16 replies to this topic

[Sphinx]

Thanks to a forum member, I scaled my car and properly set the ride height to 4 5/8" (1.6 w/ 99 hats) as the JD setup guide recommends.

 

So, riddle me this:  before scaling, the car was sitting too high and all jacked up.  The last time I went to the track, I tied zip ties on the shock shaft to see how much travel I'd get and they moved all the way up to the bumpstops.

 

Now, with the car properly lowered, I can't get the width of a finger between the top of the shock body and the bump stop (at least on the RR), and I have small fingers!  I have more room on the LR (w/o driver).  Is this right?  So, the shocks are almost at full compression most of the time anyway?

 

If this is right, then we are always on the bumpstops anyway.  Why then the admonition of not going lower?  Sounds like we are already there, no?

[fishguyaz]

i use this method because as opposed to measuring somewhere on the body, this measurement will not get altered by a body damage.

 

i ALWAYS do this with the = of my body weight in the car.

 

i use 3/4" which is what i read somewhere on this site. i made a small "feeler gauge" out of scrap material and measure from the bumpstop to the first edge of the zip tie.  this is my baseline height, then scale the car, maintaining ride height as told in the JD manual.

 

find the RH that works for you.

i made a prototype gauge out of some wire bent at 90 degrees and then cut to length( 3/4" in this case).

I plan on making more "gauges" in different lengths and keeping them handy, maybe out of scraps of sheet aluminum next time.

 

edit: i remove the zip ties from the car after it is set up on the scales. 

[Keith Novak]

Hard to say much from your details.  Last I remembered, the JD setup was for Toyos.  People shaved most of the tread off them so at the ride height he lists as the baseline, the shorter tire height requires higher spring perches and more shock travel to get to that ride height than at full tread.  You're also measuring without weight in the seat which will throw off your measurements probably around 1/8".

 

The shocks are mostly compressed but with the spring rates as high as they are, it takes quite a bit of load to compress that half inch.  You will find yourself on the bump stops but it makes a big difference if you ease onto the bump stops or hit them suddenly.  My car is set up where I can't fit my full finger between the shock and bump stop in front and can in back.  That works well for smooth tracks.  I'll get on them a bit but the suspension has taken a set and it's more in a static position than wanting to move up and down much.  I drive that setup on a track with sudden bumps and it will slam into the bump stops which is really uncomfortable and when mid corner, makes the car want to break loose very quickly and unpredictably.

[Tom Sager]

Thanks to a forum member, I scaled my car and properly set the ride height to 4 5/8" (1.6 w/ 99 hats) as the JD setup guide recommends.

 

So, riddle me this:  before scaling, the car was sitting too high and all jacked up.  The last time I went to the track, I tied zip ties on the shock shaft to see how much travel I'd get and they moved all the way up to the bumpstops.

 

Now, with the car properly lowered, I can't get the width of a finger between the top of the shock body and the bump stop (at least on the RR), and I have small fingers!  I have more room on the LR (w/o driver).  Is this right?  So, the shocks are almost at full compression most of the time anyway?

 

If this is right, then we are always on the bumpstops anyway.  Why then the admonition of not going lower?  Sounds like we are already there, no?

You'll get varying opinions but I think you have the car too low.  3/4 of an inch of travel with driver in the car as was stated is better.  The zip tie exercise is not a good one IMO as you'll compress the shocks a lot on bumps, curbs and under heavy braking.  You can compress the bumpstops about 1/4 of an inch without the spring rate going through the roof.  If you have the opportunity to test, you might try raising the car to a point where the travel is greater and then lower the car in full turn increments and see what the car likes best.  Note though the your camber and front toe will change with ride height.  Rear toe stays pretty constant with modest ride height changes.  

[Jaime Florence]

Sphinx,

I'm no setup expert but it sounds like you might be a little bit low. You should probably be able to get your finger in there and maybe a little wiggle. If you don't have the FatCat bump stops then even more so.

Don't forget the part ( in JDs setup guide) about never losing a race from being too high, but for sure you can be too low.

