45 replies to this topic

[Saiph]

Hi all,

I've recently been running the '69 cars at a few circuits, and I've had some problems at tracks where the suspension is under heavy load. I can't seem to stop the tub from scraping the track. I realise that the wings may add to the downward stress on the suspension, but I've mainly been testing with high-speed, low-downforce setups, with the wings at either 4 degrees (neutral lift) or 5 degrees (minimal downforce).

I've tried increasing ride height, increasing spring rates, and of course increasing the bump rubbers to maximum, but I still get sparks appearing. And in some cases, the scraping is still so bad that it puts the car into a spin. I've tried going back to the '67 and '66 cars, and although they scrape and spin with standard setups, I can remove the problem by making adjustments to the items I mentioned previously. But with the '69s I can't seem to shake the problem.

Incidentally, the circuits where I've noticed the problem most are the Avus versions (on the banking), and Blue Mountain.

Anyone have any advice/ideas? Thanks in advance. (And while you're thinking, I'm off to sit on an ice block for an hour or so.   )

Edited by Saiph, Jun 10 2010 - 11:50 AM.

[miklkit]

From a thread on RSC some years ago I remember that the bump rubbers are cone shaped with a maximum stiffness of 131 N/cm or 75 lb/in at the base.  I don't remember what they are at the top.  I now run them all the way up, all cars and all tracks.  The 67 BRM still bottoms badly at the Nurburgring even with 280 N/cm or 160lb/in rear springs.  What is your definition of stiff?

[MECH]

I wonder if the 69mod has the same bump settings as the stock 67's. Never checked that before.

Given the fact that the winged cars put more stress on the suspension i would expect the setup to have the option of setting it higher than the other wingless mods.
I guess it will be a matter of choosing the right mix between wing settings and bump settings per track. There are also some tracks that have such difference in altitudes it requires just driving slower on that particular section.

P.S. Have you checked what speed you drive in the problem area compared to the 67's? I've often had the feeling i was driving a 69 slower somewhere while in fact i was driving it faster than the 67. The difference in engine sound put me on the wrong track

[miklkit]

This is one area that is just begging for more research.  It appears that the F1 teams discovered bump rubbers in the early sixties.  Even though the 65s had larger tires and more powerful engines, the 61s had much stiffer springs.  I remember seeing pics of 70,71 CanAm cars in the pits and they had these big fat bump rubbers that were color coded for stiffness.  Red, yellow, green, blue, etc..

[Saiph]

miklkit, on Jun 10 2010 - 02:18 PM, said:

...............  What is your definition of stiff?

Well I didn't mention the word "stiff" in my post, but obviously that's where my setups end up heading towards. This is one of those subjective areas where ones man's "stiff" might be another man's "sloppy"! At the moment, my Avus and Blue Mountain springs are set to 160lbs front, 220lbs rear (280 N/cm & 385 N/cm). That seems to minimise the problem to a manageable level, although I can still get problems at Blue Mountain turns 10/11/12 "The Chicane", and on the Avus banking (especially leaving it) if I'm not careful.

MECH, on Jun 11 2010 - 04:54 AM, said:

...............
P.S. Have you checked what speed you drive in the problem area compared to the 67's? I've often had the feeling i was driving a 69 slower somewhere while in fact i was driving it faster than the 67. The difference in engine sound put me on the wrong track

Y'know, that's probably a very good point. I haven't had time to check this out with Replay Analyser yet, but now that you mention it, my lap times with the '69 cars are usually the fastest of all the mods, and often several seconds faster than the '67 cars. So obviously I must be driving them faster generally and putting more stress on the suspension.

Thanks for the comments chaps.    I might try increasing the spring rates a bit more, to see if that [a] helps the grounding problem, and [b] introduces any undesirable handling effects. Other than that, I might just have to consider moderating the lead boot on my right foot just a little bit! Completely alien to my nature of course, but sometimes maybe it just has to be done!  

[Lee200]

Missed this thread from before, but here is some information for you based on previous posts by Gene Fryman and discussions we've had in the physics forum.

All mods use the same bump rubbers.

The bump rubbers produce a maximum force of 179 pounds/800 newtons when fully compressed and are adjustable from .5 to 2.5" in the player setup menu.  The bump rubber comes into play at this length before the maximum suspension travel of 19 cm/7.5" is reached.  So if you set a 2.5" bump rubber length, it starts adding force to the wheel/spring force beginning at 5" of suspension compression.

The force is not linear but varies with the cube of the compression distance.  The formula is:

Force = Resistance * Compression ^3 / Bumper Length ^3

where Resistance is 179 pounds/800 newtons
Compression is how much the bump rubber is compressed
Bumper Length from the setup menu

For example, if a 2.5" bump rubber is compressed by 1", then it only adds a force of 11 pounds.  At 2", the force will be 92  pounds.  And at full compression of 2.5", the maximum force of 179 pounds is applied.

