Finding the instant center on a leaf spring car

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John_Heard
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Re: Finding the instant center on a leaf spring car

#46 Post by John_Heard » Wed Apr 30, 2014 9:51 pm

Good deal, that ought to work better for ya.

grandville455
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Re: Finding the instant center on a leaf spring car

#47 Post by grandville455 » Sun May 04, 2014 10:01 pm

Update,
Car was AWESOME today!! worst 60 ft 1.486, best 1.464.
New personal best 10.508@126.33, new best 1/8th 6.65@ 102.43, Front shocks at 2 off full loose, rears at 16 off full tight on EXT AND 15 OFF FULL TIGHT ON COMP.The car is lifting and carrying the wheels with authority now! I've never had so much fun launching her,lol.. Leaving right from idle. No adj today as it was a bracket day, went to the semi's.

here's half speed 1.47
https://www.youtube.com/watch?v=G-0nTB_xqfk

here's reg speed 1.466
https://www.youtube.com/watch?v=UDQZxWx3zsA
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

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Re: Finding the instant center on a leaf spring car

#48 Post by John_Heard » Mon May 05, 2014 7:00 am

Looks like it's working pretty good. If I remember right it looks like it's getting more extension in the rear now too.

grandville455
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Re: Finding the instant center on a leaf spring car

#49 Post by grandville455 » Mon May 05, 2014 7:20 am

Thanks John, Yes I think its seperating better also, It looks like the comp could go a tad more tho?
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

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John_Heard
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Re: Finding the instant center on a leaf spring car

#50 Post by John_Heard » Mon May 05, 2014 7:56 am

It looks like it's dead hooked now best I can tell. Now the question is how much tighter can you set it to work for the track conditions you're running on. Tighter extension will react quicker and it will show both in reaction time but also in 60'. However you'll run the risk of going too tight which of course may not hook at launch but also may cause the car to break loose on bumps down track. Don't get too wrapped up in making compression tight regardless of those that keep telling you to run it full tight, they really don't need to be overly stiff on comp.

grandville455
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Re: Finding the instant center on a leaf spring car

#51 Post by grandville455 » Mon May 05, 2014 7:59 am

Yea, I guess Ill wait and see how the track is next time out and if hooking try 2 clicks tighter on EXT just to see, I will leave comp for now then, These are slicks and not radials
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

grandville455
Posts: 176
Joined: Sat Sep 13, 2008 9:11 am

Re: Finding the instant center on a leaf spring car

#52 Post by grandville455 » Tue May 06, 2014 1:25 pm

Now that I have a baseline , will it 60 better by either tightening the front ext some , or by foot braking instead of flashing it? I can only go to maybe 2000 before it starts pushing it.
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

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John_Heard
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Re: Finding the instant center on a leaf spring car

#53 Post by John_Heard » Tue May 06, 2014 3:15 pm

Anything you do to reduce reaction time will show up either in reaction time and/or 60' ET. Tight extension is quicker than soft, as long as it doesn't hurt the launch or upset the car. Same for stiff sidewall slicks or radials, less time for the sidewall to deflect before the car moves.

grandville455
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Re: Finding the instant center on a leaf spring car

#54 Post by grandville455 » Tue May 06, 2014 3:18 pm

Think I'll make a pass then tighten front two clicks, see what happens, still leave at idle next race..
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

grandville455
Posts: 176
Joined: Sat Sep 13, 2008 9:11 am

Re: Finding the instant center on a leaf spring car

#55 Post by grandville455 » Tue Jun 10, 2014 8:00 am

So cars been working great, tightened front shocks to 6 clicks on my qa1 r series. And these are my runs.
1.496,1.487,1.487,1.480,1.484,1.491,1.493, then car sat for 2 hours cause of a mishap from another car shearing his wheel studs on the hit, and landing on guard rail. Anyway car did what it did way back in april after sitting. I'm thinking it was cause I didn't heat the tires enough? Spun 1.54, then next round worse,1.61, I got even less heat in that burnout cause of a poor water box. Think that was my issue?
Up til then it was pulling the wheels and was deadly dialing it.
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

