Calculation of motion ratio

[Joe911]

[size=3]Is the denominator for calculation of the motion ratio the distance from the inner control arm pivot to A) the ball joint/upright pivot or B) to the center of the tire? So it would vary with different wheel offset?[/size]

[Purple Frog]

Hum...

I should not go where Mitchell, Pare, et. al. go better. But my layman's explanation:

What you are chasing in motion ratio is to know that for a given vertical movement of the tire patch in relation to the chassis, how much did the shock piston move.

So at the ICP seminars we were taught to set the chassis up on a table, pad, whatever, at proper ride height etc, but without the springs installed. Set the axles at the height they would be in relation to the chassis when loaded and at hot racing tire temps/pressures. Then from that static position move the axle up a fixed measurement, let's say .25". Then measure how much the shock compressed. In this example if the shock compressed .25", then you would have a 1/1 motion ratio in that range of motion.

Realize that the motion ratio for the first .25" of movement may be different than the second .25", and both my be different than the third .25" of movement. And upward movements may have different ratios than downward. I have seen those that are real serious plot it in .10" movement increments or smaller.

In many modern cars you might only graph the motion ratio for 1" vertical movement of both sides of static, because you don't see axle movement more than that on the track. Older cars with maybe softer springs, you may want to graph a larger range of movement.

Did that help?

[RoadHazard]

According to the venerable Carrol Smith, whose book I just happen to have right here under my pillow:

Motion Ratio = wheel travel / spring travel

So as Frog says, move your wheel vertically (the way it would normally move) and measure how much your spring would extend/compress. The ratio of these two is your motion ratio. On my car, the spring moves about half as much as the wheel, so my motion ratio is 2.0.

If what you're calculating is wheel rate, that's something different.

Wheel Rate = Spring Rate / (Motion Ratio * Motion Ratio)

In Smith's example, a 400-pound spring mounted on an arm with a motion ratio of 1.5 yields a wheel rate of 187 lbs/in. (400 / 1.5^2).

None of this involves measuring the length of your control arms. Just the motion of the tire and the (missing) spring.

[Purple Frog]

Things are nicely different in Pacific Grove than most of the rest of the world. 17 mile drive, seals on the rocks... damn I'd like to be there.

But, Hazard, would not yours be 1/2 ? not 2/0.

Where is Pare when I need him to jump in and save me from myself....

[DaveW]

Motion ratio is as Froggy said. In addition, it is usually defined as shock travel divided by wheel travel. (Some people define it as the inverse of that.) Modern cars use MR's of 0.7 (damper moves 7/10 as much as the wheel) up to well over 1.0. Larger is better for damper control of chassis motions, but smaller usually results in less friction. So it's a balancing act between control and grip, which friction degrades.

Also, there are increasing and decreasing (during compression) motion ratios. This results in an increasing or decreasing wheel rate. An increasing motion ratio results in better control, but can decrease grip. Decreasing motion ratio may enhance grip, but controls chassis position less.

Everything (including this) is a compromise.

[James Hakewill]

I did say only yesterday that I was going to keep my crazy ideas to myself, but I've got pictures and everything...

The picture shows my setup for measuring camber curves and motion ratio (and bump steer). What you don't see is the scheme for measuring vertical movement of the wheel - which is a long steel rule.

The wheel movement should be measured at the center of the contact patch I'd have thought, but I just used the distance from the bottom edge of the bump-steer plate to the ground - so the wheel offset would affect it a bit. I suppose a block bolted to the plate could correct for that.

As mentioned in the previous post, zero is set for the static position of the suspension at ride (measured from seat-to-seat at the shock), then go up and down by your chosen increments of wheel movement, and measure shock travel.

Next time I think I'll use a dial guage for vertical motion - my first set of measurements were not accurate enough to tell if I'd got the rockers in the right spots for equal motion ratio on each side...

James

[Joe911]

[size=2]I am new at racing formula cars so I am learning about set-up with information from Apexspeed threads and reading C. Smith and Harney’s books. My goal is to learn enough so I can actually make sensible adjustments at the track that improve the cars performance. I have set alignment/ride height/corner balance over the past several weeks using levels/strings/Ruggles and was pleased that they were close to where I set them following the practice day at Sebring. Recent discussion on the website has discussed spring rates, motion ratios, wheel rates, and springs binding so I figure a good way to understand these issues are to go out into the garage and calculate/consider them on my car. Possibly I may discover changes with potential to improve things. It is a 1975 Titan with everything outboard so I don’t think it is possible to achieve a motion ratio of greater than one.[/size]
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[size=2]Most definations I have seen define motion ratio as a ratio of static distances rather than a ratio of the movement, but I can see that measuring it dynamically is the way to go. Plus measuring the movement will take into account my shocks being at a 45 degree angle to my control arm which I would have to adjust for if I was calculating the motion ratio based on length of the control arm to the shock mount and the wheel. I will also calculate it for a range of suspension travel and average it in making considerations about possible spring rates[/size]
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[size=2]Pare and Harney both recommend actual spring rate:corner weight of 1:1. Why are most of the older cars using much softer springs than this? I would think that newer cars (i.e., FF) would have better camber – bump steer curves, particularly with their longer control arms and could run softer springs.[/size]
[size=2] [/size]
[size=2]I will definitely attend the next ICP seminar and thanks again.[/size]
[size=2]Joe Riley[/size]