GPL's springs are different.  In the real world, the wheel rate and the spring rate are normally different and vary in relation to each by the mechanical advantage afforded by the suspension geometry.  So in the real world, 1" of wheel travel does not necessarily equate to 1" of spring travel.  In GPL, the wheel rate and spring rate are the same which is why in the setup menu, you are adjusting not only the wheel rate, but also the spring rate by the same amount.

As the suspension compresses in bump, the wheel/spring force is added linearly.  So each inch of compression increases the spring force the same as the prior one.  In other words, GPL does not have progressive springs.

When the bump rubber comes into play, its force is added to the spring force for the total force acting downward on the wheel.

One of the issues we've found with the upcoming sports car mod is that there really isn't a limit on the maximum amount of suspension travel.  There is no equivalent of a mechanical stop as real world cars have.  Although the normal suspension travel is 19 cm/7.5", under extremely heavy load the suspension may compress even more than this.  It's not completely clear what GPL does for spring/bump rubber force at this point.

If you are experiencing chassis bottoming, the only way to prevent this is to raise the ride height, increase the wheel rate, and/or increase the bump rubber length.

Hope this helps.

Lee

Edited by Lee200, Jul 30 2010 - 07:00 AM.

[Lee200]

Double post.  

Edited by Lee200, Jul 30 2010 - 08:26 AM.

[Frenchy]

Thank you very much Lee.

I've always been a little confused exactly how the bump rubbers come in to play so its nice to have it set out so clearly.



Cheers
David

[Lee200]

I might add a few points about setup wheel/spring rate, chassis weight, and ride height.

On startup, GPL determines the amount of suspension compression by computing the sprung weight on each wheel from the chassis weight, driver weight, fuel weight, and weight distribution.  It then divides the sprung weight on each wheel by the wheel rate to determine the static suspension compression.

As mentioned before, the normal suspension travel is 19 cm/7.5" so if your goal is to have the static suspension loaded to, say, half its range, you'd want to set the wheel rate so that the static suspension compression is 3.75".  This would give you 3.75" of suspension travel in both bump and droop.

Each car weighs differently, but an approximation is the weight given in the team info menus.  Add in the weight of fuel at .75 KGs/liter or 6.26 pounds per gallon and you have a reasonable estimate of the chassis sprung weight.  Then compute the wheel rate using these formulae:

Front Wheel Rate in Pounds = (Chassis Sprung Weight * (1 - .6 )/ (2 * 3.75)
Rear Wheel Rate in Pounds = (Chassis Sprung Weight * .6) / (2 * 3.75)

All GPL cars use an approximate Weight Distribution of 60% so .6 is used the above formulae.

You may want slightly more bump travel range than droop range so higher wheel rates would do that.

As to ride height, on startup GPL goes through a pretty complicated computation to attach the top of the suspension/spring to the point on the chassis so that the actual ride height is equal to the setup ride height based on static suspension compression, vertical size of the chassis, and tire size.

Lee

Edited by Lee200, Jul 30 2010 - 09:21 AM.

[Saiph]

Interesting stuff, thanks Lee.

[John Woods]

Yes, thanks Lee, I just can't read enough techical stuff about the cars and really appreciate that info.

Lately I have been tweaking bump settings to assist braking, and think I've found using a slightly lower setting on the rear is a big help, imagining the car is settling in on the stops under heavy braking without loading up the front suspension as much.

Seems like that's happening. Does anyone have some ideas if this is reasonable, accurate, a correct approach? Or as usual, am I just late, confused, incorrect, and incomprehensible?

Edited by John Woods, Jul 30 2010 - 05:18 PM.

[Lee200]

John Woods, on Jul 30 2010 - 05:17 PM, said:

Yes, thanks Lee, I just can't read enough techical stuff about the cars and really appreciate that info.

Lately I have been tweaking bump settings to assist braking, and think I've found using a slightly lower setting on the rear is a big help, imagining the car is settling in on the stops under heavy braking without loading up the front suspension as much.

Seems like that's happening. Does anyone have some ideas if this is reasonable, accurate, a correct approach? Or as usual, am I just late, confused, incorrect, and incomprehensible?

Hi John,

Not sure how lowering the rear bumper length could help under braking.

Under braking the car's weight transfers from the rear towards the front; thus the front suspensions compress while the rear suspensions unload.  So the rear bump rubber shouldn't be coming into play at all unless your rear wheel rate was so low that you were always riding on the rear bump rubbers anyway.

