Minimum front ride height

jstoezel
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Joined: September 19th, 2010, 6:21 pm

Minimum front ride height

Post by jstoezel »

Hi:

What is the minimum recommended front ride height? I guess this is somehow car dependent, but under breaking, what is a typical front ride height change?

Thanks,
Jean
Jean-Sebastien Stoezel
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tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

I guess ideally it would be just enough to avoid hitting the ground!

It really depends on the car, how high the beam is mounted relative to the frame, how soft the springs are, etc.. On my car, I have hit the ground several times under.. err.. vigorous braking maneuvers :P
jstoezel
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Re: Minimum front ride height

Post by jstoezel »

When I say ride height change, I mean change of distance between the bottom of the frame (belly pan) to the ground.

My understanding is that is pretty much the same on all cars (even if the beam is mounted at different heights with regards to the bottom of the frame), since we all have the same spring rate (different shocks though).
tiagosantos wrote:I guess ideally it would be just enough to avoid hitting the ground!

It really depends on the car, how high the beam is mounted relative to the frame, how soft the springs are, etc.. On my car, I have hit the ground several times under.. err.. vigorous braking maneuvers :P
Jean-Sebastien Stoezel
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tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

spring rates can be different.. We don't all run the same spring, or mount it the same way! I don't know if running a stock spring is a rule, but I've seen a few variations..
brian
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Re: Minimum front ride height

Post by brian »

Anything under 2.5 inches and you'll have to control compression with stiffer springs, higher compression on shocks, or limiters. I have found my car does not like a higher spring rate on the front. I use a stock spring, uncut, with an adjuster to set height. I also have a rubber bumper on the shock shaft to prevent hitting the ground. There are indications that some cars, like the Caracal and Vortec, prefer a bit of rake. Meaning that the front is lower than the rear. That can be a challenge if the rear is low as well. Hope this helps.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
fvracer27
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Re: Minimum front ride height

Post by fvracer27 »

I have found my car does not like a higher spring rate on the front
How did you get a higher spring rate?

Mark
Mark Filip
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jstoezel
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Re: Minimum front ride height

Post by jstoezel »

Hi Brian:

Thanks for the reply. Yes your reply helps, since I was looking for a minimum ride height number, thank you for providing one.

Initially I was looking for a minimum ride height value, but this seems to be highly dependent on front spring rate. I thought we were somehow all equal but it appears not...

I wouldn't mind trying a higher spring rate at the track to feel the difference. For my own understanding, could we take it from the basics of what spring rates mean and do? My understanding is that spring rate controls ride height variations and behavior over bumps (how much you launch off a bump)... While shocks control front to rear weight "transfers".

As for modifying the spring rate at the front: you mention running a higher compression setting on the front shocks, limiters (rubber stops?). How about the spring pack? It was mentioned in a previous post that installing 2 ride height adjusters would increase the rate by around 20%...

Jean

brian wrote:Anything under 2.5 inches and you'll have to control compression with stiffer springs, higher compression on shocks, or limiters. I have found my car does not like a higher spring rate on the front. I use a stock spring, uncut, with an adjuster to set height. I also have a rubber bumper on the shock shaft to prevent hitting the ground. There are indications that some cars, like the Caracal and Vortec, prefer a bit of rake. Meaning that the front is lower than the rear. That can be a challenge if the rear is low as well. Hope this helps.
Jean-Sebastien Stoezel
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smsazzy
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Re: Minimum front ride height

Post by smsazzy »

I run 1.75" and do not use any bump rubbers or limiters to make the car stiffer. My car uses the anti-dive built into the VW front suspension.
Stephen Saslow
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fvracer27
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Re: Minimum front ride height

Post by fvracer27 »

My car uses the anti-dive built into the VW front suspension
Can you expain this please.

I'm playing with a fine line on my car with rubbing and I'm sure it has something to do with me being 95lbs heavy :cry:

Mark
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Re: Minimum front ride height

Post by brian »

As the turkey legs move upward, the leverage on the spring declines, hence a rising rate. Instead of running the four split springs that are on the top and bottom of the spring pack, you can substitute one or two solid leafs from another pack and increase the rate.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
smsazzy
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Re: Minimum front ride height

Post by smsazzy »

fvracer27 wrote:
My car uses the anti-dive built into the VW front suspension
Can you expain this please.

I'm playing with a fine line on my car with rubbing and I'm sure it has something to do with me being 95lbs heavy :cry:

Mark
Picture the front end under braking. If the turkey legs are at a 45 degree angle up like this: :/ (but only 45 degrees - (:) is the cross section of the beam, (/) is the turkey leg) and you are braking with 1G, the weight shift down is counterbalanced by the tire essentially pulling backwards on the turkey leg like a lever.

If your car was not designed with this, you would have to lower the beam on the frame to accomplish this. This is a very delicate balance as it can also cause wheel hop as the spring rate and leverage of the arm can cause an oscilation and wheel hop.

