Suspension tune and handling journey

[andy harwood]

"I wish I had a place to test that wasn't on city streets!"
http://virnow.com/experience/club/
Not far!

[GavinT]

My ride heights are F: 4 1/2", R: 5 3/4". That's lower than most TCs I've heard of and the rear is higher than the "1/2" higher than the front" Richard suggested to me. Playing with ride heights may be something to look at. As for changing the rear springs, I do have a pair of 100# rear springs I could swap for my 130# springs...

I'm inclined to agree with 'madness' but go a little further to ensure the difference is noticeable and also significant.
So . . I'd set both F and R ride height to be the same.

You can always retreat back to the usually recommended 1/2" difference, but as you say, your car is different and that may not apply as readily.

[BDA]

Brian, you're way ahead of me on wheel frequencies. Other than possibly using them to guestimate damper settings, I'm not sure what their significance is. I did a little bit of poking and found how to calculate it but nothing that explained much more than that! I did see that it requires you to know your unsprung weight. I'm not about to take my rear suspension apart (again) and weigh all that stuff! As analytical as you are, I guess I shouldn't be surprised that you figured that out!  I did calculate the rear spring rate that would be the same multiple as 250 is to the stock front spring rage and it came to 160# so I guess that is another way of getting to the same thing. I think you're right that the 100# springs are not the way to go.

I'm not sure I can measure the ride height at the frame. Reaching under that small a space and operating a tape seems impossible - or I'm too lazy to try - so I went the easy route and measured at the body behind the front wheels and in front of the rear wheels.

I think everybody is right (and it seems so obvious now!) that the next move would be to work on ride height. I really like the way the front looks now - It was such a shame that head light regulations required the TC's nose to be set so high! - so I think I'll try lowering the rear a bit. I've intended to measure the camber change of the rear through it's travel. Determining the roll center would also be a nice project.

Andy, I've thought about taking my car to VIR or even an autocross. I'd have to borrow a helmet and then I'd have to get head and helmet into the car in such a position that I could drive! Even with the bump in the roof, that may be a tall (I couldn't help myself) order, but might look further into that.

Thank you, everybody, for your input!

[Pfreen]

When we talk about ride height, I think we need to be consistent.  BDA, you referenced in back of front wheel and in front of rear wheel.  I use the front lower A arm pivot bolt and the transmission lower link bolt for the rear.

I just measured what my car is set to for all four points.

Front A arm bolt 6”.  Behind front wheel 4.25”
Trans Lower link bolt 6.5”.  In front of rear wheel 6.25”

So, I think my ride height is close to BDA’s, and slightly lower than the ride height referenced in http://www.lotuseuropa.org/LotusForum/index.php?topic=1104.msg19964#msg19964

I personally think the lower link and A arm pivot bolts is the most consistent since we know the fiberglass bodies could be slightly different.

[BDA]

I couldn't agree more. Brian has made the same point at least to me. Using the body is OK when you're comparing heights on your car but Lotus quality control being what it was and the fact that we jack the car where we're measuring and that could cause problems, measuring at a hard point is a better reference when it might refer to other cars. I'll try to remeasure using your points and post the results.

[EuropaTC]

Brian, you're way ahead of me on wheel frequencies. Other than possibly using them to guestimate damper settings, I'm not sure what their significance is. I did a little bit of poking and found how to calculate it but nothing that explained much more than that! I did see that it requires you to know your unsprung weight. I'm not about to take my rear suspension apart (again) and weigh all that stuff! As analytical as you are, I guess I shouldn't be surprised that you figured that out! 

I know very little about frequencies other than how to do the sums and a very rough guide as to what the numbers feel like in practice on my own cars. For example, the original Elan was 1.18/1.34 and that was noted for it's soft suspension whilst still handling well.  When I modified mine I spoke with Pat Thomas, a UK long time Lotus dealer who has raced Elans and he asked about how & where I wanted to use the car. The end result was that he recommended a set of springs/adjustable dampers which gave 1.8/1.93 rates which gave me my second rung on the ladder of how the numbers feel in practice. (as an aside, the Elan is now absolutely brilliant, I love it all over again !)

