27 replies to this topic

[Parity]

I've seen several comments about measuring ride height from the perches rather than pinch welds. It gets sort of busy under there especially with the car loaded. Anyone care to share specifics about how you're maesuring. Tools? Pics?

 

TIA

[LarryKing]

Some folks have made coat hanger "gauges". I use a "finger gauge".

[Parity]

Yeah, I was hoping for something a little more accurate. 

[LarryKing]

More accurate than a bent coat hanger ?!

 

Picky, picky, picky.

[FTodaro]

I do both, but the most accurate gauge i use,  is my finger.  after having done a ton of alignments, i can look at a car to tell if its high or low to start with.  I do use the pinch welds as i know what number i am looking for on my car but every car can be a little different.

 

measuring the distance between the the top of the shock and the bottom of the bump stop is the most informative.

 

Do you want me to post a picture of my finger?

[LarryKing]

Coat hanger gauge looks kinda like this: l\__________

[Bench Racer]

As my member title says, different strokes for different folks.

 

Use whatever ingenious measuring apparatus that makes you feel all warm and fuzzy. For the front measure from the bottom of the shock hat to the top of the lower shock mounting bolt. For the rear measure from the bottom of the shock hat to the top of the shock mounting bolt head clearance hole in the lower control arm. Keep in mind the first 3/4 inch compression of the FatCat bump stop is usable and after the first 3/4 inch the bump stop spring rate heads straight to infinity.  Yup, I tried a setup that went beyond the 3/4 inch bump stop travel. don't try it.

 

Because I'm an economical racer I don't have linear transducers to monitor bump/rebound. I've implemented a little pin slide to measure maximum bump keeping I'm mind if I hit a hole, monster gators of slightly off track.

[Alberto]

I do both, but the most accurate gauge i use,  is my finger.  after having done a ton of alignments, i can look at a car to tell if its high or low to start with.  I do use the pinch welds as i know what number i am looking for on my car but every car can be a little different.

 

measuring the distance between the the top of the shock and the bottom of the bump stop is the most informative.

 

Do you want me to post a picture of my finger?

 

Could you?!  That would be swell!

Which finger do you use?

Maybe also measure the thickness with some calipers so we can all compare.  

 

happy friday all!

[Bench Racer]

Could you?!  That would be swell!

Which finger do you use?

Maybe also measure the thickness with some calipers so we can all compare.  

Aimed at Frank, I get the finger measuring, it's the flesh compression that screws up the measurement.

[Parity]

 

Oh you mean like this...Wait

[SaulSpeedwell]

Finger gauge at the perches.  Much more "accurate" than pinch welds that literally have nothing to do with the distance between the tire and the bumpstop, especially after the donor was totalled twice, then had the rollcage welded in while it was twisted on jackstands, then got wrecked a few more times while making impressive first turn passes from 23rd to 21st, etc.

 

The key is getting the springs posed and paintmarked in the right place where your finger can be a quick and easy feeler gauge, and repeatable by having your finger in the same orientation for every corner.  But if you want to spend more time, you can squish modeling clay, etc., too.   Any measurement at the bumpstop is better than any measurement at the pinch weld, if you ask me.

 

But convince yourself - measure 2 or 3 cars, and see if the "ride height" correlates at all to the "shock travel", which is what we care about.  But if you can come up with the right correlation of shock travel to "ride height" for your personal car, I don't see any reason you can't use "ride height" from then on.

 

The best part about measuring shock travel?  Your setup isn't a function of tire height/tire of the year, tire wear, etc.

[RWP80000]

I have found the use of a Starrett #237 "T-square" tool to measure and record from the bottom of the threaded shock adjuster sleeve to the bottom of the spring support (the one with the three holes used to adjust the spring height setting) to be an easy and reliable way to characterize the chassis setup.

 

The tool has a 1/8 inch wide, 6 inch long rule with 1/64 and 1/32 graduations and costs about $60 although there are Harbor Freight quality devices out there that can be had for significantly less.  I always record the 1/64 graduation reading so I do not have to deal with a mix of fractions.

 

I have found this captures the chassis setup and is independent (not affected) by fuel load or tire pressures as opposed to flange heights or jounce bumper gaps. This makes incremental changes at the track easy to capture and therefore easy to back track if a change doesn't work out.  I also record the accompanying "top of shock body to jounce bumper gap" but here again this is a function of fuel load and or chassis setup characteristics (camber, rake and wedge).  