Being 95 pounds over weight though is a MUCH bigger concern. Spend your time and money there and just put some bump rubbers on the shocks to stop the car from bottoming out.
Stephen Saslow
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tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

Anyone has any ideas how to calculate CG height for our cars? Or a general idea of what it would be on most vees? Give or take 2 inches should work for this purpose :) Knowing the CG height, calculating the angle of the turkey legs to give us enough anti-dive would be easy..
smsazzy
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Re: Minimum front ride height

Post by smsazzy »

I would estimate it to be a few inches in front of the roll bar (based on the angle of the car when picked up on a tow hook) and likely about the height of the front pulley of the engine.

That is my guess.

If you take your front to rear weights you should be able to confirm the fore/aft percentages and find that part. Height is a little trickier.
Stephen Saslow
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Re: Minimum front ride height

Post by tiagosantos »

yup.. unfortunately the height is really the tricky part for the weight transfer formula!

Anyway, making some assumptions.. With a wheelbase of 83 inches and a COG height of 20 inches.. (this is a really wild guess!) and assuming 1.2G's of braking (which is the best I've ever been able to do, for a worst case scenario as far as hitting the ground goes..), the extra weight at the nose of the car under braking would be:

(1.2G*1025lbs) * 20in / 83in = 296.4lbs..

The front spring would let the car hit the ground with waaay less weight, but like Stephen said, there is anti-dive built into the suspension. With the turkey legs parallel to the ground, the car dives rapidly, but as the angle increases, it counteracts the weight transfer. Assuming an angle of, I dunno, 15 degrees, the same 1.2G braking force and a front bias of 60%, the anti-dive force would be 197lbs.. Or the other way around, the angle at which the anti-dive equals the added weight transfer is about 21.8 degrees. That's quite a bit! But of course this doesn't take into account the spring rate, shocks and whatever.

Just having fun with numbers guys, carry on :) It still seems like the generally accepted practice is to have the turkey legs parallel to the ground at normal race weight, which is what I've been doing so far.
jstoezel
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Re: Minimum front ride height

Post by jstoezel »

Back in 2000 I worked as a summer student in the former Formula 1 team Prost Grand Prix. I witnessed mechanics measuring the height of the center of gravity of the F1 car, by raising either end of the car and measuring the weight on each axle.

It's a fairly easy procedure, If you have scales and a way to lift either end of your car quite high. I've also read it's quite empirical...


tiagosantos wrote:Anyone has any ideas how to calculate CG height for our cars? Or a general idea of what it would be on most vees? Give or take 2 inches should work for this purpose :) Knowing the CG height, calculating the angle of the turkey legs to give us enough anti-dive would be easy..
Jean-Sebastien Stoezel
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smsazzy
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Re: Minimum front ride height

Post by smsazzy »

tiagosantos wrote:It still seems like the generally accepted practice is to have the turkey legs parallel to the ground at normal race weight, which is what I've been doing so far.
The generally accepted practice is not always best. ;-)
Stephen Saslow
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tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

Jean, that's clever.. Yup, I can see how they would do it, but I don't have scales and I wouldn't trust myself around a car sitting on scales at an angle :P It would end badly!

Stephen - not always, no :P Just don't want anyone to be impressed by my awesome math and start running their cars with the turkey legs at a 21 degree angle, with the nose scraping the ground down the front straight..
tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

By the way, if anyone wants to try it.. http://www.msgroup.org/forums/mtt/topic ... PIC_id=137
SR Racing
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Re: Minimum front ride height

Post by SR Racing »

Some of this has been said but: Your front ride height is generallly going to be dictated by your chassis. The swing axle trans is fixed in the chassis and with your preferred camber and tire size your front ride height is what it is. On most cars you are going to want at least 1/2" of rake at speed. (1/2" lower at the front) This will vary somewhat on the cars aero package.
YOu will never want the front to be higher than the rear. So assuming 1/2" rake is what you want, the only fix for bottoming out is a stiffer spring and/or in combo with preloads, shock pressures etc.

"Most" cars will end up with turkey legs parallel to the ground, but certainly not mandatory. And there is some science to this since the angle can make for a decreasing or increasing spring rate. (Not much, but I am sure some people consider it for handling.)

Also, you CAN get creative with the trans/engine angle to change the rear ride height (ala XTC, Lazer, and a few others,) This even lowers the CG of the car overall.
hardingfv32-1
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Re: Minimum front ride height

Post by hardingfv32-1 »

SR Racing wrote: And there is some science to this since the angle can make for a decreasing or increasing spring rate. (Not much, but I am sure some people consider it for handling.)
With a front wheel rate of 30-40 lb/in, you would be luck to feel a doubling of the wheel rate much less the few pounds you are talking about from angling the control arms. If the driver does not know of the change, he is never going to be able to tell.

Brian
jpetillo
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Re: Minimum front ride height

Post by jpetillo »

jstoezel wrote:Back in 2000 I worked as a summer student in the former Formula 1 team Prost Grand Prix. I witnessed mechanics measuring the height of the center of gravity of the F1 car, by raising either end of the car and measuring the weight on each axle.