So my take is - up to 1.5, soft, comfort. 2-2.5 feels like a modern sporty car and is probably as far as I'd want to go on the road because the springs & dampers are going to get harsh on potholes, uneven surfaces.  Above that you're looking track cars and I've no idea what the upper limits are or do.

Second thing is the F/R ratios. There's internet opinions that you should have them identical and others that say you should have different rates.  When you hit a bump at speed then the F will bounce first and be in a cycle when the rear hits the bump and starts it's own cycle.  If they are slightly different the theory is that the car stabilises better than with F/R identical rates, whereas I've seen plenty of views saying when you're on high rates it doesn't matter. 

I decided initially to follow the "same both ends" and went for 1.8 all round. I didn't like it and hence where I am now, 1.8/2.0 which (doh) is a similar relationship to both the OEM Elan & Europa and Pat's modified Elan settings. Sometimes you have to stick your hand in the fire to see if it's hot. 

I'm not sure I can measure the ride height at the frame. Reaching under that small a space and operating a tape seems impossible - or I'm too lazy to try - so I went the easy route and measured at the body behind the front wheels and in front of the rear wheels.

As I said, I can get into the workshop pit and roll the car over me to measure heights but mostly I use internal calipers, photo attached. It's very easy to get them roughly right, lie down and reach under the car to the chassis central spine and adjust the screw thread to take a reading.

Brian

[BDA]

Thanks Brian! That helps me understand some of the frequency business. As you Brits say, everyday is a school day!

When you said you had a pit, it became obvious how and why you chose the center of the frame for your ride height reference point. I am jealous of people like you who have a pit and Certified who has a lift! Both are pretty much impossible in my garage.

[Fotog]

This is all quite interesting to me.  I haven't driven my car in a coon's age (or longer), but I will.  My recollection is of a lot of understeer; feeling like maybe there's no weight in the front of the car keeping the tires on the road.  But I haven't checked-out anything, not even tire pressures, so lots of good food for thought here.

Brian:  do you have a suggested place to get instructed about how to estimate the natural frequencies of the suspension?  In other words, "how to do the sums".

Also:

When we talk about ride height, I think we need to be consistent.  BDA, you referenced in back of front wheel and in front of rear wheel.  I use the front lower A arm pivot bolt and the transmission lower link bolt for the rear.

So I understand  clearly, is this to the centerline of the bolt?

[BDA]

The centerline of the bolt would probably be the best reference point. I haven't been able to measure mine yet so I'll see what's practical.  I might even try using my inside calipers like Brian!

[Pfreen]

Centerline of the bolt.  Again, look at europaTC’s post in http://www.lotuseuropa.org/LotusForum/index.php?topic=1104.msg9212#msg9212.  It shows where he measures from.  This is where I also measure from.

[BDA]

Well I gave my inside calipers to a buddy but I was able to measure my ride heights with a scale. I'm at 6.5" in the front and 6" at the rear (measured to middle of the inside suspension pivot bolt).

Brian, what were you unsprung weights? Did you actually measure your unsprung weight or did you use a value that you found?

[EuropaTC]

Brian:  do you have a suggested place to get instructed about how to estimate the natural frequencies of the suspension?  In other words, "how to do the sums".

There's quite a few websites around these days which cover suspension calcs and it wouldn't surprise me to find an online calculator now.  I started with a book by Alan Staniforth on "Competition Car Suspension" but there are many others and these days they tend to be easier going for someone starting out. You can even get the basic maths from the internet with some google searching - just try "suspension calculations" and you'll be inundated ! example:  https://eibach.com/us/p-101-suspension-worksheet.html

I've added a cut down version of my spreadsheet as an upload because the last sheet contains all the maths formula you'll need for a basic stab at it. This isn't complete because you should consider the antiroll/sway bars but for what I was doing I wasn't bothered. The other caveat is that it contains assumptions which may/may not apply to your car. As I wanted to compare OEM with new setups it didn't really matter because I was looking at differences in rates with the same car, not trying to get an absolute number.

