Track forum general chit chat.

derbo wrote

342/685 lbs/in is the off the shelf Ohlins RT setup.

Yes, these are the rates I've got

P1zzaboy wrote

You can loosen the 4 front subframe bolts and center it. There’s enough play there to get more than 1deg camber difference each side

This is not true. The front subframe is pinned. I confirmed this and there isn’t enough play to affect camber more than a fraction of a degree. Plus there are 8 bolts holding it up.

tlrid3r wrote

This is not true. The front subframe is pinned. I confirmed this and there isn’t enough play to affect camber more than a fraction of a degree. Plus there are 8 bolts holding it up.

There's almost zero play in the front subframe. I tried to get rigid collars installed once and it was a bitch cos the tolerances in the bolt holes were super tight.

tsk94 wrote

I'd say this is true, except if he's on stock springs (or a mild lowering spring). Then a front sway bar can help get you more front end grip just by limiting the camber loss mid-corner. I did this on my 128i, and added on the adjustable Hotchkis M3 bar and it helped on track. But, that was with some Eibach off the shelf lowering springs, so still very soft rates.

Otherwise, yes. If you already have a coilover setup with decent spring rates, adding on the front bar alone will just make the car understeer more.

Accurate. I am using the Dinan front bar with a 300-ish lb front spring, stock ZCP rear spring (X6 code I believe, been a while since I looked), 2.2-5 degrees of front camber and ZCP-ish ride height. Of course there are other faster better-handling solutions, but they demand other compromises. So it really depends what you're trying to achieve. My car is set up to be comfortable on the street, be able to clear transitions, bumps, etc., and not destroy its tires immediately on track and it mostly accomplishes that goal. The car would push like a broom without the bar, particularly on a good tire it really needs the extra roll resistance to get the nose in and keep your options open mid-corner on such soft springs

Has anyone experimented with bump springs up front? I need a solution that does not require increasing the front spring rate over 900 lbs since I like the current feel, but my splitter is getting trashed at Buttonwillow.

okusa wrote

Has anyone experimented with bump springs up front? I need a solution that does not require increasing the front spring rate over 900 lbs since I like the current feel, but my splitter is getting trashed at Buttonwillow.

Love this! I assume the splitter thrashing is coming from aeroload near the back end of straights and into initial braking?

DRLane wrote

Love this! I assume the splitter thrashing is coming from aeroload near the back end of straights and into initial braking?

I think it's happening all over, but in particular on Off Ramp (nasty off camber right hander) and Riverside (high speed sweeper).

okusa wrote

I think it's happening all over, but in particular on Off Ramp (nasty off camber right hander) and Riverside (high speed sweeper).

Hmmm, please let us know where you land between a spring vs rubber. But if you feel under sprung in corners, then you’d want to go up in spring rate (which you said you didn’t want to do) or dial up the sway bar? I’m not a suspension engineer, just trying to visualize. Hopefully someone competent pipes in.

okusa wrote

Has anyone experimented with bump springs up front? I need a solution that does not require increasing the front spring rate over 900 lbs since I like the current feel, but my splitter is getting trashed at Buttonwillow.

i listened to a "tune your car's handling" podcast by ross bentley and race engineer jeff braun a while back. they touched on dual rate spring rates and how complicated they can be. i don't have much to repeat about that.

they did touch on how easy it can be to tune spring rates and bump stops using spring rubbers. the key point i remember (its been a while) was using a bumpstop that wasn't too firm because it can cause a sudden loss of traction. if the bump is too firm, it can result in a sudden loss of traciton.
the cool part is, these parts aren't expensive and can be easily swapped out for a test 'n tune day.
https://www.resuspension.com/bump-stops.html

roastbeef wrote

i listened to a "tune your car's handling" podcast by ross bentley and race engineer jeff braun a while back. they touched on dual rate spring rates and how complicated they can be. i don't have much to repeat about that.

they did touch on how easy it can be to tune spring rates and bump stops using spring rubbers. the key point i remember (its been a while) was using a bumpstop that wasn't too firm because it can cause a sudden loss of traction. if the bump is too firm, it can result in a sudden loss of traciton.
the cool part is, these parts aren't expensive and can be easily swapped out for a test 'n tune day.
https://www.resuspension.com/bump-stops.html

Bump stop and bump springs serve the same purpose, with the spring being what I’d prefer to play with. To install they’ll both require you to drop the suspension from the top. Likely need to disconnect the caster lower control to get enough clearance to drop it assuming you’re running non OEM arms. It can definitely be done trackside but it’s not a 5 min change.

Bump rubbers are little devices you insert into the spring to in effect reduce the spring by a full coil thereby increasing the spring rate. Those are easy to install. Can also be a solution to getting non permanent ride height for loading on a trailer.

There are some excellent HPA materials on bump stop / spring tuning. Their 10 min overview is at a bare minimum helpful to orient to concepts.

i sort of remember it being said you could cut/zip tie the bump stops on to experiment and tune, and then do a proper install once you find something you're happy with.

i'm not at this point yet, i'm just relaying what i've learned about. zero personal experience here.

roastbeef wrote

i sort of remember it being said you could cut/zip tie the bump stops on to experiment and tune, and then do a proper install once you find something you're happy with.

i'm not at this point yet, i'm just relaying what i've learned about. zero personal experience here.

If you cut a bump stop vertically, to say slip it around the damper and then zip tie it closed. I think it would just collapse and provide no measurable change. Again, I think you’re thinking of rubbers. Or you can add/remove shims to effectively change the engagement point of the bumpstop.

