2016 SM (Street Mod) Autocross Discussion
May 26, 2016, 09:26 PM
#106
Yessir, its much harder to predict what happens day to day than on a track.
Ive always felt that with the Tire/weight ratio difference between the front and the rear plus the mac strut vs multilink that its all about sticking the front. Rear has excess grip if you want it but theres never enough front grip. I will eventually put effort into the rear but there are a few other projects between that I think are more important especially when considering $/laptime.
Ive always felt that with the Tire/weight ratio difference between the front and the rear plus the mac strut vs multilink that its all about sticking the front. Rear has excess grip if you want it but theres never enough front grip. I will eventually put effort into the rear but there are a few other projects between that I think are more important especially when considering $/laptime.
May 27, 2016, 09:39 AM
#108
hey get in line dude!
I'd also prefer bare aluminum. anodizing and powder coating will hide any cracks that might form until it's too late to do anything about it. worst case I'd hit them with some etching primer and a light coat of spray paint.
I'd also prefer bare aluminum. anodizing and powder coating will hide any cracks that might form until it's too late to do anything about it. worst case I'd hit them with some etching primer and a light coat of spray paint.
May 27, 2016, 02:28 PM
#110
One of these days I'll make a separate thread for the uprights, maybe after I get the next set made.. I could just start sending files to my local guy for a set of 10 but that feels risky to not at least make the next set first just in case.
A little torn because I know a bunch of people are wanting them now. I'm making a few drawings for the parts now to send out so should be able to get some quotes next week at least. Fingers crossed they come in where I'm hoping the do.
A little torn because I know a bunch of people are wanting them now. I'm making a few drawings for the parts now to send out so should be able to get some quotes next week at least. Fingers crossed they come in where I'm hoping the do.
May 29, 2016, 10:35 PM
#113
So I've had a couple people ask questions about RC behavior based on the modeling I've done. I finally got around to opening up the model, keep in mind I haven't looked at this in a couple years so the details of the exact setup I have in the model is a little foggy. I feel it's worth comparing numbers here though if you're willing. The results I see now are much more curious, after taking a break from it and putting a couple more year into learning about this stuff.
For reference, a lot of my model was built around a particular wheel/tire setup. 18x10 RPF1 with 38mm offset and a 20mm spacer with a 275/35/18 tire. Basically trying to get the max tire for a ~2640lb SM car. Because of this selection, I have a couple things that might not line up with many other's setups. For one, this requires setting the camber at the hub to max positive to maximize tire/strut clearance, then moving the top strut mount in as far as needed to get the negative camber needed (about -3* is the max with the camber plates I had in the model). I also maxed out caster with the offset PSRS and then rotating the camber plates to get around 7-7.5* total (squatting in the rear too). Also, these are numbers I quickly pulled out of the model with no real correction done to deal with alignment changes or tie-rod heights. It will affect the actual numbers, but the overall trends will be similar.
It looks like what I have in the model is 2" of height correction on the balljoint and a ride height of 4.7" to the pinch seam. Keep in mind, that's based on a perfectly round tire with no deflection, actual ride height would be lower.
Roll center height at ride is slight below 1" below ground level and the instant centers are WAY out there. With 1.5* of roll, the roll center migrates around 40" to the low side of the car and sits right around ground level. Full droop puts the RC about 3" above ground. With the tire crashing into the modified frame rail, RC is about 8" below ground.
With 1" of correction, RC sits about 4" below ground level at ride height. 1.5* of roll causes about 12" of RC migration and puts it about 3" under ground. Full droop puts it about .5" above ground, and RC height is about 12" under ground when the tire runs out of travel.
For 2.5" of correction - RC is at ground level at ride height. 1.5* of roll induces 180" of RC migration and raises it to 8" above ground (instant centers are WAY WAY WAY out there at ride height). RC is around 4" above ground at full droop and 6" underground at full travel.
Am I crazy to think that maybe less correction is better on this car? Just for the sake of keeping roll center migration to a lower level? Or does RC migration (in particular, lateral migration) not mean a whole lot?
For reference, a lot of my model was built around a particular wheel/tire setup. 18x10 RPF1 with 38mm offset and a 20mm spacer with a 275/35/18 tire. Basically trying to get the max tire for a ~2640lb SM car. Because of this selection, I have a couple things that might not line up with many other's setups. For one, this requires setting the camber at the hub to max positive to maximize tire/strut clearance, then moving the top strut mount in as far as needed to get the negative camber needed (about -3* is the max with the camber plates I had in the model). I also maxed out caster with the offset PSRS and then rotating the camber plates to get around 7-7.5* total (squatting in the rear too). Also, these are numbers I quickly pulled out of the model with no real correction done to deal with alignment changes or tie-rod heights. It will affect the actual numbers, but the overall trends will be similar.
It looks like what I have in the model is 2" of height correction on the balljoint and a ride height of 4.7" to the pinch seam. Keep in mind, that's based on a perfectly round tire with no deflection, actual ride height would be lower.
Roll center height at ride is slight below 1" below ground level and the instant centers are WAY out there. With 1.5* of roll, the roll center migrates around 40" to the low side of the car and sits right around ground level. Full droop puts the RC about 3" above ground. With the tire crashing into the modified frame rail, RC is about 8" below ground.
With 1" of correction, RC sits about 4" below ground level at ride height. 1.5* of roll causes about 12" of RC migration and puts it about 3" under ground. Full droop puts it about .5" above ground, and RC height is about 12" under ground when the tire runs out of travel.