Just my 2 cents. I hope this helps.


Jaime

[Jaime Florence]

Oh yeah...and the zip ties test...I think you'll find that they will end up shoved into the bump rubbers every time in these cars.

[Bench Racer]

http://www.fatcatmot...36_46_NA_63.JPG

[Sphinx]

You'll get varying opinions but I think you have the car too low.  3/4 of an inch of travel with driver in the car as was stated is better.  The zip tie exercise is not a good one IMO as you'll compress the shocks a lot on bumps, curbs and under heavy braking.  You can compress the bumpstops about 1/4 of an inch without the spring rate going through the roof.  If you have the opportunity to test, you might try raising the car to a point where the travel is greater and then lower the car in full turn increments and see what the car likes best.  Note though the your camber and front toe will change with ride height.  Rear toe stays pretty constant with modest ride height changes.  

 

Thanks for the tip.  I'll take a look at it and adjust accordingly.

 

Oh yeah...and the zip ties test...I think you'll find that they will end up shoved into the bump rubbers every time in these cars.

 

Exactly.  I read it on here and thought I'd try it and realized that it told me that even with my back-marker driving, I was bottoming out anyway (with the car raised much higher).

[Glenn Davis]

http://www.fatcatmot...36_46_NA_63.JPG

Does this imply that there is no real spring rate in the first 1/2"+, or that they start already compressed?

[Keith Novak]

The spring rate there is low enough that it doesn't measure a load on the test rig set up.  The travel of the plunger pressing on it is accurate at that range and measures the travel but the load cells aren't.  You see that a lot when you test flexy things for deflection vs. load.

[Bench Racer]

My real point was to show that the FatCat does not have nearly the harsh rate that the OEM bump stop has. If you have 1/2 inch to 3/4 inch clearance your on the FatCat stops in a hurry. How much travel does the top of shock travel after hitting the stop I don't know.

 

I presume when we tie rap, the shock top pushes the rap up and the bump stop pushes the rap down. Anyone???  Maybe I'll make a sort of dip stick that will travel from above the shock hat through a hole down to the top of the shock top with a bend in the dip stick. Some how I'll be sure the dip stick stays where it's pushed so I can measure after a few laps. 

 

 

With the bump stop force vs distance chart I'll agree to disagree with Keith. My understanding of the chart is that all three bump stops start at zero force and zero distance. The 36mm and 46mm bump stop lines are moved to the right to clear up the chart. To not have lines crossing other lines. I'll comfirm this with Shaikh of FatCat on Monday. 

 

 

This bump stop issue sent me to the garage to do some measuring. The bump stop extends below the FatCat housing by 11/16 inch. The FatCat housing has a bore diameter of 2.010 inch. The shock body has an outside diameter of 1.975 inch diameter. The radial clearance between the shock body outside diameter and the FatCat housing bore is .0175 inches. In my design mind no one would expect the shock body to freely fit withing the FatCat housing bore. As the 11/16 inch bump stop is compressed to zero inches the shock body would bind/bottom out on the FatCat housing end surface. If one were to set the ride height to zero bump stop clearance the shock body top would travel 11/16 (approx 75 pound bump stop force) inch before bind/bottom on the FatCat housing end surface. Once this bind/bottom out occured there would be no spring involved, everything would be stacked solid metal. But if the ride height were set to 3/4 inch bump stop clearance there would be total travel of 1.437 inch before this bind/bottom out condition occured. The 3/4 (or 5/8 inch) inch bump stop clearance now makes total sense to me. I'll also talk about this bind/bottom out condition with Shaikh of FatCat on Monday.

 

 

Dave Wheeler, Mike Collins, Jim Drago, Tom Sager, Keith or anyone else please feel free to jump in. Agree, disagree???  

[Keith Novak]

It will be interesting to see what Shaikh says.  Usually you do overlay the plots.  Otherwise its like comparing DAQ data with you data off 100ft.  It makes it hard to compare discrete points.  Plus 75# to fully compressed doesn't really sit well with my gut feel indicator.

