Any issues with excessive SAI on a non-steering wheel?

[mike944]

Hey everybody,

I'm looking to understand if there will be any issue with excessive Steering Axis Inclination on a non-steering wheel? Or rather, should i say, what problems will this cause?

I'm building a mid-engine race car, and using the front driveline in the rear of the car. I was considering using the strut front upright, and turning it into a double a-arm suspension, by attaching a spherical bearing to a bracket on the upright. It will make the SAI upwards of 22 deg. Based on the wheels i'm using, this makes the scrub radius pretty close to zero, which is good for low moment around the SAI, but the SAI just "looks" excessive, compared to a normal double a-arm suspension.

Since this is not a wheel that turns around that SAI, what, if any issues is this going to cause with my geometry?

Thanks!
Mike

 

[golfpin]

Hi Mike, I don,t know if this would be of much help but if you Google "Terrapin Forum" you will find some postings of mine re sports car with Golf as donor car, built around the concept that you have described. The pics on the sight under, Golfpin might help, failing which please feel free to contact me via E mail schultzp@telkomsa.net by that time I might have got my head around your question but what You have in mind sounds good to me and the full support of the late great Alan Stanniforth author of many design and concept books.
Cheers Golfpin

 

[GregLocock]

Do you mean front view SAI, ie KPI in English, not castor which is side view SAI?

I can't think of a bad first order effect, it does couple camber change into any toe, but that seems to be rather a small effect.

Normally this would mean a very short upper arm, ie lots of camber-in-roll, but you probably want that.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Most multilink rear suspensions that were designed as rear suspensions (!) have connecting points to the spindle that bear no resemblance to a front-end "steering axis". It's irrelevant as long as the camber and toe behaviour with suspension travel is what you want it to be and the deflection response to applied forces is what you want it to be. Suspensions that are variations of the Ford "control blade" design - essentially a flexible trailing arm with transverse links guiding its path - includes VW Golf Mk5/6, Chevrolet Equinox, and most likely others - have a "steering axis" that varies widely with suspension up/down travel because of the changing angle of the trailing arm, again pointing towards "it doesn't matter".

 

[mike944]

ok, thanks for the help!

To answer the Greg's question above, yes, i mean front-view SAI (aka kingpin angle)

I couldn't think of any negative effect, but then again, i don't have a lot of suspension design experience, aside from the research i'm doing now, and a few Formula SAE cars I worked on in college in the late 90's. I normally design jet engines, not cars. This is just a fun project for me.

In the car i'm building, the formerly front strut suspension isn't going to package well in the rear, due to the extremely low ground clearance, and short car, the strut tops would be getting close to the roofline! Plus, i'd like the beneficial camber change of a double a-arm anyway (to match the front).

I've been reading a LOT on the subject, and pretty sure i know what i want. there was just this one open item that seemed odd, and i didn't know what to make of it.

I'm looking for a near-zero scrub radius, a good amount of negative camber gain on bump (to counteract any roll) a roll center a few inches above ground, and zero to slightly inward toe change on bump. Also, fairly well-matched front and rear roll centers, camber curves and roll stiffness (i think that's everything)

I'm sure i'll eventually have other questions (which i'll sometime post later)


Mike

 

[evox]

Keeping the scrub radius as close to zero is a must have for traction related compliance(s). Otherwise your KPI should be defined by your desired suspension geometry (kinematics).

Be sure to consider traction (accel and braking) when you define your rear camber gain (dont have too much!). You can also use anti- geometry to keep the squat/lift under control but beware of excess. Lot of roll stiffness can help too (but has its own caveats).


Also...roll axis inclination. The rear roll center height is typically higher than the front. This makes the car roll more with the front, which helps you 'feel' the loading on the steering tires. Relate it to your CG heights front and rear too! Some older JDM production vehicles do the opposite (lower relative RC in the rear) and can feel a little strange in turns.