For 2.5" of correction - RC is at ground level at ride height. 1.5* of roll induces 180" of RC migration and raises it to 8" above ground (instant centers are WAY WAY WAY out there at ride height). RC is around 4" above ground at full droop and 6" underground at full travel.
Am I crazy to think that maybe less correction is better on this car? Just for the sake of keeping roll center migration to a lower level? Or does RC migration (in particular, lateral migration) not mean a whole lot?
Last edited by 03whitegsr; May 29, 2016 at 10:48 PM.
May 30, 2016, 12:25 PM
#115
Lol, leave it to Marshal to call it like it is.
When it comes down to it, our front geometry is effed. Honestly I still dont understand what it means when the instant center transports to outside of the car other than the car transitions to camber loss in bump.
On that subject, is your model looking at the slider axis or steering axis for roll center? (The right answer is slider axis
)
Its a rally car and not intended to run as low as we do, actually the car is pretty dang happy at stock ride heights which matches what Marshal said, static RC ~1" above ground. I cant pretend to really understand all the complex interactions but have my assumptions based on bad characteristics and playing with so many different combinations.
If theres one thing I really believe in for a front heavy macstrut car, its that the more rear bar you can effectively run, the more front grip you'll have. Sticking the rear is easy, but keeping the outside front from diving and planting inside front.. That tops my setup goals.
Skin the cat how you want to, regardless of class rules, thats what I would try to do. In stock if I had to run stupid shocks and weird camber/toe/tire pressures to make it work, big *** rear bar! Debate, GO!
When it comes down to it, our front geometry is effed. Honestly I still dont understand what it means when the instant center transports to outside of the car other than the car transitions to camber loss in bump.
On that subject, is your model looking at the slider axis or steering axis for roll center? (The right answer is slider axis
) Its a rally car and not intended to run as low as we do, actually the car is pretty dang happy at stock ride heights which matches what Marshal said, static RC ~1" above ground. I cant pretend to really understand all the complex interactions but have my assumptions based on bad characteristics and playing with so many different combinations.
If theres one thing I really believe in for a front heavy macstrut car, its that the more rear bar you can effectively run, the more front grip you'll have. Sticking the rear is easy, but keeping the outside front from diving and planting inside front.. That tops my setup goals.
Skin the cat how you want to, regardless of class rules, thats what I would try to do. In stock if I had to run stupid shocks and weird camber/toe/tire pressures to make it work, big *** rear bar! Debate, GO!
Jun 1, 2016, 03:40 PM
#116
With the zero ****s given method, 2.5" of correction is about right, ha. I think those numbers are also based on the Whiteline RCK and PSK being installed too, not that the PSK matters for roll center but it puts total height correction around 2.8" from stock if I recall correctly. Also, the minimum camber at the hub isn't needed either as originally I was trying to fit the +38 with no spacer, it was close in the model but in the real world, it wouldn't work and would need a minimum of 8-10mm spacer. You could dump in a little extra camber and probably end up around -4* with the parts I had at the time.
I had that hang up long ago with slider vs. steering axis, it's based on slider axis.
I don't know the answer on the rear bar as a bigger bar is effectively taking away grip from the rear? I assume this is coming more from the classical racecar dynamics mindset of set your spring rates based on ride frequency and then adjust bars to get the car to handle the way you want?
I don't disagree with the thought that the geometry is so jacked up on these cars that it probably doesn't matter all that much, particularly for an autoX car.
I had that hang up long ago with slider vs. steering axis, it's based on slider axis.
I don't know the answer on the rear bar as a bigger bar is effectively taking away grip from the rear? I assume this is coming more from the classical racecar dynamics mindset of set your spring rates based on ride frequency and then adjust bars to get the car to handle the way you want?
I don't disagree with the thought that the geometry is so jacked up on these cars that it probably doesn't matter all that much, particularly for an autoX car.
Jun 26, 2016, 01:43 PM
#117
Greetings. I'm checking in to see what weight reduction, specifically interior removal is allowed in SM. I've read the rules a couple times, but I'm not 100% sure what is allowed.
So far I've pulled all the carpet and panels in the trunk, removed the rear seat and the hatshelf, 3rd brake light, speakers and grilles, all rear seat-belt tensioners and the baby seat attachment points etc.
I'm NOT allowed to remove the inner door covers. I think. What about the headliner? Main cabin carpeting? Rear part of the center console? Glove box? Anything on the front of the car I can remove for weight?
So far I've pulled all the carpet and panels in the trunk, removed the rear seat and the hatshelf, 3rd brake light, speakers and grilles, all rear seat-belt tensioners and the baby seat attachment points etc.
I'm NOT allowed to remove the inner door covers. I think. What about the headliner? Main cabin carpeting? Rear part of the center console? Glove box? Anything on the front of the car I can remove for weight?
Jun 26, 2016, 04:57 PM
#119
Other than removing rear seat belts and seats, needs to look like the interior of an RS (Google for images). You're also taking out stuff from the rear when you need to get creative up front.
Lithium battery
Relocate or remove ACD pump
Remove dash pad
ultralight brake rotors
Light motor mounts
Baller billet hubs (shameless...)
Then start thinking about mid car since its split front/rear
Replace seats
Remove airbags
Remove sound deadening
Everything stereo related gone
Rear is easy on an Evo but front is where it matters.
Lithium battery
Relocate or remove ACD pump
Remove dash pad
ultralight brake rotors
Light motor mounts
Baller billet hubs (shameless...)
Then start thinking about mid car since its split front/rear
Replace seats
Remove airbags
Remove sound deadening
Everything stereo related gone
Rear is easy on an Evo but front is where it matters.
