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Rear Spring Bumpers

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6.7K views 37 replies 6 participants last post by  docmart  
#1 ·
Hey all.

I'm starting this thread to share information.
Having never seen a new set of these bumpers available anywhere, I'm casting some.

Currently the plan is to make copies of the model, retaining shape, without the tip being hollow at the bumper end. It will have the cavity inside where the metal cap is on the original. The metal cap will not be in it though.
Casting will be Urethane 60A; https://www.mcmaster.com/8644k24

This video shows the repair of the bottom seat of the original bumper for use as the master.
It is currently clamped together for the glue to dry.

Mostly, updates will include links to videos, so everyone can see what is going on.
Please offer any suggestions, if you can think it will improve the bumpers. Such as; Should I remove the hollow tip portion (about 3/4" off the tip) of the bumper and make it more flat on the top? Should the cavity be only as deep as required to sit on the mounts button?.

I'm going to begin making the base for the model to sit on. Should begin laying on the clay in a few days.
 
#4 ·
The mold came out great. I've been cleaning it up and getting it ready to pour the 2nd part.

I've ordered the poly, 60A. Will give it try and see if that is the ideal durometer. I ordered a pound. I think that will be enough for two of them. I'll fill the mold with water to see how much it takes. I'll guess maybe less than 6oz each.

Anyway, that is where I am.

I'll post another video sometime. Sorry I don't have any video editing software. Maybe I'll try to find something and do a short video using clips from each process. Problem is, finding time to learn how to do it :(
 
#5 ·
I'd be interested in a set or two. I thought about doing this but I was worried about the spring going through the poly. Just a tip for casting, you should mix the parts together then put it in a vacuum chamber to get the air out. It only takes one bubble to ruin a mold. It gives a lot better results and you can make one with a pressure cooker. That is what I did.
 
#6 ·
cavalier3400z said:
Just a tip for casting, you should mix the parts together then put it in a vacuum chamber to get the air out. It only takes one bubble to ruin a mold. It gives a lot better results and you can make one with a pressure cooker. That is what I did.
Click to expand...
Good tip and certainly required for most types of silicone. I thought about making one when I was watching videos on mold making. I have a vacuum pump. Smooth-On says the Mold Star 30 doesn't require degassing. That's the only reason I didn't get the stuff to make a chamber.
 
#7 ·
Well the 2nd half of the mold is poured.

Spent some time building up the spring end seat. Should work well. I used 24 Oz for this pour. Brought it right up to 1/16" from the top of the box. I'll unbox it in the morning and see how it goes.

I did record the work I did on the spring end of the master. It is long though, so I'll figure out how to edit a video sometime in the near future.

If all goes well, I'll cast the first copy this weekend. I can only do one at a time, so I'll have to divide the urethane.
 
#8 ·
Opened it up this morning. All is good 8)
The master came out without too much trouble.

Poly is supposed to be here today. So I'll cast one this weekend. I'll probably leave it in the mold for at least a couple days. It says it can be de-molded in 2 hours though.
 
#10 ·
I just filled the mold with water to see how much it held. It came to about 6.75 ounces. According to the data sheet that came with the 60A poly, it mixes 100.00 to 10.00. One pound will be enough to make 2, with a little bit wasted. Since it is so close to using all of it, I'll just divide the poly and hardener by half to make the two pours.
I'll weigh in grams once I'm ready to mix it.

Hopefully everything will turn out well.
 
#11 ·
Just poured the first one. 1/2 the product brought the level up to the very bottom of the fill hole. One pound appears to be the perfect amount for two of them. I've got exactly the same amount left, 200gr of resin and 20gr of activator.

I'll let it sit overnight, de-mold in the morning and pour the second one. I'll video it.

Hopefully there won't be many bubbles in it. I pour it slowly in a fairly long stream. While the Mold Star 30 silicone did not need degassing, the urethane could have used it. But it starts to thicken pretty quickly. One would have to work fast to get it mixed, degassed and pour in the time you'd have.

Here are a couple pictures pf the fill hole and where the poly came to.
 

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#14 ·
They are mounted and all seems good. Did a test drive including going over uneven side road turn offs, curves and bumps. No negative effects.

Will drive a few hundred miles over the next couple weeks or so to see how they hold up.

I measured the height of the car at the rear wheel wells before I changed them out. These raised the rear of the car about 3/8-1/2 inch. Roughly the thickness of the base. The originals were worn down to nothing, so the base of them were not really holding the spring off the axle any amount.

I'll call it a success, so far anyway.
Cost quite a bit though.
Mold making rubber - $77.80
Clay (reusable) - $16.20
Casting compound (took every bit of the 1 pound size) - $44.97
Mold release (still have most of it leftover) - $17.00
Misc items - $5.00
Total $160.97 :eek:

Though now with the mold made, making more bumpers will pretty much just be the cost of the casting compound and a shot or two of the mold release. However, if I buy the 10 Lbs size of the compound, the price for making more of them will probably be a little less. Though shipping 10 Lbs may cost a bit. I don't know. I'll need to order some before long, as all my cars need these replaced. 10 pounds will make 20 of them.
 

