Suspension Question - GT coils vs 620 lb coils - Vintage Mustang Forums

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post #1 of 18 (permalink) Old 04-18-2012, 06:28 PM Thread Starter
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Suspension Question - GT coils vs 620 lb coils

I'm finally resurrecting my 66 GT and going through the suspension. I ordered new GT springs front and rear for my car from Mustangs Plus, but now wonder if I should put 620 coils on the front instead. I took it to a local shop to do the Shelby UCA mod and install my new front coils (among other things) and he strongly believes these new springs will be too soft for my liking.

I plan to drive the car regularly on the street, and maybe 1 track day a year. I do drive on the assertive side (ok, aggressive), and I like a firm but comfortable ride and want the car to corner flat. I don't want a harsh ride that bounces over every bump and pebble in the road, and don't want it to feel like it's skipping all over a rough road.

So, those of you with 620 springs, what do you think? How do your cars ride? How do your wives and girlfriends like the ride. Both my BMW 328 and Pantera have firm but supple rides, and that's what I'd like from the Mustang. BTW, the car has KYB Gas-Adjust shocks.

Thanks!
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post #2 of 18 (permalink) Old 04-18-2012, 06:40 PM
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Hey Garth,

I hear your concern well. In short, you will be pleased the GT coils. If I recall correctly, the GTs have are 550# as opposed to the 620#. I originally had the 620s on my car (an aggressively driven daily driver) and they were WAY to harsh. I swapped them for a pair of 550s and these are MUCH nicer, and also seem to match better with my stiff rear springs. I have a 1" sway bar and the same shocks you do, and the car handles excellently with almost no body roll. So, be OK with your purchase and don't look back.

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post #3 of 18 (permalink) Old 04-18-2012, 08:35 PM
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I m have similar thinking. I am thinking of ride height too.I think there could be more messing around cutting GT springs for ride height where the 620 1 inch down would be closer to what I what. It sounds like from what I heard here, that you can get away with harsher spring rates like the 620 if you go the roller perches also.


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1965 Fastback-Scat 331, Dart heads, FPA headers, Air Gap intake, 650 Ultra DP, Cobra pan, Lunati VooDoo cam 61003, C-4, 10 inch converter, Reverse manual valve body,Detroit Truetracw/3.50s, subframe connectors.
1968 S-code Torino Fastback= project
1966 A-code Mustang coupe=Basket case not sure what to do with it used some parts for the fastback.
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post #4 of 18 (permalink) Old 04-18-2012, 09:58 PM
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I'll trade you my 620 's !


64.5 Coupe (owned since 1998)
302 / T5, 9inch Rear 3:50 Traction Lok, Shelby style glass packs, Granada Disk, Full MSD Set Up, Nightmist Blue
GT Wanna Be
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post #5 of 18 (permalink) Old 04-18-2012, 10:27 PM
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The Mustangs Plus 620 spring is not a 620 rate, but a .620" wire. They have a spring rate ~540.
GT springs are rated ~275 lbs. IIRC.

I'm with the mechanic if the car will be driven hard. Talk to Mustangs Plus and get their opinion.

Something else to remember is that ride quality comes from the shock, not the spring. The spring dictates how far the suspension moves when you hit a bump. The shock dictates how FAST the spring will react. If the spring reacts slowly over a bump, the bump will feel harsh because the spring didn't have time to compress. If the shock lets the spring react too fast over the bump, the ride will feel mooshy, soft and uncontrolled. People often cheap out on shocks then blame the spring.

If I told you my spring rate you'd think I was nuts (way more than 620), but the ride is on par with a stock S200 or Nissan Z. The ol lady and kids have never complained about the ride. I have good shocks though.

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DD, mild 331, T5, torque arm
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post #6 of 18 (permalink) Old 04-18-2012, 11:34 PM
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I have 620's and it is fairly smooth.

Fitting 315's on the '66
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post #7 of 18 (permalink) Old 04-18-2012, 11:36 PM Thread Starter
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Rorin67 - Thanks for the feedback. Are those stock "GT" springs from Mustangs Plus? They claim to be the only vendor that actually reproduce the GT spring.

65since88 - The GT springs are 1" lower ride height than standard springs. Therefore they're the same ride height as the 620 coils.

64NAHALF - Why? "Inquiring minds want to know!" Is the ride quality too harsh?

Campindog - Thanks for the clarification on the spring rates - very good info! FWIW, I did talk to MustangsPlus when I ordered everything. I was actually planning to order the 620 springs with the 4.5 leaf mid-eye rear springs and the Mustangs Plus guy talked me into the GT springs insisting I'll be much happier with them. But my local Mustang Shop is insisting they'll be too soft for my car and driving style. I've also had several other friends offer the same feedback (although I've never ridden in their Mustangs).


Thank you everyone for your feedback. It's all valuable information. Now if I could just find someone local to me with a set of 620 springs on a 65-66 Mustang who could give me a ride, I could make a decision based on first-hand experience.

I'm so anxious to drive this thing again - it's been 10 years! Watch for a new thread with pics detailing the resurrection.
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post #8 of 18 (permalink) Old 04-18-2012, 11:40 PM Thread Starter
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Quote:
Originally Posted by Campindog View Post
If I told you my spring rate you'd think I was nuts (way more than 620), but the ride is on par with a stock S200 or Nissan Z. The ol lady and kids have never complained about the ride. I have good shocks though.
So what shocks do you have? Sounds like a critical piece of the puzzle.

