So I wanted to determine what the natural frequency of the suspension is and to do this I require both the sprung and unsprung weight.

Does anyone have any methods or links to measuring the unsprung weight on the rear of a DC2?

It would be reasonable to weight the parts individually and then calculate the weight but the rear-trailing arm has me stumped. It pivots on one end with the compensator arm etc.

Any thoughts?


Dave
IP2 GSR

 

The simple way to deal with things like the trailing arm that has one side fixed to the body (unsprung) and the other side moving with the sprung items, it to just weight it and let 1/2 the waight be unsprung and the other 1/2 is sprung. Don't forget your spings are like this also because only 1 side is free to move.

Hope that helps. I have seen this methode used, and I think it's the common way of doing it.

 

Yea I know what you mean.... Like for the Lower control arm and the upper control arm it is a simple matter to just remove them from the car, make the measurement and use 50% of that weight in the calculation.

Where I have a problem is with the rear trailing arm. It is located at the front by the compensator arm, in the middle with the large rubber bushing and then located but the upper and lower control arms at the rear. Both the compensator arm and the large bushing are mounted to the chassis so by definitions are sprung. I assume that this means more of the trailing would be considered sprung weight. Sigh but how much?

I know with a strut suspension the weight can be measured using a scale under hub with the sway bar disconnected. When I tried this I measured 22 lbs at full droop, and 56 pounds with the suspension set to near ride height?? Don't try that at home kids.

To make matters worse it's the trailing are where the majority of the unsprung weight is.

Thoughts?

Dave
#72 IP2 GSR

 

When I took my vehicle dynamics course from Pi Research, Claude Rouelle went over this...he said all F1, CART, F3000, Atlantic etc. teams use the 50/50 method to calculate the suspension components portion of unsprung weight.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by F1HONDA &raquo;</TD></TR><TR><TD CLASS="quote">When I took my vehicle dynamics course from Pi Research, Claude Rouelle went over this..</TD></TR></TABLE>

Name Dropper!

I'm just curious IP2GSR - what are you going to use the results of your calculations for?

Are you going to constrict yourself (thru your spring rate choice) with some outmoded upper bound on natural frequency (and spring rate) and drive a slow and sluggish setup around until you see the light? You wouldn't be the first person that happened to.

And what the hell do you care about a closely estimated value of unsprung weight for? Run light wheels. Run light calipers. Lighter is better - there's little more to be said (or calculated). I can't think of a practical use for a numeric value - unless you're supplying it to someone who's using a quarter car program to spec some valving for you - in which case you're participating in an unneccessary excercise in reinvention of the wheel - which is fine if you know what you're doing and your intention is to enjoy the journey - otherwise you're wasting your time and money - you are not going to reinvent the wheel successfully relative to what Mugen, OPM/Koni, or Moton can hand you off the shelf.

Scott, who thinks if you've got questions this basic about how to add up the unsprung weight, you're not equipped to do anything meaningful with it...which you would know if you knew...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR &raquo;</TD></TR><TR><TD CLASS="quote">Name Dropper!</TD></TR></TABLE>

?? Whatever.

I mentioned Claude to lend some corroboration to the previous post about using such a method. He's as experienced as the late Carroll Smith in the world of motorsports. Trying to add some meat to the discussion is better than "I think...I think...I think..." crap.

 

Well said RR98ITR. Make everything as light as your budget will allow! I dont exactly see why you would go through all the trouble for the numbers, either.

 

Call me Dave! Only the people that don't like me call me IP2GSR....

Well if you really want to know, it started with me ordering some double adjustable alloy, dampers (Where I took their advice rather than re-invent the wheel on spring rates). Then, as I pulled them out of the box, I thought, "Man these things are light". So I figured I might as well get out the scale and see just how much. Well that's all good!
That lead me to wonder...... what percentage of the unsprung weight I have saved. Well since the suspension is totally apart on the floor for inspection, I might as well take notes and some measurements. Better now then later when I have to take it apart again. I've kept a "setup book" with tons of stuff that I measured taken notes on, measured again, taken pictures for future consideration. Will I use the data? At this point I don't think so, but that won't stop me from spending some time trying to answer the question and enjoy the journey as I proceed. I enjoy working on the car as much as driving it in competition.