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#16 ·
docmart said:
Looks great Butch. Thanks for all the videos. I'd be interested in getting a set if and when you care to produce some.
Click to expand...
Thank you for the kind words.
I'm going to test these for a while to make sure they hold up and are safe to use. Once I'm confident they are, I'll buy 10 Lbs of poly and make 10 sets. I'll need to keep 3 sets for my other cars, but will sell the rest. Price will be around $60.00 a pair, I imagine. I'm going to keep the price as low as possible.

I'll keep you posted.
 
#18 ·
Sure thing Brad, thanks

Been driving the car daily for over 2 weeks now, around 600 miles. All seems fine with them. Haven't taken one out yet.

Been looking at poly and thinking 70A may add to the life of them. Saw this type made by Smooth On.
https://www.smooth-on.com/products/pmc-770/
I'd want to talk to them about it though. Price is well bellow McMaster.
The PMC-770 is $207.05 for 24 Lbs. Whereas the McMaster is around $217.22 for 10 Lbs of their 60 or 80A compounds.
Don't have shipping quotes for either though.

24 pounds is too much though. The shelf life is something to consider. It would need to be used rather quickly, to form a quality product. I'd need to find other things to make with that much poly. Would really lower the price per pair of the bumpers though, but 24 sets is quite a few to be making. Are there even 24 cars left that might need them? :lol:

Anyway, I'll get some more miles on these and take them out in a couple or 3 weeks to inspect them closer.

Anyone want to share your opinions on the durometer? Hoping for long lifespan yet not so hard that they become brittle with age?
 
#20 ·
Thank you. I appreciate your kind words. I'm not into making videos, but it is cooler than taking a lot of pictures, especially for something like this.

I've put 4,000+ miles on them now. Visual inspection appears all good. I'll take them out this weekend to get a better look. I'll get a video of them.

I'll also need to contact Smooth On to get their thoughts on using the PMC-770 product for this application.

I also want to find out about their dyes.
https://www.smooth-on.com/products/so-strong/
The 770 is Amber color, but they make various colors of dye that can be added. I suppose BLACK will suit everyone? Now, if anyone did want a set in something other than black; It should be possible to make them in any of the colors, especially if they sell the dye in small amounts. Might be a problem getting both bumpers the same tint/shade though.

I need to find out the demold time too. I didn't see it mentioned on their site. Hopefully it doesn't have to remain in the mold for the entire cure time of 16 hours. They need to be heat cured 4 to 8 hours too. I've lots of questions for them.
Need to find out the shelf life of the cured product. I'd want to know how long they can be stored without deterioration.

If this product is suitable for these, price will be dirt cheap.
 
D
#21 ·
Again, thanks Butch.

The site says:

Cure time can be reduced with mild heat or by adding Smooth-On "Kick-It®" Cure Accelerator.

XTEND-IT® Dry Gas Blanket (available from Smooth-On) will significantly prolong the shelf life of unused liquid urethane products.

This is rather vague information but you can ask about it. Of course, it costs more to use these products.
 
#22 ·
docmart said:
Again, thanks Butch.

The site says:

Cure time can be reduced with mild heat or by adding Smooth-On "Kick-It®" Cure Accelerator.

XTEND-IT® Dry Gas Blanket (available from Smooth-On) will significantly prolong the shelf life of unused liquid urethane products.

This is rather vague information but you can ask about it. Of course, it costs more to use these products.
Click to expand...
Thanks for taking the time to look this up and pointing them out. I'll add them to my list of things to ask about when I call.
 
#24 ·
There is a Smooth-On distributor in Orlando. I see these people are experts in casting products.
I'd much rather speak to them in person than to call.
I'm going over there today. I'll take the old original bumper (I used as the master) with me.
 
#25 ·
Well, not so good.
This video explains it all.

But never fear. Were there's a will, there's a way!
Figuring out what material to use, the hardness and putting together a vacuum chamber. I'll be ordering more poly soon. Lots of studying still to do.

These were made with McMaster-Carr 60 Shore A Urethane Casting Compound. Failure to degas is likely cause of breakdown of material.

60 Shore D is a very hard, simi-rigid material. It might be very good for these. Or it might be too hard.

80 Shore A may be fine too. It is hard, from seeing my engine mounts.

McMaster-Carr has either one. However, they both have very short work time, especially since degas is required. They say 15 minutes, but it begins to thicken up in much less than 15 minutes, (if it reacts like their 60A did). Degas would have to be done very quickly. I don't much like that.

I'm going to keep researching and order something by tomorrow evening.

Three things I'm looking for;
Relaxed working time.
Quick demold time.
Correct durometer to use, be it Shore A or D.

Anyone happen to know what durometer Energy Suspension Urethane Bushing are? Do you think it would a good hardness to use?

Here is a shot from Fibre Glast. It looks sweet, but is very expensive. Colors offered are Black or White.
 

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#26 ·
Bit of an update.
I've not given up! Just lots going on and too many things to do.

I've got a vacuum chamber together, along with a dedicated oven for post curing.

I'm going to try a 45 Shore D urethane. They will be quite hard, semi-rigid. They should be extremely durable. The color is transparent amber. It can be dyed black, but I'm not going to use any in the first set, at least. I like the idea of the poly being transparent, for inspection.

I'm going to do my best to get the set made the weekend of the 15th. If I can't get them made then, it will be the weekend after the 4th of July. I'm going to do my best to get them made and installed in the car I'll take to NC for 2 weeks. That will put at least a couple thousand miles on them, if not close to three. I'll probably be driving around a lot while up there.
 
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