User avatar
John_Heard
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Joined: Thu May 12, 2005 11:20 am
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Re: Finding the instant center on a leaf spring car

#56 Post by John_Heard » Tue Jun 10, 2014 8:27 am

With it repeating that well earlier in the day it seems likely it was track prep, burnout procedure, tire pressure something simple that changed on those later passes.

grandville455
Posts: 176
Joined: Sat Sep 13, 2008 9:11 am

Re: Finding the instant center on a leaf spring car

#57 Post by grandville455 » Tue Jun 10, 2014 8:32 am

Yea, I checked pressure after that first run and it was over 17, so i figured thats why it spun and went 1.54, aired back down to 16 , but got a shitty burnout, and it was even worse.Thats what Im leaning towards anyway. I'm Putting a little convex mirror on my door so I know where to stop, screw these young kids and not giving a shit where they stop u! It did like the tighter front tho, even in 73 degrees, was still pulling them off :)
It should be fun come fall!
74 Pontiac Grandville
7.33- 1/8th @ 94.55/ 11.501 @117.74- 4550 pds
1.60 60 ft....all bests

74 Firebird 3580#
2016 1.377 60ft, 6.33@106.87- 10.02@ 132 mph

duke66
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Re: Finding the instant center on a leaf spring car

#58 Post by duke66 » Thu Oct 16, 2014 8:41 pm

This a quote from John Heard:
"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."

When I talked to John Calvert about my son's 68 Camaro, he told me to use upper hole and 1/4 turn preload, extension full loose. All the info I see about setting Caltrac's(Yellow Bullet forum), says the more preload, the harder the hit. Your quote from Calvert says add preload to calm the hit.

My son's car crushes the sidewall, from what you say, if we added preload, it would calm this down? Other question I have is most Stock Eliminator stick cars I see, have the bars in the lower hole. I told Calvert I thought a tranny brake hits as hard as a stick and he disagreed. What would be you opinion on this?

Thanks for any thoughts you have.

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Re: Finding the instant center on a leaf spring car

#59 Post by John_Heard » Thu Oct 16, 2014 10:51 pm

Note that conversation with those guys at Calvert was 9 years ago lol... I'm not sure I agree with all that now.

I think if you add preload you're going to tighten it up and hit the tire harder. Think about it, with the bars with zero preload, when the bar pushes the pin into the spring it's going to deflect a bit which is going to soften that motion. If you have preload on it, it's not going to deflect as much when the force is applied. It will be stiffer action, things will happen faster and it won't absorb as much energy. So no, I don't agree. More preload is going to hit the tire harder initially.

The upper hole in the plate will do the same thing, it lessens the leverage the bar has to deflect the spring making everything stiffer and faster acting. Lower hole will cause more spring deflection and let the housing rotate more dampening the hit at the tire some.

Full loose extension will also hit the tire MUCH harder.

I hate to disagree with those guys but everything you're doing is contributing to crushing the sidewall. Tightening the shock extension is going to dampen that more than anything. I'd back the preload off some also, 1/4 or a 1/2 turn ought to be plenty.

It's kinda weird seeing discussions from so long ago :thumb:

duke66
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Joined: Thu Oct 16, 2014 8:13 pm

Re: Finding the instant center on a leaf spring car

#60 Post by duke66 » Fri Oct 17, 2014 7:28 pm

Thanks for the response. This is still a hot topic.
What you are saying is what John Calvert to me this summer when I spoke to him. Top hole and preload is the hardest hit on the tire. We only have 2 flats of preload in it now. Our AFCOs have 40 clicks of adjustment and they are 20 extension and 12 Compression.
We do use a combination of shocks, bar adjustment and tire pressure to adjust reaction times on the car.
We are just bracket racers and always keep looking to make the cars more consistent. During the heat this summer on video, it looked like the car had about an extra turn of the tire in the sixty foot, instead of dead hook. Went from 1.46 - 47 to 1.50 - 51. I see lots of stockers with the bar in the lower hole running better sixty foots than us. Car runs mid 6.60's the way it sits. Wonder if lower hole would be better?
Thanks for the discussion.

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