A rider on the measurement points. Yep, I have used bolt centres but the dimensions given earlier on this thread refer to the floor/chassis spine distances.  The reason I moved to that was because I wanted to get a feel for the downward angle of the chassis and felt the chassis measurements over a few feet were more reliable than trying to work it out from bolt centers. (I don't know the relative heights of the lower wishbone bolts and the inboard rear lower link bolts) 

Whatever you use doesn't matter as long as you are consistent because you've got relative numbers directly relating to your car.

[BDA]

Thanks, Brian! I think you sent me the full version of that spreadsheet a while ago so I'll be looking closer at that.

Was the value for "Wheel Weight (per axle)", row 7, observed? If I add the last three values in the "Unsprung Weight Data", I get less than the 72# found on row 7 so I'm assuming they are observed.

I agree that all the ride height references points are valuable if we're talking about the same car. I think that Pfreen's suggestion to use the center of the inboard lower suspension bolt as a reference is a good idea when talking about this on the forum because it is pretty easy to measure and eliminates a lot of manufacturing variables. Measuring at the center of the frame 'T' is not a simple operation for those of us without a pit!

Serge posted this link that looks like a nice calculator several years ago (http://farnorthracing.com/autocross_secrets16.html). I haven't tried it yet but it looks interesting.

[EuropaTC]

Was the value for "Wheel Weight (per axle)", row 7, observed? If I add the last three values in the "Unsprung Weight Data", I get less than the 72# found on row 7 so I'm assuming they are observed.

   hmmm, it was a long time ago, which is another way of saying I can't remember 

I do remember weighing all the wheels on bathroom scales and also the latest discs/calipers.  I can't be sure but I think I guessed the weight of the rear calipers and said "about the same as the front".  I don't think I weighed the rear hubs because I'd have had to dismantle & don't remember doing that.  The angle for the front springs I do remember measuring but the rear I just said "90deg" and didn't bother checking if it was 90 or 88, at that point it doesn't make much difference.

Vehicle weights and F/R balance came from road test data, I went through a couple of Brookland's books and took an average.

As I said in the disclaimer, there's some assumptions in there so don't take it as absolute truth in the calculated numbers.

Specifically on Row 7, I guess you're looking at HTU's columns and that's 2x36lbs for the MGF alloy wheels. I ran the 56lb in Row8 (2x (13+15)) across the lot, accepting that the OEM car wouldn't have had those front discs and would have used rear drums so there's another assumption/inaccuracy. 

If you accept that you're looking for comparisons, being a few lb out doesn't make that much difference, you'd need 50lb or so to notice. For some reason I didn't weigh the 205/50x15 on the rear tyres but used the weight for the 195's. It's probably 2/3lb heavier per wheel but if you change the 72lb for 80lb it won't make any significant difference.

Brian

[BDA]

It's been a long time since I've updated this thread. The short of it is that I've increased my shock stiffness quite a lot. Without being able to find the settings I last mentioned, I'm up to 11 clicks in the front and 9 in the rear. You would be forgiven for not remembering that the SPAX tech guy told me that I may have only 14 points of adjustment. I'm thinking that I actually have 24. Turning the adjustment screw is pretty easy and when I made my first adjustments, I could hardly detect the "click." That leads me to believe that the clicks will get harder to make and that at the top of the adjustment, the tension on those clicks would be pretty stout. If you have experience in this, please feel free to chime in. Anyway, I will be going stiffer yet (after I get my turn signal situation squared away). The increased shock stiffness has made my car more stable at highway speeds and not uncomfortable to drive. However I think there is more to be gained. One day, I will do a study of the suspension geometry and wheel rates and all that fun stuff but that will have to wait for a while.

The real reason I'm making this post is to attach a table of handling characteristics, causes, and effects I shamelessly transcribed from Carroll Smith's book, Tune to Win. Of course the emphasis is on race cars so there is a lot that most of us wouldn't car about but then there are some of us who do race. I hope it's useful to someone.