Lengthening the front bump rubbers should result in a more gradual loading of the front suspension; however, normal braking is usually so abrupt that I doubt the bumper lengths makes much difference.

But if it works for you...

Lee

[John Woods]

Thanks Lee for responding.

Well, my hypothesis/theory has been that under the load of braking the front bump stops would increase spring rate until relative to the rear the front would be stiffer, causing load transfer to be slowed at some point, or requiring some to stay on the rear, while preventing the front suspension from compressing more, or as much as it would otherwise. Just trying to explain in a little more detail...and it only seems to work, (I think), so the reason for bringing it up here was to not to make a claim but more to learn if possile what others think about this idea, or if there is any actual technical connection/validation. Also, this is in the context of running fairly even f/r spring rates, close to 50/50.

Edited by John Woods, Jul 31 2010 - 08:41 AM.

[Lee200]

John Woods, on Jul 31 2010 - 08:01 AM, said:

Thanks Lee for responding.

Well, my hypothesis/theory has been that under the load of braking the front bump stops would increase spring rate until relative to the rear the front would be stiffer, causing load transfer to be slowed at some point, or requiring some to stay on the rear, while preventing the front suspension from compressing more, or as much as it would otherwise. Just trying to explain in a little more detail...and it only seems to work, (I think), so the reason for bringing it up here was to not to make a claim but more to learn if possile what others think about this idea, or if there is any actual technical connection/validation. Also, this is in the context of running fairly even f/r spring rates, close to 50/50.

Hi John,

Well GPL can be strange and we don't fully understand all the physics calculations it does, but years of testing and experience has given us a fair idea of how it works.

Remember that the amount of load transfer has nothing to do with the spring or bump rubber settings.  For a given acceleration/deceleration in G, the load transfer only depends on the CofG height, weight, and wheelbase and WILL ALWAYS BE THE SAME (yes, I'm shouting.   ).  The springs and bump rubbers only affect how MUCH the suspension compresses which in turn has an effect on tire camber and toe in/out.  The dampers/shocks only affect the RATE at which the load is transferred.

GPL does model suspension compression effect  on tire camber...it's set for all mods at about .5 degrees of camber change per inch which is consistent with real world cars; however, GPL does NOT model suspension compression effect on  toe in/out.  To be perfectly accurate, it may model toe in/tout change in the code, but all GPL cars are set with a zero rate of toe change per suspension compression.  So in effect, GPL does not model bump/droop steer.

Real world cars must run springs and dampers rates that are soft enough to be compliant with bumps in the road as the goal is to keep the tire in contact with the road surface as much as possible for maximum grip.  As GPL does not model road bumps, it's entirely possible to run extremely high spring and damper rates with no apparent ill effect.

I recommend you use wheel/spring rates of 40% front and 60% rear.  The reason for this is that all GPL cars have a fore/aft CG of approximately 60% so it makes sense to set the wheel/spring rates in the same proportion so that the front and rear suspension compressions are the same.  With your 50/50 split, your front suspension compression will be less than the rear.

Lee

[John Woods]

Lee, thanks for this two-post boatload of info. I've read through it twice alrady and will again. Is there somewhere else this kind of Grand Prix Legends specific technical info is presented? I'd like to spend more time trying to figure it out without carrying on with novice technical questions if they are already answered elsewhere.

Allison and Nate Hine, (Eaglewoman and GRE inventor, respectively), were once pretty much all there was as far as sources for Grand Prix Legends engineering, and when first reading through their presentations I was struck a bit when they recommended 40/60 f/r spring rates because that seemed contrary to what I had learned about cars from sources like Road & Track magazine and books I'd read.

I started learning more about the real Lotus 49 and found that it is indeed 40/60 dry, but almost all fuel and most of the driver sat on the front half of the car, and when adding that weight the fully loaded weight distribution becomes close to exactly 50/50.

The only test I could think of was to look at average tire temps for each wheel given a schedule of test runs while adding fuel between each run, and found tire temps increased with fuel load evenly and consistently across all four tires, so it seems Grand Prix Legends does not model actual (real) effects of fuel and driver load even on a car sitting still.

So the question became, well, what next? Should I blindly follow the Hine's advice, or believe what I had learned everyone else in the real world was trying to design and build into not just race cars but all cars if possible? And what does that have to do with Grand Prix Legends, and why would it not model a car fully loaded, (a race ready archtype), rather than dry?

Eventually, I gave up caring that much because its more fun to drive, all I could do is guess, and others way faster than me posted setups using 50/50 or close to it, (P. Jackson, for instance), but now I'm thinking about it again, entirely because of incredible experts at srmz and this thread in particular.

Edited by John Woods, Aug 01 2010 - 05:36 AM.