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Author Topic:   Different Height Rear Springs??
Randy 37
Dirt Roller

Total posts: 15
posted February 17, 2005 09:55 AM  
How does installing different height rear springs effect weight transfer to the rear. For Instance, if you normally run 13" springs, then change to 11", then readjust ride height, you should gain weight transfer because the top of the spring is lower in relation to the center of gravity. The opposite should occur when installing a taller spring. Am I way off base on my thinking???
Thanks, Randy

zeroracing
Dirt Forum Champ
Total posts: 1875
posted February 17, 2005 10:09 AM  
you are thinking correct.. if on a spring cup and you bring the top cup down to compenstate, but if your on sliders and they act on the frame where the upper hiem is, so you would have to lower the hiem mount.
this is called the spring table, and there is potential on dryslick on the rr especially, not many people mess with it but some major dirt late models have experimented and i have a little but plan to work with it more this year.
also just lowering the 13 spring down in the car would do the same, if you could move both the top and bottom down in your mounts.
i comend you most people never think of anything like that, that is very good thinking.

[This message has been edited by zeroracing (edited February 17, 2005).]

Randy 37
Dirt Roller

Total posts: 15
posted February 17, 2005 10:20 AM  
Zero, thanks for the kind comment....sometimes I think too much and make something easy far more complicated. I'm looking at trying this on a limited mod that has a stock metric rear. The springs have to sit on top of the rear and we have an adjustable spring cup on top.

wfoondirt
Dirt Forum Champ
Total posts: 505
posted February 18, 2005 12:24 PM  
Can you bring any tech on how this is supposed to effect weight transfer? The generally accepted answer is that it doesn't. Physics is physics.

The sprung mass of a racecar is one entity and the unsprung masses (front and rear) are another. The only thing that matters with reguards to weight transfer and the connection of the sprung and unsprung masses are the distance between them.

I have heard people talk about this "theory" but noone has been able to substantiate the claim. Nothing I have read or seen has been able to prove it either.

NOBODY
Dirt Roller

Total posts: 11
posted February 18, 2005 12:54 PM  
It's like an old hand-held valve spring compressor,if you screw the bolt down too far it makes it harder to compress the spring.The head weighs the same and the spring has the same rate,but the aplication of force has been changed.

mod52
Dirt Full Roller

Total posts: 30
posted February 18, 2005 05:10 PM  
I know that there are engineering marvels in racing that I have not experienced. And most that I have, I could understand. But I can't see this one. Although, I would consider any attempt at an explaination!!

mod70
unregistered Total posts: 30
posted February 19, 2005 06:51 AM           send a private message to mod52   Edit/Delete Message   Reply w/QuoteBBBBBB
I think what zero was refering to is the vertical point at which the springs are working. If you are using 2 16" 200# springs and switch to 2 11" 200# springs the spring rate won't change, but the height at which that change takes place does. (lowering the spring "table") I don't think it would be a huge change to make in a car, more of a dial in kind of change. Did I read you right z?

tilley88
Dirt Forum Champ
Total posts: 879
posted February 19, 2005 08:33 AM  
Then why the latest trend to longer springs? I would think they do act like valve springs, in the fact that longer springs allow more compression before coil bind. If you really like to roll over on the right rear, then a longer spring would allow more travel than a short spring before coil bind occurs. Does this seem correct?

superdave
Dirt Freak

Total posts: 280
posted February 19, 2005 08:49 AM  
I think the long spring trend has to do with using lighter rates. When you have a 150 where you used to run a 225 then it takes more turns and compression to get the car back to ride height. Alot of the spring height is used up in compression just while setting the ride height. Your 150 spring would be compressed much more with the same corner weight and with the car back at ride height.

I also agree that lower spring mounting points on the rear promote front to rear weight transfer. The weight gets transfered on top of the srping and rear as apposed to just rearward.

Cars have so much rear percentage that nowadays the trend is to have the rear end driving up under the chassis (getting up on the bars) and not so much front to rear transfer. The weight is already on the rear.

Good luck,
Superdave

wissota3x
Dirt Forum Racer

Total posts: 86
posted February 19, 2005 09:09 AM  
There is no difference in the same rate springs that are different heights ( a 200# 13" spring gains 200 lbs per inch of compression and the same for a 16" spring.)The difference would be in less coil bind for the 16" spring. I think the trend to run longer springs would be in the left rear . The live movie clips on here that show the left rear 4 bar prove that the spring is not holding anything when the cars get up on the bars . This would be hypothetical but a 250 pound 13" spring in the left rear that would be replaced by a 150# 16" spring with the same amount of LR bite. The 150# spring would be compressed further for the same left rear bite but would follow the wheel alot further as the car hiked up.JMO.

speedy46
Dirt Maniac

Total posts: 165
posted February 19, 2005 09:32 AM  
the reason some guys are putting longer springs in the LR is so that u don't see the spring just laying in the cups when the car is on the bars it helps push the car over on the RR more so the LR spring actally does something.

leapinlizard
Dirt Freak

Total posts: 402
posted February 21, 2005 10:21 AM  
If you picture a kitchen table, that stands four feet tall, and you cut the legs and place springs between the kerfs, it won't matter wether the spring is located between the floor and the leg or between the table top and the leg. The height won't change asuuming spring free standing height and rate are all the same.

