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Old 08-17-2018, 10:09 AM   #1
01RSTI
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Default RSTI scrub radius

I am in the process of a new RSTI build that will spend a good part of its post build life on the track.

I'm looking for information regarding changes to the scrub radius from the additional 10cm of width. I'm asking because I will be adding new wheels as well and would like to calculate the additional wheel offset to line up the new geometry.
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Old 08-17-2018, 12:14 PM   #2
jamal
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There's basically no change, as the SAI decreases with the wider control arms. A wagon and sedan still come with the same offset wheels.

What tire size do you plan to use? You'll likely need to compromise anyway just to provide clearance on the inside.
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Old 08-17-2018, 10:33 PM   #3
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Even running quite wide wheels, you'll be able to reduce the effect to a certain extent by increasing negative camber (edit: camber bolts won't help any) with the longer STI control arms and using camber plates to move the upper pivot point inboard, lowering ride height can help too.

when you say 10cm added width - is that the bodywork? track width?
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Old 08-18-2018, 12:30 AM   #4
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Originally Posted by T-37 View Post
when you say 10cm added width - is that the bodywork? track width?
02-07 wrx/sti sedan control arms and lateral links are about 10mm wider than the GC and wagon parts. So actually 20mm wider overall.

I wonder about increasing SAI in order to bring scrub closer to zero though. A little scrub is maybe not so horrible but sai basically counteracts caster and means less camber gain with steering angle.

Reality is that I never heard any complaints from the driver/owner of a pretty fast time attack car and we had a pretty low overall offset to use 10.5" wide wheels all around. I think just keep it as close to the inside as possible for your given tire width. Maybe even go with a higher offset than fits and bring it out with extended studs and spacers.
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Old 08-18-2018, 01:14 AM   #5
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The scrub radius won't be any different than the sti. The 1993-2007 bed pan on the Impreza is the exact same. The sti just uses a +53 offset to pull the rubber in a tad.

You can do the same and run 245 width tires under the stock fenders. Just grab some adjustable rear lateral links and crank them in 1/4in. A rear camber plate will allow you to pull the strut inward to make room for more tire.
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Old 08-18-2018, 02:12 PM   #6
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Thanks to all - This is excellent information. The donor car is an 06 that came with Ohlins colilovers. The RS will have the wheel wells cut and fenders added.

Some additional info on the setup I’m considering based on the results of my research here.
Wheels Enkei GTC01RR
9.5 wide with (35 or 45 offset)... but what I really want is 10 inch with 22 offset.

Switch the top hats, left to right and then rotate them for the purposes of adding caster and camber.

If is necessary to add more camber (lost from rotating the top hats) install a camber bolt.

@ T-37, you mentioned camber bolts will not help. Can you explain why?

Check out this post on Scoobynet it has different but similar info talking about alignment setup and adding mor castor, that you may be interested in.

https://www.scoobynet.com/suspension-12/918196-geometry-settings.html

Hearing others doing time attack haven’t complained running such a large offset change gives me more confidence.

Lastly the rear tower will be modified by drilling new holes for proper fit any - thoughts or experiences here? I haven’t seen anything negative about this.

Last edited by 01RSTI; 08-18-2018 at 02:27 PM.
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Old 08-18-2018, 02:46 PM   #7
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Quote:
Originally Posted by AcquaCow View Post
The scrub radius won't be any different than the sti. The 1993-2007 bed pan on the Impreza is the exact same. The sti just uses a +53 offset to pull the rubber in a tad.

You can do the same and run 245 width tires under the stock fenders. Just grab some adjustable rear lateral links and crank them in 1/4in. A rear camber plate will allow you to pull the strut inward to make room for more tire.
I love this idea! but the ohlins don’t have this option. In the search for rear top hats that didn’t require the drill / slotting modification, I asked Rallysport and Ohlins if the Ohlins are compatible with other manufacturers. So far this is still and unknown.
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Old 08-18-2018, 04:07 PM   #8
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Quote:
Originally Posted by 01RSTI View Post
I love this idea! but the ohlins don’t have this option. In the search for rear top hats that didn’t require the drill / slotting modification, I asked Rallysport and Ohlins if the Ohlins are compatible with other manufacturers. So far this is still and unknown.
I have a set of ohlins coils for the GC in a box in my garage and am currently on ohlins FPS with rear camber plates with a cusco bearing.

I can double check this week.
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Old 08-18-2018, 04:29 PM   #9
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Quote:
Originally Posted by 01RSTI View Post
@ T-37, you mentioned camber bolts will not help. Can you explain why?
Camber bolts won't help with Steering Axis Incline, it's only dependent on the upper and lower pivots points (tophat and ball joint.) Camber bolts are still useful for camber but they wont help reduce scrub radius.