[Joe911]

[size=2]Just finished checking. At 1½” bump = 0.5 shock to wheel movement, 1” bump = 0.5, ½” bump = 0.5, -½” = 0.5, and at -1” it calculated at 0.6. So my wheel rate is: 500lb springs x .25 = 125 lb/in. for wheel rate. So if I have 220 lbs on corner balance at each front wheel I should have 880 springs to approximate the recommended ratio of 1:1. Is the rear also 1:1 or is the rear typically softer?[/size]

[DaveW]

Pare and Harney both recommend actual spring rate:corner weight of 1:1. Why are most of the older cars using much softer springs than this? I would think that newer cars (i.e., FF) would have better camber – bump steer curves, particularly with their longer control arms and could run softer springs.

The reason to run higher spring rates is to get platform (chassis) rake and ride height control without excessive damping or droop limiting, both of which have the downside of possibly losing grip. A secondary reason is for control of unwanted suspension motion.

See my Comp clinic presentation for more info:

http://www.neohio-scca.org/comp_clin...ut%202005a.pdf

[AJWALKER]

Joe you may want to mention what car you have. This will possibly help find what others have found to work on a similar car.

[T.A. Treat]

Here's my low buck solution:
I went to the local hardware store or hobby store and found two 1 foot pieces of small brass tubing that slid inside each other. Drill a hole in each end and bolt the thing in the shock location. Then when you move the tire up and down from ride height just put marks on the tube with a sharpie and measure them. I used pieces of wood flooring to stack under the tire to get 1/8" increments. If you plot out an inch on each side of ride height, you will have the data you need. This method is very quick and handy especially if you have multiple mounts points on your bellcranks.

[Joe911]

[size=3]The car is a 1975 Titan Mk9 – outboard suspension on both ends. This model has always been heavy in the rear so I am trying to move weight forward and optimize the spring rates as I do this. I will calculate the rear motion ratio tonight.[/size]


[size=3]T.A. - The tubing idea is a good one and probably more precise than my tape measure. Nevertheless, the measurement I took at each 1/4” of wheel movement were pretty consistent at 2:1 until I got to 1” droop.[/size]

[size=3]Is there any typical relationship front to rear or is this just a tuning issue?[/size]

[Purple Frog]

Don't get too hung up about achieving Wheel Ratios (WR) 1:1 on an older car. Tires are much better than 30 years ago, and that accounts for a lot of our ability to stiffen. Also we just know a lot more now.
What I did 3 or 4 years ago when developing an older car to my style was to stiffen WR evenly front to rear in increments. First it bumped up to around .62:1 then .75:1 then .84:1.... each time testing it. It was surprising how much better it got with each change. Part of the improvement was because as it got stiffer it was also able to get much lower. That particular car, with rockers and piss-poor motion ratios was just too much a handful in the WR range of 1:1, not enough shock movement to dampen the springs. I seemed to be happiest around .88:1. Your mileage will vary.
In CFC in the last couple of years, Mark Hatheway has done very well with an '84 Reynard. He's done much development, but I suspect the biggest improvement came when he started fitting it with stiffer and stiffer springs.

[LolaT440]

I think I need to go to a seminar.

[Brian]

Another part of the stiff is better equation is the actual stiffness of the chassis, which has imporved dramatically from 1975. If you stiffen your wheel rates too much, you can get to where the chassis becomes the totally undamped spring. So on older cars with large motion ratios, there are a lot of factors that mitigate against stiff wheel rates.

Brian

[Joe911]

Good point about frame flex or other components limiting the range of spring rates useful for older cars. I would expect this is what Mr. Frog was refering to when he described uping the wheel rate incrementally but that as he approached 1:1 it got worse. Plus the shocks could not keep up. I figure that if I am better educated about these things, I will make better decisions about what to try first as I tune the set-up.

[RoadHazard]

Well, Smith's book (TtW, pg. 65) defines motion ratio as wheel travel divided by spring travel, so on my car that's about 100 mm divided by 50 mm, or about 2.0. Or 2:1, if you like. Or II in Roman numerals.

'Course, as Dave W points out, some people use the inverse of that number, so I've got an equal chance of being wrong. Typical.

Time to spend some more quality time with the seals on the rocks, I guess.

[EYERACE]

not like i've got a lot of springs laying around but off the top of my head at the office i'm thinking the Crossle 32F currently has front springs of about 220 and rears of 260 ????? going to try going to 240 front and 300 rears someday

[Daryl DeArman]

Eye,

I had 350/400's on my Crossle 32. On American Race tires this seemed to be the standard set up out here with MANY fast Crossles. That would also keep the f:r ratio very close to what you currently have and what you are proposing.