It's a fairly easy procedure, If you have scales and a way to lift either end of your car quite high. I've also read it's quite empirical...
Agreed. The math is straightforward based on measuring the wheel loads by raising one end of the car up. Since it's based on measurement, it's empirical. I think I have a spreadsheet somewhere. You don't need to raise the car that high, but then that depends on the accuracy or repeatability of your scales because the forces don't change that much. If you instead tilt the car from the side, the forces change faster and it may be a better way to get this measurement with less accurate scales. Also, I'm jealous of your summer student job! John
Last edited by jpetillo on November 20th, 2011, 10:41 am, edited 1 time in total.
jpetillo
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Re: Minimum front ride height

Post by jpetillo »

smsazzy wrote: Picture the front end under braking. If the turkey legs are at a 45 degree angle up like this: :/ (but only 45 degrees - (:) is the cross section of the beam, (/) is the turkey leg) and you are braking with 1G, the weight shift down is counterbalanced by the tire essentially pulling backwards on the turkey leg like a lever.
Steven, that's correct, but I believe there's a bit more to it than that. There is a torque from the tire contact patch that will try to push the turkey legs down, where they mount to the beam - so the beam has an additional force pushing down. The more the suspension is loaded down towards that 45 degree upward tilt, the more leverage that torque has to shove the beam down. The effect is similar to putting the rear brake on and the nose going down. But in this case you do have the force like you mentioned pushing up. I wouldn't know what wins out - the pushup that you're talking about or the push down - without doing a few calculations. I'd bet it's the pushing up (my car's apart so I can't look out and see) what wins.

Okay, some rough numbers for 45 degrees tells me that the force down is about equal to the force up. For angles less than the upward tilt of 45 degrees, down always wins, even if by a little. Over 45 degrees and the anti-dive wins. What I mentioned has nothing to do with weight shift. However, at 45 degrees, Tiagos' 296 lbs of weight shift is counteracted only by the highly rising rate front spring that Brian mentioned, and I think 296 pounds pushes it down more than 45 degrees, and the anti-dive begins to beat the push down of the braking torque.

John

Sorry, I got home late and had to clear my mind. Fun thread.
tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

John, are you saying that under braking, the contact patch is basically trying to stop rotation of the wheel, which "tries" to spin the whole spindle/link pin carrier assembly..? I think that's what you mean by torque at the turkey legs. Sorry, I'm good at math, but never studied physics or mechanics in English, so I get lost in terminology very easily!

If that is what you mean.. Even if my COG height is way off.. A COG at 15 inches would still mean about 220 extra lbs at the nose under braking.. Considering our very low spring rates, even with the rising rate from the turkey leg angle going past horizontal - if the torque cancels out the anti-dive forces, what else is stopping the nose from smashing into the ground? That is a genuine question, by the way :) Is it the shocks? I am completely honest when I say that I don't fully understand what shocks do. I know what the idea is, but.. I just don't understand them. Gonna do some reading today!

edit: re-read your post and noticed that you had sort of answered my question. I need to go look at my car and take some measurements, how long are the turkey legs anyway? Damn garage is way too dark and cold, I've been staying away from that place for a while.
smsazzy
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Re: Minimum front ride height

Post by smsazzy »

The act of the tire under braking is pulling the tire backwards in relation to the direction of trasvel of the car. That is what is pushing the nose back up in the air. At 1025 pounds and 1G braking, you have 1025 pounds of force pulling backwards on that lever. Figure out the ratio and you have the exact angle at which the two equal out. The weight pushing down is only 200 some-odd pounds based on the previous post, so it does not have to be at 45 degrees to win out.
Stephen Saslow
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tiagosantos
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Re: Minimum front ride height

Post by tiagosantos »

I don't have a turkey leg to look at handy, but I'm imagining that the distance from the link pin to the front beam is about 6 inches.. Is that remotely close?

Someone else mentioned that our front spring rates (or wheel rates) are roughly 30lbs/in. So starting with the turkey leg parallel to the ground, the spring rate would be 30. If the weight transfer is 220lbs, the suspension would quickly move down the first inch. The angle would now be roughly 10deg and the spring rate = 30.4lbs. Total force from the spring = 60.4lbs. Still not enough, so lets keep going.. Another inch, roughly 20degs and a spring rate of 31.95.. Total = 92.35lbs. Another inch, right around 30degs and a spring rate of 34.6, for a total of 126.95. Hmmm.. At 40degrees, the spring force would be around 170lbs, so either the anti-dive forces are significant or my math is terribly screwed up! Are our turkey legs a lot shorter than 6in? That is, in the plane parallel to the car's centerline.

Do our shocks play a big role in stopping movement from weight transfer? I sort of assumed that they would only slow the movement down, rather than minimize it.. If you're braking at 1G for long enough, the car would end up in the same position regardless of the shocks, wouldn't it? Well, assuming it wasn't bouncing around.. :)
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