I was aware of the cost difference, but we're open to anything at the moment.

I will definitely keep the notes on losing properties when welded in my mind. Thanks for the info!

 

[Bud92gsx]

We plan on removing the fiberglass bumper, along with the styrofoam piece (obviously). This unit will replace that, as well as replace the two mounting brackets (that the bumper bolts to) that slide into the frame rail.

We're trying to avoid welding a lot as we don't have a welder or the skills to do so. Some welding may be required (or preferred in some areas), but we're trying to make it more bolt-in. Plus, heaven forbid something gets bent/broken/deformed during use, you'd have a lot more work to replace it if things were welded in. Then if someone ever asks for one to be made, the price wouldn't skyrocket because of outsourcing welding. But that's a distant thought and I don't want to steer this thread that way right now.

We'd also like to keep everything as removable and non-permanent as possible. Hence the minimal welding.

You must not have gotten what I was putting down..I think were on the same page..The whole thing I was talking about would be easily removable...Unboltable..The only welding needed..Is the plates and tabs..Where the tabs are welded onto the bumper, is where the whole thing can be easily unbolted..

Your picture was almost exactly what I had in mind..Your thinking too hard on this one Brian..

 

[gixrman]

Online metals Online Metal Store | Small Quantity Metal Orders | Metal Cutting, Sales & Shipping | Buy Steel, Aluminum, Copper, Brass, Stainless | Metal Product Guides at OnlineMetals.com is a good supplier for AL and has a good breakdown on the different grades of AL. I don't see why T6 wouldn't be strong enough. Also you guys don't live that far away do you? I can TIG something up for you.

 

[99gst_racer]

Great idea for a new part. Looks like an awesome project, guys.

 

[turbosax2]

Something not pictured (or even really thought too much about yet) is how to support the bumper up near the headlights at the factory support points there. A simple piece of aluminum will be doing that job; I don't foresee too much difficulty with that.

That model will weigh 13.35 lbs. in steel, 5.00 lbs. in aluminum (w/ steel tow hook). The dimensions of that model are just educated guesses at this point, and those weights don't include any hardware. I estimate the stock fiberglass bumper with both steel frame rail pieces weighs 15 lbs.

And here are a bunch of reference pictures of the area:
http://s104.photobucket.com/albums/m167/turbosax2/Bumper/



We're open to all ideas, so if you think there is a better way to attach a tow hook to the front end, we're all ears.

 

[greengoblin]

I would use steel. If you build the bumper right it will be about the same weight as an aluminum version and more fatigue resistant.

 

[snowborder714]

Your picture was almost exactly what I had in mind..Your thinking too hard on this one Brian..

Ok. It sounded like you suggested welding a plate to the frame, to which the bumper would bolt to. Maybe I misunderstood. As long as we're on the same page, we're good!

Online metals Online Metal Store | Small Quantity Metal Orders | Metal Cutting, Sales & Shipping | Buy Steel, Aluminum, Copper, Brass, Stainless | Metal Product Guides at OnlineMetals.com is a good supplier for AL and has a good breakdown on the different grades of AL. I don't see why T6 wouldn't be strong enough. Also you guys don't live that far away do you? I can TIG something up for you.

Thanks for the website! I was looking online a bit earlier for metal stock, but didn't have much luck. Granted, I looked for about 5 minutes.

You're correct, we're about 1-1.5 hours from Maryland. I think it takes us an hour or a bit more to get to Tom's house, if that helps

If we don't take you up on that, I might make a trip down anyway to get some instruction/pointers on TIG if you wouldn't mind. It's something I'd love to learn.

 

[99gst_racer]

I agree with Kevin. Mild steel over aluminum.

Who's constructing the fixture and doing the bending and welding?

Onlinemetals.com is an excellent site to order from. I've been buying from them for a few years now. I like their site layout and transit times are excellent now that they've recently opened up an Ohio location.

 

[snowborder714]

One step at a time, Paul We're trying to get a solid design down before we start looking at sourcing stuff and constructing.

Not sure who would be bending the tube.

Also have to figure out who would cut out the necessary pieces (mainly plates and tow hook).

As for welding, Chris offered to possibly do it. Or we have a great local guy who does all our other welding.

 

[99gst_racer]

Right on. I was just curious.

Instead of fabricating the hook, you might want to consider designing it to retrofit one of the existing available universal tow hooks. They're cheap and readily available, and it might save you time and money.

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[turbosax2]

I modeled a tow hook from here:
Racing Tow Hooks

3/16" steel - it's one of the better OTS ones I've seen. But it's actually quite cheap to have that same part custom cut.

 

[snowborder714]

Yup, we're not dead set on fabing our own. It would be better to use an off-the-shelf one as it'll probably be much cheaper. Just a matter of making sure bolt holes line up, length is good, etc.

 

[99gst_racer]

I wonder how well aluminum would hold up as a tow hook. That same design in aluminum is on ebay for about $22 shipped.

 

[turbosax2]

It's definitely a possibility, although I have my doubts. That particular one would stick out way too far, though. Also to note, it's 9mm, which is almost twice as thick as the ones I linked to.

 

[Calan]

When you get some definite dimensions and are sure of the loads and material, fire me off a model and I'll run some FEA on it like we did for the fire extinguisher brackets.

 

[turbosax2]

Can that be done with models of assemblies too? I now have bolts/nuts/washers and a tow hook spacer added to the model, and I want to make sure none of them will be the weak point.

In any case, I need to redesign the flat plates with holes; the bumper is hanging way too low!

 

[Calan]

I can do assemblies, but it's probably best to constrain it as one fixed part to find weaknesses in the structure itself, making the assumption that the fasteners are infinitely strong (i.e. a welded connection). We can then do some additional analysis that focuses on the fasteners if needed...although I'm not sure it would really be needed as long as the fasteners have the right yield strength and are clamped down properly.

Based solely on your model above, the vertical plates will most likely see the most bending stresses, and I have a hunch that they may need to be gusseted, depending on how much force the whole thing sees. In simple terms, it's going to want to bend the vertical plates and peel them off of the tubes that bolt into the factory frame.... I think.

 

[turbosax2]

Sounds good. By the way, I just started using SolidWorks two days ago, so it may take me a bit to modify this thing each time