 

I have been using this for a two car team and find there is very close correlation of this dimension between both vehicles.  Although you have to make this measurement from under the car, you can do it very quickly and it doesn't matter if the car is on a platform hoist or up on jack stands which makes it easy to make controlled and repeatable recordings/adjustments at the track from a baseline set up. For reference, nominal readings should be in about a one inch overall range from around 1inch&16/64ths  to 2inch &16/64ths  depending on how you set your vehicle up.  There is a good, but not exact correlation to the "top of shock body to jounce bumper gap" but this will vary relative to cross weight setting, wedge and rake.

 

This is my "go to" setup reference characteristic which I have used for several years and provides a historical picture of how our setups have evolved over time.

 

Rich Powers

[FTodaro]

Not a bad Idea Rich. Am i the only one that notices that the car does settle and compress after coming back from the track. to minimize this

 

Before i set the car up for the next weekend, at the conclusion of the prior weekend, I like to check the RH first before i uncompressed the suspension to get a true reading. If i take the weight off the springs the car seams to not settle right away, so the most accurate measurement IMO is taken when the car comes off track before you jack it up.

[steveracer]

I set up my stands then let the car sit on them overnight before checking tide height.

[Steve Scheifler]

Similar to Rich we record the perch height, bottom of threaded collar to bottom of perch, but only before and after each event unless we "get lost" making changes at the track. As Mark and others have said, the most important measurement is probably available shock travel, but once we get everything the way we want it at the shop, the perch heights provide by far the fastest and most accurate way to get back to a particular baseline whether at the track or as the first step when preparing for the next event.

To be clear, equal perch heights does NOT necessarily mean equal ride heights or shock travel. Unless you have very equal weight side-side they will be a bit different at "level" or equal travel, and front/rear are not only very different weight but also different shocks and springs. What the measurements do is make it easier to reproduce a desired setting, and provide a sanity check if you lose track of turns and direction when making adjustments at the track.

[Johnny D]

I was thinking about posting this. This is as good as any. Sorry no cut and paste.

http://grassrootsmot...chassis-tuning/

 

J~

[SaulSpeedwell]

Not a bad Idea Rich. Am i the only one that notices that the car does settle and compress after coming back from the track. to minimize this

 

Before i set the car up for the next weekend, at the conclusion of the prior weekend, I like to check the RH first before i uncompressed the suspension to get a true reading. If i take the weight off the springs the car seams to not settle right away, so the most accurate measurement IMO is taken when the car comes off track before you jack it up.

 

The car definitely "settles" after a period of time (resulting in high camber numbers overnight, especially), I presume because the gas in the dampers slowly migrates from one side to the other.  My conclusion was it was more accurate setting the car up as soon as it was lowered onto the scales and/or after "pumping" the car up and down.  That more closely replicates the reality of your on-track situation.  This assumes you have something resembling "slip plates" in your alignment rig.

[RWP80000]

How much of this settling is related to the tire taking a set (flat spotting)?

 

Whenever I leave the car setting for an extended period or overnight, it is with the car rolled off the scales.  That way when the car is rolled back on the scales it is not rolling onto a flat spot caused by sitting in the same spot on the scales.  While I frequently roll the car back off the scales to "bounce it after adjustments, I swear by the use of thin greased slip plates under all four wheels and credit their use for the biggest gain in obtaining consistent/repeatable results.

 

I also set the tire pressures to off-track target pressures to simulate the dynamic tire cross section however I have not been able to detect any measurable cross weight affect from doing this.   

 

Rich Powers

[FTodaro]

Interesting thoughts on Settling issue. I have never tested this now i will, but I assumed that once you make your first lap or two, the car set up reverts back to that lowest ride height number, that is why i was trying to capture and set the car up at that lower number, so now i am not that sure.

[SaulSpeedwell]

Interesting thoughts on Settling issue. I have never tested this now i will, but I assumed that once you make your first lap or two, the car set up reverts back to that lowest ride height number, that is why i was trying to capture and set the car up at that lower number, so now i am not that sure.

 

I see it the other way.  The wheels are hammering up and down, the car is rolling, the car is pitching under braking and acceleration ... thus the longest time it is "settling" is down the straightaway.  In any case, if you set your car up using the same rig every time, then it shouldn't matter - over time you will learn what works and you will converge to the best setup for your setup tools and processes.