Thanks!
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post #9 of 18 (permalink) Old 04-18-2012, 11:59 PM
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Originally Posted by Garth66 View Post
So what shocks do you have? Sounds like a critical piece of the puzzle.

Thanks!
Ya, I had 750lb progressive springs and adjustable struts on my 89 hatch and it was a kidney buster. I'd like to know this spring rate too.

Fitting 315's on the '66
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post #10 of 18 (permalink) Old 04-19-2012, 07:37 AM
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Originally Posted by Z06killinsbf View Post
Ya, I had 750lb progressive springs and adjustable struts on my 89 hatch and it was a kidney buster. I'd like to know this spring rate too.
Keep in mind that when comparing different cars, the spring rate doesn't say much if you don't take the leverage of the control arms into account (motion ratio = inch of spring compression per inch of wheel movement). With the motion ratio known, you can calculate the wheel rate by multiplying the spring rate with the motion ratio squared (because the leverage increases both the force on the spring and reduces the compression). 79-93 Mustang lower arms have a motion ratio of 0.5, so with a 750 lb/in spring, you'll get a wheel ratio of 0.5*0.5*750= 188 lb/in. A vintage mustang has a motion ratio of 0.6, so a softer (~520 lb/in) would result in the same wheel rate. This also explains why the rear leafs are so much softer than the front coils. The leafs attach directly to the axle, so the move up and down with the wheel at the same rate, which makes the motion ratio 1.0. That means that the leafs can be about 3 times as soft as the front coils for the same wheel rate.

Actually, wheel rates don't mean so much either. If you really want to compare different cars, you need to look at the ride frequency, which also takes the unsprung weight into account. As I understand it, it's the frequency that more or less determines how comfortable the springs are. A nice article on this topic can be found here (springs and dampers part 1): Technical Papers - OptimumG

The original GT springs were around 290 lb/in I think. Boss 302 coils were about the stiffest the original springs got at around 350. A quick calculation shows that the ride frequency of both is very similar (1.2 vs 1.25 Hz), because the Boss has stiffer springs but is heavier too.

The frequency goes up with root of the spring rate, so installing a 600 lb/in instead of 290 lb/in GT springs doubles the wheel rate but ride frequency goes only up 40%. Enough to be objectionable, but not as much as you might expect by just looking at the spring rates.

Since it's largely a matter of preference, you would need to test them before you know what you like and since you already have the GT springs, I would install those and see how you like them. If you don't, switching to stiffer coils isn't that expensive or time consuming.
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post #11 of 18 (permalink) Old 04-19-2012, 07:41 AM
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Originally Posted by Garth66 View Post
64NAHALF - Why? "Inquiring minds want to know!" Is the ride quality too harsh?


Yes they are for me , My style of driving has changed since I installed the 620 1" drop springs , I now just want to go out for a nice cruise , wromp on the loud pedal once and a while , but I'm not pushing the 30mph turns at 60 anymore, I have new Grab A Tracs 4.5 standard in the rear and I'm happy with them but I would like to soften the front a bit , the 620's are so tight you don't need a spring compressor to install them


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302 / T5, 9inch Rear 3:50 Traction Lok, Shelby style glass packs, Granada Disk, Full MSD Set Up, Nightmist Blue
GT Wanna Be
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post #12 of 18 (permalink) Old 04-19-2012, 10:00 PM
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+1 to what Helmatel said. Thanks for the article. All I really know about suspension frequency is that front and rear should match. I gots some reading to do.

I've had 750 springs that where underdamped and the car rode like a 69 caddy. I was into the bumpstops just going down the freeway. I aint giving out MY combo cause someone will try it half way and break a tooth or rupture a kidney. Besides it makes no sense for the OP and that's not what I would recommend for him.

To the OP, if you are expecting comperable performance to your BMW & Pantera out of a $30 shock (1/2 that to manufacture), I think you will be dissapointed.

For srings in the 300-500 range, you should look at the Bilstien street valved shocks. They are available from Street or Track (a sponsor here) or just up the street from you at Maier Racing in Hayward. They are ~$350 a set and worth the $ for both comfort and handling.

BTW, I have a pair of the fronts sitting sitting on a shelf if you're interested in used gear.

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post #13 of 18 (permalink) Old 04-20-2012, 03:20 AM Thread Starter
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Originally Posted by Campindog View Post
To the OP, if you are expecting comperable performance to your BMW & Pantera out of a $30 shock (1/2 that to manufacture), I think you will be dissapointed.
Never said anything about a $30 shock. The KYB's are just whats on there now. I also have a set of new Koni's to put on. Thanks for the recommendation on the Bilstiens.

Was only looking for other's experiences with their combinations on 65-66 Mustangs.

Thanks everyone who replied! All good info.
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post #14 of 18 (permalink) Old 05-29-2012, 06:19 PM Thread Starter
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Suspension update:

If you've been following the "66 GT Barn Find" thread, then you already know this...

As far as springs and shocks, I decided to install the Mustangs Plus reproduction GT springs. They set the car at 1" lower than the standard Mustang springs. I also installed a set of Koni orange shocks (set on the softest setting). Took the car for a shakedown run this weekend; was pleased with the combination.

Also installed the TCP subframe connector kit with cross member and driveshaft hoop, Traction Master underride traction bars, and did the Shelby UCA mod.

Now need to address the sloppy steering - way too much play in the wheel, and the ratio is too slow.
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post #15 of 18 (permalink) Old 05-30-2012, 04:39 PM
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I aint giving out MY combo cause someone will try it half way and break a tooth or rupture a kidney.
This might be one of the most ridiculous things I've read on this forum in 10 years.
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