Edit: i see the flat disk style bump stops that you’re likely referring to roastbeef. I take it back!

DRLane wrote

If you cut a bump stop vertically, to say slip it around the damper and then zip tie it closed. I think it would just collapse and provide no measurable change. Again, I think you’re thinking of rubbers. Or you can add/remove shims to effectively change the engagement point of the bumpstop.

Edit: i see the flat disk style bump stops that you’re likely referring to roastbeef. I take it back!

i was thinking about that... the cut bump stops just blowing off the zip ties and the bumps falling off on the first strike. i'm not sure how these things behave, i'm speaking from inexperience.
i did see these bump stop cups. it looks like you install these at the end of the shaft and stuff a bump stop into it. i suppose the walls would prevent it from blowing out. i'm not sure how you'd get it out quickly or without destroying it though... mabe lube it up and install a pull ribbon?
https://www.resuspension.com/bump-stops/bump-rubber-cup-625-x-1-00-re-brcup-625-1.html

DRLane wrote

Bump stop and bump springs serve the same purpose, with the spring being what I’d prefer to play with. To install they’ll both require you to drop the suspension from the top. Likely need to disconnect the caster lower control to get enough clearance to drop it assuming you’re running non OEM arms. It can definitely be done trackside but it’s not a 5 min change.

Bump rubbers are little devices you insert into the spring to in effect reduce the spring by a full coil thereby increasing the spring rate. Those are easy to install. Can also be a solution to getting non permanent ride height for loading on a trailer.

There are some excellent HPA materials on bump stop / spring tuning. Their 10 min overview is at a bare minimum helpful to orient to concepts.

As you know, one of the main differences between them is bump stops have a progressive spring rate vs. a linear rate bump spring. This makes bump stops more forgiving if your suspension travel calculations are off slightly and/or you encounter an unexpected offroad excursion that would result in an over-travel condition. A bump springs stiffness change is abrupt when it hits its block height and results in an abrupt loss of grip.

Bump stops are cheap and bump shims can be easily added/removed to change damper travel (could do this with bump springs, too, but it’s likely there’s no no available length to incorporate them). Bump stop are more compact and much easier to integrate (bump springs are quite long compared to a bump stop). Not saying bump springs don’t have advantages because they do, such as a wider range of available stiffness, stiffness not affect by temperature, length and stiffness remains constant/repeatable with abuse, etc.

For keeping a splitter from bottoming out, I’d choose a bump stop over a bump spring especially if you’re running with a 5” main spring - bump springs are ~+/-4” long so you’ll start using them more frequently and, therefore, alternating your main spring rate. Bump stops will better protect the splitter in unexpected events, running different lines, etc.

This is a very helpful discussion, but unfortunately way above my tech level. That said, it sounds like it would be better to start with bump stops. I was just concerned about them unsettling the car but, again, that is due to my lack of technical understanding.

okusa wrote

This is a very helpful discussion, but unfortunately way above my tech level. That said, it sounds like it would be better to start with bump stops. I was just concerned about them unsettling the car but, again, that is due to my lack of technical understanding.

Worth a listen: it’s under 10 mins.

Does anyone have an accurate value for the motion ratio of the damper?

M3SQRD wrote

Does anyone have an accurate value for the motion ratio of the damper?

I have to figure out where I got these numbers from but they are in my spreadsheet.

Spring motion ratios
Front 0.96
Rear 0.576

Damper motion ratio
Front 0.96
Rear 0.813

Sway bar motion ratio
Front 0.96
Rear 0.56

derbo wrote

I have to figure out where I got these numbers from but they are in my spreadsheet.

Spring motion ratios
Front 0.96
Rear 0.576

Damper motion ratio
Front 0.96
Rear 0.813

Sway bar motion ratio
Front 0.96
Rear 0.56

Thanks!

I was looking at the spring MR and thinking that doesn’t look right…until I realized I was mixing E92 and F82 MRs There’s a similar main spring/bump spring discussion going on in the F8x forum. The F8x MR is higher than the E9x MR.

tlrid3r wrote

This is not true. The front subframe is pinned. I confirmed this and there isn’t enough play to affect camber more than a fraction of a degree. Plus there are 8 bolts holding it up.

I’m not inventing it. Maybe I don’t remember the number of bolts correctly. I loosened my subframe to ease the installation of new engine mounts. Right after that went for an alignement and couldn’t get similar camber readings on both sides. Went back home, loosened the subframe again, pryed a little to one side, went back to alignment and was able to get it perfect.

P1zzaboy wrote

I’m not inventing it. Maybe I don’t remember the number of bolts correctly. I loosened my subframe to ease the installation of new engine mounts. Right after that went for an alignement and couldn’t get similar camber readings on both sides. Went back home, loosened the subframe again, pryed a little to one side, went back to alignment and was able to get it perfect.

I didn’t go back and read all old posts…do you have camber plates installed?

As for the subframe, there’s practically no play in the front subframe. If there’s as much play as you believe there is, wouldn’t recentering your subframe from its recently reinstalled position to its newly recentered position most likely cause the camber to increase on one side while reducing it on the other side?

derbo wrote

I have to figure out where I got these numbers from but they are in my spreadsheet.

Spring motion ratios
Front 0.96
Rear 0.576

Damper motion ratio
Front 0.96
Rear 0.813

Sway bar motion ratio
Front 0.96
Rear 0.56

Is the rear sway bar MR listed above correct? Unless I’m not looking at the sway bar motion relative to wheel motion correctly, shouldn’t the rear sway bar MR be closer to the damper MR than the spring MR? Above, the sway bar MR is lower than the spring MR.