I also though that this might be an interesting question to discuss on the board. As I mentioned, I would rather learn from others, than re-invent the wheel. If someone knows, then all the better.

So hopefully that has answered your question.

You mentioned at the end of your post that it was a basic question. Do you have a basic answer other than 50% of the trailing arm weight despite it having two attachment points?

Thanks

Dave
#72 IP2/ITS GSR

 

Okay....Dave,

You've been told the conventions - and they're good enough. I'm sure you can "safely" neglect the toe link.

Glad you're not fixed on reinventing the wheel - without knowing what you were thinking that was the most probable path to take.

Scott, who wishes he didn't have such a clear idea now of how little F1Honda learned at Claudes seminar....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR &raquo;</TD></TR><TR><TD CLASS="quote">Are you going to constrict yourself (thru your spring rate choice) with some outmoded upper bound on natural frequency (and spring rate) and drive a slow and sluggish setup around until you see the light? You wouldn't be the first person that happened to.</TD></TR></TABLE>

Can you please elaborate on this (perhaps in another thread or an IM)? Is there a better way to determine better/best spring rates than natural frequency? Thank you

 

Simply put: whatever makes you go fastest. Keep going lower and stiffer till you don't go any faster or the car becomes undrivable. All along the way the demands on the dampers will change and it's a real challenge to isolate factors and successfully converge on that last few percent.

I liked how Carroll Smith put it: "...frequencies...we don't need to know about them.."

Crack open one of the classics. It'll say don't go over something like 2-3 cps. The basis for this is old research on human performance and tolerance. It has nothing to do with the performance of an automobile.

Then go look at what a fast car of our type is running - an RTR ITR for example - and when you calc out their setup you'll see much higher frequencies - especially at the rear.

Then if you have the opportunity suggest to them that they should be running much softer springs. I don't know exactly what kind of response you'll get, but it won't be a sincere "gee, I wish I'd thought of that, thanks."

Scott, who has read an awful lot of books on this stuff, talked to an awful lot of people over many years, and has alot of personal experience in a range of suspended wheeled vehicles...but I could be wrong...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR &raquo;</TD></TR><TR><TD CLASS="quote">Simply put: whatever makes you go fastest. Keep going lower and stiffer till you don't go any faster or the car becomes undrivable. All along the way the demands on the dampers will change and it's a real challenge to isolate factors and successfully converge on that last few percent.</TD></TR></TABLE>
I see... Unfortunately I don't have the money to do the trial and error method. Darn, I was hoping there was some kind of formula, like budget x bling factor = spring rate.

 

listen to scott, it will save you (and possibly him) some headaches. i fought with him tooth and nail last year about this stuff, but the long and short was that i was wrong. he's got a pretty damn good handle on it. and if you don't have the budget to test on your own (you have the budget, you just don't know it. testing is 75% of the fun in racing), copy someone else's setup that is racing the same car in the same form of racing as you with the same comfort restraints as you and that is most importantly FAST. and remember that you can't (well, you can but you really shouldn't) copy an SPU road racing setup on a CRX onto your SOLO2 STX ITR and expect it to work.

nate

 

