Getting my vintage mini truck ready for towing - Page 5 - Fiberglass RV


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Old 07-10-2013, 04:46 PM   #57
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Originally Posted by chuyler1 View Post
Everything I've ever read says the rate increases if you cut it. I think the WD hitch will help...but it's just a bandaid.
I too have cut coils to lower a vehicle. It did firm up the ride which I wanted. An AMC Pacer with cut Olds coils.......
http://img.photobucket.com/albums/v2...05/pacer11.jpg

I don't think I would call the WDH a bandaid. It really is an upgrade to your combination. It is the way I would go first. Then see where you are at.
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Old 07-10-2013, 04:55 PM   #58
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The springs I found that fit are MOOG CC250. Which have a rate of 412 and a load of 786. Free height is 13.44" and installed height is 11.50". The only info I have on the OEM REPU springs is that they are 11 7/8" tall with a spring rate of 512 lbs.
Excellent info, Chris.

First, for those not familiar with springs...
  • The rate of 412 will be in pounds per inch, so each 412 pounds of load causes an inch of additional compression in length.
  • The free length/height is with no force on them.
  • The installed height is the length when carrying the designed load (the truck, sitting empty and stationary).
  • Compressing a 412 lb/in spring from the free height of 13.44" to the installed height of 11.50" (1.94 inches difference) should take 799 pounds... but the "load" value is the force at the installed height, and is slightly lower (786 lb) - explanation to follow. The total load for two springs as installed (1572 lb) should be substantially more than the empty weight on the pickup's front axle, because the suspension arms are levers and the spring location is only partway out them.

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Originally Posted by chuyler1 View Post
When I couldn't get the first set in, I cut them down from 13.44" to 12". This produced an acceptable ride height but very soft front end that would dip around corners and there wasn't enough clearance to turn the wheel full lock without rubbing the fenders. I thought like you that cutting the spring would increase the rate and make it closer to the OEM rate but that's not how it felt.
If the Moog springs were constant-rate, then cutting one down from the original free length of 13.44" to 12" would make it 1.12 times as stiff (12% stiffer, 11% shorter). That's 461 lb/in, still softer than the stock 512 lb/in spring. So cutting did make them stiffer, but apparently not enough.

The complication is that the Moog springs are not constant-rate - they're variable-rate. This is also called "rising rate" because as the spring compresses the more closely spaced turns at one end close up solid so it is as if they are no longer spring, but just a rigid spacer, leaving a shorter effective spring and thus a higher rate (stiffer spring). If the wider-spaced end was cut off, 11% of the length would be less than 11% of the spring. If the photo on the linked page is correct (it is actual part, not just a typical example) it has 6.5 free turns of coil, and cutting one off has the same effect on the starting spring rate whether it comes from the wider-spaced end or the tighter-spaced end... one turn would make it 18% stiffer than stock (487 lb/in). Also, the quoted rate is probably an average, so the actual rate at full length is lower (see the load value note above), and at the installed height the first coil might already be closing up.

The Moog springs were probably good before cutting, because they are intended for use at higher loads, and in that case they would be compressed enough to be in their stiffer range... they are probably supposed to carry a heavier load at stock height, not a stock load at raised height... or a reduced load at raised height. Of course, Moog doesn't list them for the REPU at all - they're for the Courier and other Mazdas, so they fit the suspension but maybe are not appropriate for the loading.

Also, those springs are square on the low-rate end and the spec says tangential on the other end (square means the last turn is closed up so that it meets the next turn - hard to see in the photo). Some coils have ends ground flat so they work on flat mounts, but these are the more typical version that needs a seat shaped to hold the coil end. If the square end is just cut off it becomes tangential, and some force will be required to collapse that first turn down to match what the factory-prepared end always looks like.

The way to change height without changing the rate of the springs is with spacers... but they take up space and thus may limit compression travel. Would the stock spring fit with a spacer, or would it close up entire and block further movement before it should?

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Originally Posted by chuyler1 View Post
I'm thinking the WD hitch will pull the front down to a safe height.
WD would work, but it just seems like a lot of hardware and operational hassle to fix a problem with wouldn't even exist with the front suspension at its stock height.

Back to the fundamental nature of the situation...
The longer springs are in there to raise the ride height moderately from stock. Doing this solely by a spring change - any kind of spring change - means moving the normal ride point higher in the same range of travel, and that means giving up droop travel - that's a problematic combination with reduced front axle load, even though the load is still above that of an empty truck.

The solution for raised trucks is to not just extend the spring, but relocate the suspension mounting points downward - or from the other perspective, lifting the truck off of the suspension. I doubt that's worthwhile in this case. The slickest systems lower the bottom control arm, leave the top control arm where it was, and connect them with a longer spindle carrier;however, there is not going to be an extended spindle carrier for a REPU... or anything from Mazda, or anything from 1975.

Next not-as-wild idea: use the stock springs, add cheap air bags (Airlift 1000 or Firestone Coil-Rite) inside them, and pump them up to add just an inch or so of ride height. These bags are not intended for lifting, but presumably by staying within the stock suspension travel range they would be acceptable. They can be let down (to low but not zero pressure) to correct ride height when towing without WD (or for the original "low rider" attitude). Firestone doesn't list applications this old and obscure, so a suitable match would need to be found based on spring inner diameter and length.