 

I'll agree to disagree too.  When I did a bunch of R&D essentially measuring the spring rate of plastics and foams to compare against fiberglass and carbon fiber, we would always see a lot of flex before the machine read anything for load and the plots looked like that.  It's not really linear flex at all.  My thought was that he cleaned up the graph by deleting all the dots on the zero force line.

[fishguyaz]

I edited my post but thought it was worth a repost.

 

the zip ties, are used only in the shop as a starting point for setting up the car.

step one: set up ride height of the car.

step two: remove zip ties, unless you want to remeasure.  I leave them on the car until i have set the car up completely. then remove them before i drive the car.

 

seems some guys are making this way to complicated. IMO you either want to be hitting the bumpstops, or you dont. I know that i dont. at an event my car was way too low, I qualified 40th/41st. I am normally midpack something was wrong.

 

someone very knowledgable looked at my car and said i was way too low. could not fit a palm of the hand between the fender and the tire while at rest............

I followed the advice and raised the car one turn on each corner, this was right before the race.

I gained ~ 4 seconds/lap and ended up midpack as i should have.  this is how i know i dont want to be hitting the bumpstops at all.

 

this event also made me realize that  i must learn how to be a test driver(not just a race driver).

i have since learned what i need to know to rely on myself to set up the car.

 

 

the zip ties are useful for changing/finding your baseline set up, but they should be used the same way as a ruler or any other  tool to be used as a measuring reference.  when you are hitting the bumpstops you will know because ( on all tracks i have raced on)eveyone else will be passing you.

 

the only reason i choose use the zip ties on the miata, is that the distance being measureed wont get wrinkled up in a incident. 

 

Josh

[Bench Racer]

It will be interesting to see what Shaikh says.  .

Left a message at FatCat. I'm sure their very bussy.

[Bench Racer]

Response from Shaikh of FatCat Motorsports.

 

 

My point number one to FatCat was on force vs compression graph, is the graph showing compression of the three different bump stops only?

 

 

FatCat response:

 

 

"True, I offset the graphs for clarity based on when the bump stops would engage relative to a stock 90-97 63mm bump stop."

 

 

My point number two to FatCat Motorsports.

 

 

I provided shock and FatCat isolator dimensions, then asked the following question.   In my humble judgment the shock would bottom out or bind at the interface between the top of the shock body and the bottom surface of the isolator. With this stated occurence the bump stop would compress only .75 inch before the shock bottomed out.

 

 

FatCat response:

 

 

My words, FatCat has an a work around for this ^ issue.

 

 

FatCat words, "let me know your thoughts and if you want to propose this to the forum I can take it up with the SM board."

 

 

My response to FatCat Motorsports is that I would appreciate very much for FatCat Motorsports to communicate with the SM board because the work around improvement by FatCat is positive to all Spec Miata racers.

 

David Dewhurst

[FTodaro]

You are better off having more RH than you need then not having enough. When you get into the bump stops you’re not going to like driving your car, it’s likely
you will be spinning.

 

So Point one better to error on the side of too much rather than too little.

 

The second point is how much shock travel you need really depends on your track and if you use the curbing, speed in the turns and so on. I need more RH
at road Atlanta then I do at Mid Ohio.

 

with your wt in the car 3/4 of an inch of shock travel should be more then enough and it would give you room to raise or lower the car at the track.

 

[Bench Racer]

Have you ever had any clunking when going some place really rough on track or gators?

 

When you speak of spinning, is that because your on the bump stop 11/16 of an inch or are you spinning because the shock body has bottomed out on the bottom side of the isolator? 

 

Compressing the 3/4 inch the bump stop sticks below the isolator is only an extra 75 pounds or reference the compression graph, approx 25 pounds pre 1/4 inch of travel.  What is our front spring rate, 700 pounds per inch. 

 

If I want the chassis low, when it comes to the shock body bottoming out on the isolator one should ask themself the question, do I want a higher bump stop rate or do I want a instant infinity rate at the bottom out point?

 

I here about these infinity bump stop rates. Hmmm.....

 

We need to have factual information to make setup decisions.