If you're curious about RTR' ITR rates, I asked Nathan from RTR what they were running this year for their RTR at Mosport. He told me 900 lb/in. front and 1400 lb/in. rear. They used to run 800 lb/in front and 1200 lb/in. rear. They are still using the Mugen N1 shocks, and he says he likes the fact that they are so reliable. He talked to me about the Motons they are using on the RSX, and said they had to be re-built regularly, and this made them a pain (great shocks though). I asked about buying some used Mugen N1's, but he didn't have any. This year you can buy a complete ITR from them for $30,000, as it may be the last year they race them, and they are having a clearance sale! He also said the ITR and RSX were about the same speed around the track. Guess the driver who won at Mosport (Kleinubing in the RSX) makes all the difference. In any case, if you calculate the resonant frequencies with 1400r/900f you will get some pretty high values. This is not a car for the street, as I would imagine it would rattle your teeth out. I suppose it's all about limiting the roll angle overall to minimize camber change and keep the tire at the optimum camber. Could be camber control is more important than bouncing over bumps for lap-time. Besides predicting front to rear balance requirements for a front driver would require knowledge of the traction while coming off the corners, since in a pure vehicle dynamics/tire management point of vue, you would run roll stiffness closer to weight distribution, but this is not the set-up that has made RTR and other's ITR's faster around the track. Stiffer in the rear and inside wheel lifting don't optimize cornering traction but they do optimize lap time. If you set it up for best skid-pad or theoretical performance, you won't come up with the same set-up as if you test it on the track. In any case, everyone knows stiffer is better!

I also have a few books on the subject, but I haven't seen anything very good dealing with set-ups for front wheel drive. Seems most authors think a real race car should put the power down where the weight transfers, not the opposite, and there haven't been any recent books that recognize the fact that the world is driving front wheel drive's, and if it has a motor, boys will want to race it..

By the way I have been in contact with R. Small who writes the program SusProg3D to ask him if he can model the ITR and other similar Honda suspensions. I sent him a few pictures and scans from the ITR manual. I am awaiting a reply. His software makes nice pretty pictures! I don't know if doing a 3D analysis of the suspension will make you any faster, but it may entertain you while you're pondering how to go quicker.

And by the way, just take apart your entire suspension and weigh everything! Make a list and post it. I'm sure many people would be curious to know, just maybe not Scott. hehe.

 

Thanks for the RTR info, I'd like to plug those rates into the nat. freq. equation and see what they are. Anyone know the sprung weight/wheel for a stripped Integra?
My car's a daily driver/ STS autocrosser so I won't be going with spring rates that high, but it'll be nice to know how far RTR went, maybe give me a better idea.

 

Just an FYI - to defend my basic curiousity after Df's little jab - I posted a table a year or two ago with frequencies at various spring rates in a decent little thread. I'll dig out my worksheets after the weekend is over and post it up.

Scott, who wonders if Df is gonna be hanging around here and posting more often...I'd be curious to know your H-T Aclicktion Factor....

 

One of Scott’s old posts referring to an ITR;

On a car very comparable to yours:

400F gives wheel rate of 180 & 1.53 cps
600F gives wheel rate of 270 & 1.88 cps
800F gives wheel rate of 355 & 2.15 cps
1000F gives wheel rate of 444 & 2.41 cps
1200F gives wheel rate of 533 & 2.64 cps
1400F gives wheel rate of 622 & 2.85 cps
1600F gives wheel rate of 711 & 3.04 cps

250R gives wheel rate of 140 & 1.7 cps
600R gives wheel rate of 330 & 2.6 cps
800R gives wheel rate of 440 & 3.0 cps
1200R gives wheel rate of 660 & 3.7 cps

 

Do those numbers take into account the tire stiffness?

While checking in one of my favorite car books, Driving Ambition, The Official Inside Story of the McLaren F1, on p. 127 Gordon Murray set a design target for his ultimate road car at 1000 kg with a 43%f/57% weight distribution and ride frequencies of 80 and 92 CPM front and rear respectively, or 1.33 and 1.53 Hz. There is an interesting dicussion in this book about the unsprung to sprung weight ratios on p.129. They were afraid that with a light carbon structure and the 600 or so hp, they would necessarily have large wheels, tires and brakes which inevitably weigh a lot and make up the bulk of the unsprung weight, so there was a danger of it "wagging the dog". Oh to have such problems. It turned out the unsprung corner weights were 42 kg (92 lbs) front and 55 kg (121 lbs) rear, which for a 1000 kg car gave sprung to unsprung ratios of 5.5:1 front and 5.8:1 rear according to the book. Now if we could just add 8 more cylinders to the ITR and keep the weight down!