Finding the instant center on a leaf spring car

Suspension Tuning, Troubleshooting, Design and Discussion

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John_Heard
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Finding the instant center on a leaf spring car

#1 Post by John_Heard » Fri May 27, 2005 9:35 am

Ok, I'm jealous... I've seen all the information on 4 link and ladder bar cars, the calculators, etc. for finding the instant center, percentage of rise, etc.

How do you do it on a leaf spring car?? I know the front spring mount has to be related, but how does the rear spring mount, distance to center of rear end to leaf spring, etc. all figure into this?

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#2 Post by Bobalos » Fri May 27, 2005 9:45 am

I wanna say that the IC on a spring car is right @ the front spring mount. I will check my chassis book this weekend & see what is says. I bought both Morgans "door slammers" book & Rick Jones chassis book (http://www.rjracecars.com/), & IMO, the rick Jones book is a more hands on approach & the Morgan book is more the science of chassis. one of them had something about it. I dont suppose it really much matters simply becuase you cant change it, unless you want to cut up the car & move it. Maybe try posting it on the zone & see if Al has a comment on it.


Bob

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#3 Post by John_Heard » Fri May 27, 2005 10:16 am

I've got both of those books, to the best of my memory neither one talks much at all about leaf spring cars, just 4 link and ladder bar.

I can change the height of my front spring mount at little (two adjustments), height of the rear mount, the height of the car and perhaps spacers between the rear end and spring (surely that matters too?).

I'm also curious as to how cal-trac bars affect the instant center. Maybe I ought to call up Calvert and see if they have some suggestions.

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#4 Post by Bobalos » Fri May 27, 2005 10:20 am

I have a buddy that talked with them for his Box nova & they were VERY helpfull. Too bad they are still sitting in the shop & he has not put them onto his car :(.

I wanna say that it only extends the IC by the distance that the line is projected out from the connection points between the spring eye & the line out from the lower bar on the cal trac bar (maybe 2 inches out). I will have to check again.

I wanna say that there is a VERY short paragraph in the Morgan book about spring cars. Not much, but a little bit.

Bob

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#5 Post by Craig W. » Fri May 27, 2005 11:04 am

By definition, IC is the point that the suspension rotates around. Neglecting spring deflection, that's the center of the front spring eye.

Caltracs also pivot around the front spring eye. They don't effect IC as its normally defined.
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#6 Post by John_Heard » Fri May 27, 2005 11:08 am

Ok I talked to Travis at Calvert, he was great and went over all this with me.

He said basically that on a stock suspension car, the instant center runs from the contact patch of the tire forward through the front spring mount. With the Caltracs, the front point switches to the location of the front hole in the caltrac plate. With the bar in the lower hole, the instant center is at it's most forward and lowest point.

Moving the leaf spring front mounting point down also changes the instant center down and forward. The rear spring mounting point doesn't have much effect on the instant center. The length of the spring from the rear end to the front spring mounting point also affects the instant center, longer distance to the front eye from the rear end moves the instant center forward. He also noted that increasing preload also will calm the hit down, for example with the bar just touching the spring or with a slight air gap the suspension will be more violent than with it preloaded. He said it's common on a high horsepower car to run up to 1 1/2 turns of preload to calm it down.

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#7 Post by Bobalos » Fri May 27, 2005 11:19 am

Hey Craig,

a couple of years ago I was in KY on business & managed to squeeze in the Pro race there. were you there pitted next to Chris Uratchko? I wanna say I seem to recall seeing the 2Nova's car there?

Bob

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novaz
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not about leaf springs but interesting

#8 Post by novaz » Fri May 27, 2005 12:16 pm


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#9 Post by Craig W. » Fri May 27, 2005 12:30 pm

Bobalos wrote:Hey Craig,

a couple of years ago I was in KY on business & managed to squeeze in the Pro race there. were you there pitted next to Chris Uratchko? I wanna say I seem to recall seeing the 2Nova's car there?

Bob
Yeah, that was me Bob. You've got an excellent memory!

Chris and I met 7-8 years ago and he's taught me a lot! Passed down a lot of good parts too :D

I was running Nostalgia Muscle Car that weekend. Since I've found more local heads-up series to run, I haven't run with PRO since that weekend. I stay closer to home and run with Ohio Valley and OSCA.

Are you coming to Bowling Green next weekend for the PRO race? I'll be there hanging out with Chris if he gets his car done.
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#10 Post by Bobalos » Fri May 27, 2005 12:37 pm

VERY cool. that picture tells a LOT of how the IC affects the car. I REALLY need to go back through that chassis book & then make some changes to the car & see how it affects my combination.

thanks
Bob

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#11 Post by Craig W. » Fri May 27, 2005 12:39 pm

Beyond wrote:Ok I talked to Travis at Calvert, he was great and went over all this with me.

He said basically that on a stock suspension car, the instant center runs from the contact patch of the tire forward through the front spring mount. With the Caltracs, the front point switches to the location of the front hole in the caltrac plate. With the bar in the lower hole, the instant center is at it's most forward and lowest point.
So I guess the definition either has changed a little or there is a difference between actual instant center and effective instant center (or something along those lines).

I talked to Travis quite a bit when ordering my Calvert springs and he's a lot of help. Lots of good info and willing to take the time to share it.

I've been running my bars in the bottom hole. 1/2 turn preload on the driver's side and 1 turn on the right side. That makes it go straight. I have to be careful when taking weight in and out of the car though. I took 80 lbs of ballast out of it once last fall and didn't reset the bars. It didn't want to go straight on that run!
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#12 Post by Bobalos » Fri May 27, 2005 12:48 pm

No unfortunately not. Im on the west coast & have changed to a different company so I cant "accidentaly schedule" my business trips to coincide with races. LOL. Yea, Chris is one heck of a great guy.

You prolly dont member me, but I was the tall white guy with the Xtrm 66 Ballcap on. I helped you guys load up the car when it started to rain.

PS. sorry to hi-jack your thread :(

Bob

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#13 Post by John_Heard » Fri May 27, 2005 1:02 pm

So I guess the definition either has changed a little or there is a difference between actual instant center and effective instant center (or something along those lines).
Right, that's basically what he was saying that the front bar position changed the effective instant center.

So it looks to me like a person could use the same calculations as you would for a ladder bar car on this. Not that we have much adjustment to make, I would like to see what the numbers come out at, and how moving weight around in the car affects it.

I've been running my bars in the lower hole with about 1/2 turn of preload, the car goes straight but it does a moderate amount of twist in it. I'm just trying to think of things to do to help it 60 better. Going more forward instead of up should help as long as it's hooked up.

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#14 Post by Craig W. » Fri May 27, 2005 1:07 pm

My 60' times have never been anything to write home about either. 1.47 is my best. If the converter hadn't been built for all-motor at that time, I'm sure it would have done a lot better on the nitrous. Heck, it did 1.49 on the motor only, so a 150 shot should have improved it by more than .02! LOL
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#15 Post by John_Heard » Fri May 27, 2005 1:20 pm

I've got my work cut out for me, the cars in our class are 60'ing anywhere from 1.24 to 1.30's. My best so far was a 1.42, so I've got a ways to go.

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