mod52
Dirt Full Roller

Total posts: 30
posted February 21, 2005 09:13 PM  
My take on this is that rearward wt. transfer is the result of the rear wheels driving forward & sprung wt. not wanting to.
The amount of wt. transfer is determined by the center of gravity height and the mass about it. The springs are doing two things @ this point, 1) regulating roll by spring rate 2)pass this additional wt. to axle.
Now, if such wt. transfer should produce 200 lbs. @ upper spring cup & spring is rated @ 200 lbs, then 1 inch of travel can be expected.
This 1 inch of roll movement will be experienced anywhere vertically (above or below)of upper spring mount. Therefore, reguardless of spring's location, (along this vertical line), the spring will always be working w/200 lbs of wt. transfer.
The kicker to it all, is that rarely does one ever see 1 inch of movement, due to anti-squat. Now the 200 lbs. of wt is being delivered to the axle through the anti-squat geometry, not just the springs.

Desert Mod
Dirt Full Roller

Total posts: 60
posted February 27, 2005 07:19 PM  
I'm thinking of using a 16" on the RR and keeping the 13" on the LR, in part because I want to try a lighter spring rate (125)on the RR and the 13" I had in there was compressing too much. Also, the jack bolt is about to run out of threads. Any down side to running the longer spring on the RR?

dirtracer14
Dirt Forum Champ
Total posts: 1261
posted February 27, 2005 08:57 PM  
The only problem you might have is getting the spring in with the rear dropped on the underslung. I run a 15" lr and a 13" rr i would have run the 15 rr but didnt have the room to get the spring in there.

Desert Mod
Dirt Full Roller

Total posts: 60
posted February 28, 2005 08:05 AM  
I had 16s in there before, on both sides, but went back to 13s on both. We had to use a spring compressor to get the taller ones in. Can you think of a downside to having a 16 on the RR and a 13 on the LR?

mod52
Dirt Full Roller

Total posts: 30
posted February 28, 2005 06:02 PM  
I installed a 16"-100# spring at RR and it bulged out like a bannana when I lowered the jack. So, in an attemped to square/center spring on cups, I wrapped cardboard & duct tape around inner cup to fit spring tight. This kept spring straight, but after heat, spring looked slightly curved.
I think the only hope for a tall/light spring in the RR would be to install deep cups to aid in cup alignment or use slider.
I might add that when I did this, it was a last ditch effort to overcome a bad setup decision made earlier.

Istock66
unregistered Total posts: 30
posted February 28, 2005 07:15 PM           send a private message to mod52   Edit/Delete Message   Reply w/QuoteBBBBBB
The top mount of your spring or slider should be located a little below the CGH.

The simple reason is that wt will have a greater leverage ratio between the CGH and the top mount and will load vertically onto the spring or mount.

Too low as with short springs the wt will transfer horizontally (shear).

Too high and it will resist wt transfer onto that corner.

Same goes for shock mounts.

wissota3x
Dirt Forum Racer

Total posts: 86
posted February 28, 2005 10:55 PM  
Mod 52,
We designed a spring cup that is on a swivle.The swivle is up inside the cup or the spring and forces the spring to stay perfectly straight.We used a small ball joint out of a foriegn car that is tough and greasable and sealed from the elements.We installed them on the weight jack and on the clamp bracket on the left rear that typicaly see's alot of misalighnment.This eliminated the C.O.E.and worked awsome all last year.If you or anyone else would like more info PM me.I hope I haven't steped over the line for the spam laws !!

[This message has been edited by wissota3x (edited February 28, 2005).]

mod52
Dirt Full Roller

Total posts: 30
posted February 28, 2005 11:54 PM  
That weight transfer stuff is really tricky! First it ricochets off of a tall spring, then pushes down on a spring that is not too low, then shears off into hyperspace when spring is too low!!! Sounds like the bullett that killed Kennedy.
The average CGH of a Mod is 14"-18"+/-. A 13" spring will compress 3" w/750lbs on it, making it 10" @ ride hieght. Put on a slider w/ 3" of adjustment plus 2-2" hiems. Now, give it 4" of ground clearance (8" below axle)and it all adds up to a 21" slider attachment hieght. (3" higher than the 18" CGH)
Mass,CGH & the suspension attachement points pushing at the under side of this mass creates a rearward rotation of mass. The amount of additional mass gained rearward of these pushing points is mass transfered. As long as the pushing force,mass, pushing points & CGH stay the same, the wt transfer over the axle will always be the same. The mass above the axle is solid, therefore, any point on the mass(above the axle) will have the same rotation & weight transfer. To change this wt transfer you must change CGH,mass, pushing points or distance between them, etc. Not lowering the spring attachment hieght.
If 1/2 of spring is concidered to be sprung mass, then one could argue that "raising" the spring could actually increase wt. transfer by raising mass. In turn raising CGH. Split'n' atom here!
I could

mod52
Dirt Full Roller

Total posts: 30
posted March 01, 2005 12:00 AM  
The "I could" at the end of my last post was a typo!!......I cannot split atoms...Nor did I shoot Kennedy

NOBODY
Dirt Roller

Total posts: 11
posted March 01, 2005 04:12 AM  
If your not splittin atoms, then you may never get the boom!!

speedyd1
Dirt Maniac

Total posts: 170
posted March 01, 2005 06:48 AM  
somebody shot kennedy? wow - i really should get out of the shop more.

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