Shocks were Penskes, don't ask me about valving....any notes I had went with the car.

[Joe911]

[size=2]I have now calculated the rear motion ratio to be .67. So I have a wheel rate of 125 in/lbs for the front (500 lb springs) and 169 in the rear (375 lb springs). So the next question is should I use corner weight or sprung weight to calculate wheel rate:corner weight ratios? Using corner weights of 230 and 345, wheel rate to corner weights of .54 front and .49 rear.[/size]

[size=2]Removing the springs I have estimated unsprung weight as 35 front and 40 rear per side using scales. Does this sound reasonable? I was surprised to see this much. If I use wheel rate:sprung weight I am at .68 front and .57 rear.[/size]

[size=2]Mr. Frog - from your comments it sounds like you were using similar wheel rate:corner wt ratios front and rear. Is this typical? Or is this different for every car and the goal is to achieve balance while being stiff enough to keep the car off the track.[/size]

[Brian]

is the next question. Your wheel rates are right in the ball-park as a starting point for me, maybe a little softer in rear as a percentage than I would start with. If you can get a good balance on the car with bars, then you could experiment. For me, I like to try to minimise rear bar and use rear rate to support the back end, just a thing I have when running an open diff. I also like to use the front bar to stabilize the car in roll. So I tend to have stiffer rear rate than front as a percentage of corner weight, and use front bar to get a basic balance and then add/remove rear bar to tune. But that's just my theory.

My other theory is that older cars and newer cars are very different (duh!) in how they use camber curves to work the tires. Old cars often use short arms and a lot of camber gain in bump and roll, which lets them run softer springs and still work the tires in roll. Newer cars have very little camber gain due to the long arms, and that requires them/allows them to limit suspension movement with very stiff springs. With essentially no roll compared to an older car, they work the tires at a very high frequency and generate a lot of grip. New tires respond to that, while I figure that tires made in the 1970's probably didn't. With an older car running new tires, you have to search out a compromise.

If you take that another step, then newer tires may not like variable camber. Then, your soft, rolling car with lots of camber gain may not be doing what the tire wants, so going stiff and really limiting suspension movement to try and mimic what the newer cars are doing makes a lot of sense. Plus it lets you run a lot lower. If you just drop an older car down by an inch or more, then it will be in a very different place in it's camber curve. Plus, what were the tire diameters that the car was designed to use? They may have been an inch or two taller in diameter. Then you get into shocks and the limited shock movement that you get with your big motion ratio's and stiff springs. And I'm not even going to attempt to think about that, except that I agree that it's probably why older cars tend to not respond quite as well to stiff rates as newer cars.

Get a set of tires, a bunch of springs, and play for a day testing just to see what happens. You'll get a direction to go in for further tuning, and probably have a lot of fun doing it!

Brian's two cents worth...

[Jim Garry]

I think I need to go to a seminar.

ICP plans to do one somewhere in the northeast in early '06.

[Joe911]

[size=2]I hope this thread is as useful to future readers as it has been to me. Having gone through this process (mentally and physically with the car) has help me understand the important issues and will also help understand material that will be presented when attending the next ICP seminar as will as reading the Apexspeed threads.[/size]

[size=2]What is my car doing? Generally OK balance with occasional early corner understeer with several spins following losing the rear end before the apex. Possibly from throttle off weight transfer - but it happens so fast I am not sure. Slight but controllable oversteer condition in mid-speed long constant radius corners (like carousel at RA). Some or most of this could be driver inconsistency as I now have 8 hours of track time in the past 3 months in this car + needing new tires.[/size]

[size=2]I am moving weight forward to improve bias and want to start to see what changes in the car relate to track experience/performance. Now I have an idea of which directions make sense in terms of set-up. Given about 2 months down time over the holidays it is also a good time to make these changes, acquire parts for testing, and prepare for testing sessions with specific goals.[/size]


[size=2]I am happy to hear from others that have developed “older” cars and the steps they have gone through, right and wrong - - unless you don’t want to give away your speed secrets!!![/size]
[size=2]Joe Riley[/size]

[Dave SanF 50]

Joe
Don't tune the suspension on worn out, deader than snot old tires!
In order to have consistant result for your testing, two words: tire management. The biggest and most immediate change you can make to your car's handling is changing from old tires to new. So if you start the test day on a set of fairly new tires (or even new) and run 4-6 sessions, the changes you make to the chasis may not be the cause of the changes in handling because the tires may go off during the day. Nothing is more frustating than trying to tune the chasis and finding that the tires are changing throught out the day along with the other changes you are making.
Everyone says to change only one thing at a time. Pretty impossible. Over the course of the test day, the tires change along with the amb temp and track conditions (oil, dirt, etc)-and don't forget driver fatigue. So even w/o making any changes to the suspension, things change.
I think a good rule of tumb is to set/tune the car on tires simular to what you will be racing on. If you will be racing on stickers, better set the car up on stickers. Changing the suspension to correct the way the car acts on 20 heat cycle tires is worthless if you will not use them to race on.
dave