One last techie note: the solution in oval-track "stock" cars to adjusting coil-spring rear suspension is to put the top mount on a screw jack, and turn to desired height. This doesn't change the rate, but does change the height for a given load... it's like a quickly adjustable spacer. I don't think I would try to fit one those into that spring mount area, especially with the droop stop for the upper arm.
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Old 07-10-2013, 05:01 PM   #59
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An AMC Pacer with cut Olds coils.......
http://img.photobucket.com/albums/v2...05/pacer11.jpg
Wow - a chopped Pacer! It doesn't look chopped, it looks like it was designed to be that low.

Back to the REPU as trailer tug...
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Old 07-10-2013, 05:47 PM   #60
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Awesome info Brian.

Unfortunately I don't have the stock springs and I can't seem to find anyone willing to sell them to me. The truck came with a set of coilovers that a guy on the mazdarepu forum sells. Even at their highest height they provide a 2" drop from stock. Here they are next to the MOOG springs (before I cut the MOOGs)...



You are correct about them being progressive instead of linear. They also don't have the tangental top coil like they are supposed to.

There was an intermediate step I took after cutting but before purchasing a new set of coils. I installed a spring rubber. Although it claimed to offer up to 2" of height, it only lifted the truck 1/2" more than it was with the cut springs. I wasn't able to install it near the bottom where the coils are closer. I could only get it in the top where the springs don't really compress that much. The ride got a little firmer but I was still rubbing the fenders when I turned.



So it comes back to my only real option being cutting the spring...but not as much as I did before. The rate won't be right...but I'm not sure what other options I have. Maybe if there was a business that could make affordable springs to spec...and I knew exactly what specs I wanted.
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Old 07-10-2013, 08:56 PM   #61
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Chris, you are a loooong way from Portland, Oregon. But there's a whole bunch of hot rodders in PDX. Have you checked with Benz Spring to get EXACTLY what you want? Over the years, I've purchased six sets of coil springs from them... great CUSTOMER service: Benz Spring Company Home Page
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Old 07-10-2013, 09:01 PM   #62
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Thanks for the link Donna. Maybe I'll give them a call.
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Old 07-10-2013, 09:08 PM   #63
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Thanks for the link Donna. Maybe I'll give them a call.
Phone calls are cheap compared to fitz to starts. Be sure to tell them a member of "Henry's Half-Tons" recommended them.

Good luck!!
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Old 07-11-2013, 12:38 AM   #64
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Oh and did I mention is was 90 degrees and 100% humidity? My wife overheated in the truck (no a/c) but the cooling system did not.
Wow! New record dew point for the United States!

Kidding... I'm sure it was hot and humid... Just not THAT humid!
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Old 07-11-2013, 07:33 AM   #65
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As far as drive train cooling goes very humid air helps more that very dry air.
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Old 07-11-2013, 09:29 AM   #66
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Wow! New record dew point for the United States!

Kidding... I'm sure it was hot and humid... Just not THAT humid!
Weather History for Concord, New Hampshire | Weather Underground
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Old 07-11-2013, 09:41 AM   #67
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As far as drive train cooling goes very humid air helps more that very dry air.
I agree.
That's why old British cars overheat everwhere except back in Britain, where they are fog-cooled.

Sorry, couldn't resist
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Old 07-11-2013, 09:45 AM   #68
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I agree.
That's why old British cars overheat everwhere except back in Britain, where they are fog-cooled.

Sorry, couldn't resist
LOL, I think you're on to something. Maybe that's why the tow ratings are higher for the same vehicles in Europe too!
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Old 07-11-2013, 09:49 AM   #69
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100% relative humidity is when the temperature equals the dew point. In that weather record, this happened only at lower temperatures; during the hottest time, it was 46%. The graph shows this, but way the numbers are reported in text is somewhat confusing. The whole idea of relative humidity makes sense, but I doubt many people understand it, unless they're really into weather or an area of technology such as HVAC.

Now, about those springs...
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Old 07-11-2013, 10:13 AM   #70
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100% relative humidity is when the temperature equals the dew point. In that weather record, this happened only at lower temperatures; during the hottest time, it was 46%. The graph shows this, but way the numbers are reported in text is somewhat confusing. The whole idea of relative humidity makes sense, but I doubt many people understand it, unless they're really into weather or an area of technology such as HVAC.

Now, about those springs...
Right... I other words it may have been 90 degrees that day, and it may have been 100% humidity that day... But it wasn't both at the SAME TIME that day. 90 degrees F at 100% humidity would have been a de point of 90. The record high humidity in the United States was a dew point of 88.

Relative humidity is not a very good measure of how it 'feels' outside. Dew point is the number you want. Anything over 70 is pretty nasty.

If it was 90 degrees and 46%RH, the dew point was about 67. Right now, at my work, it is 76 degrees (chilly summer) and 91%RH, which is a dew point of 73.
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