FWIW I've run 225/40/18s on 7.5 et38's on my DD bugeye for many years. Far from ideal scrub radius. The only place I notice it being an issue is on uneven, sunken/rutted asphalt, sometimes it takes some steering input to keep moving straight at lower speeds if it's very rutted. I've never noticed any negative cornering effects that would outweigh the added track and traction. Full disclosure, I have been through a few front wheel bearings though (5x100), but I pretty much commute through a minefield.

I've found the same to be true with my autox car. A little wider track in the front tuned out a little bit of understeer. (Using 10mm front spacers currently on 17x9 et45's.)

Last edited by T-37; 08-18-2018 at 05:07 PM.
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Old 08-18-2018, 06:58 PM   #10
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Quote:
Originally Posted by 01RSTI View Post
I love this idea! but the ohlins don’t have this option. In the search for rear top hats that didn’t require the drill / slotting modification, I asked Rallysport and Ohlins if the Ohlins are compatible with other manufacturers. So far this is still and unknown.
MSI has the top nuts for their plates available in a few different thread pitches, and I think their rear tops are drilled to fit both the GC and GD bolt patterns.

If you are having a cage put in, it's easy enough to get some of those reinforcement plates in the GD pattern while they're at it. If not, then I'd just get some GC camber plates to stick on there. Or use the MSIs. Shouldn't be an issue to get them on that suspension but It depends on the upper spring perches.
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Old 08-18-2018, 07:32 PM   #11
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All this is now making me think more about what will actually work best vs when enough is enough. Obviously I have a different car but am going through some of the same questions now that I'm changing from wagon LCAs to sedan LCAs and getting new coilovers in the spring to better suit my goals. But with all the new changes and possible adjustments moving from OEM style top mounts to camber plates, the ideal balance of SAI, camber, scrub radius, wheel offset, tire size, and caster angle is making my head explode.
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Old 08-18-2018, 09:44 PM   #12
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In the end they are all pretty small differences. Like, are you really going to be able to notice the difference between 15 and 16 degrees of SAI or a few mm difference of scrub? Probably not, even with a consistent driver and a stopwatch. But personally I'm still going to try to keep SAI and scrub to a minimum. But if you want to have big wide tires you'll need to provide clearance one way or another.

Anyway, main thing is to start with is getting the spring rates and swaybars and alignment right.
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Old 08-18-2018, 10:06 PM   #13
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I sort of agree with the above. Individually, no probably wouldn't notice a difference. But when I ask the questions:
Am I creating more work for my brakes?
Am I making the car more twitchy on uneven surfaces?
Am I creating unneeded tire wear?
Then they can make a difference. I do agree alignment (which this is all part of) spring rate and sway bar size make the biggest difference without a doubt. You can add stiffer bushings and chassis reinforcement in there as well. It's all part of the puzzle. But that's why I'm researching, researching, researching. I know my 9-2x will never be the 240sx I once had but that's part of the fun is learning a different platform and what works for it.

Sorry OP, not trying to thread jack. Just similar conversation to what I'm going through as well.
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Old 08-20-2018, 03:59 PM   #14
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Quote:
Originally Posted by AcquaCow View Post
I have a set of ohlins coils for the GC in a box in my garage and am currently on ohlins FPS with rear camber plates with a cusco bearing.

I can double check this week.
Are the GC rear camber plates also from cusco?
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Old 08-23-2018, 08:39 AM   #15
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Originally Posted by 01RSTI View Post
I am in the process of a new RSTI build that will spend a good part of its post build life on the track.

I'm looking for information regarding changes to the scrub radius from the additional 10cm of width. I'm asking because I will be adding new wheels as well and would like to calculate the additional wheel offset to line up the new geometry.
To put some final comments on this thread...

I have spoken with a NVAuto Tech and Jason McDaniel - Lead Engineer at Vorshlag Motorsports, both confirmed the comment by @jamal - there really isn't an issue with the amount of negative scrub radius with this setup. Jason was particularly knowledge and willing to take the time with me to discuss in detail, he has built and raced several RSTI. The reasons it isn't a significant issue is the quality of suspension having adjustable coilovers with some camber increased rigidity from stiffer bushings and some additional negative caster. He said this setup tends to have some positive camber so that adjustment is especially important. Without these changes increased scrub would be more noticeable.. Consensus is it would track ruts and increased unpredictability during breaking.

So I guess the answer is Yes it's an issue and no simple answer for all but as you would expect from this type of build it's more holistic and can be addressed through suspension mods other than changing the king pin angle.
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