Scrub Radius changing from -5 to +35. Is this too much?

[SWRT]

Hello, I'm new to this forum. I've been going through some of the post and am very impressed with the level of knowledge and experience you guys display.

I would like to get some opinions on a design issue I've encountered.

Here's my situation:

I am trying to optimize the front suspension of a front wheel drive 2700lb touring car. The suspension is macpherson strut, I'm starting with production car geometry.

My issue starts with a new knuckle that I have sourced from another market this platform is sold in. The new knuckle has a much larger bearing. I need this larger bearing to remedy a frightening pad knock-back issue we've been experiencing during high lateral cornering.

The new knuckle also applies to a car with a -15mm ride height, which partially corrects my roll centre and bump steer (my actual ride height target is -43mm).

So, to the point... The larger bearing has pushed my hub face further from the centre line of the car. This has pushed my outboard wheel face outside the fender... which is one of the only suspension rules I am bound to. So to pull the wheel back inside the fender, I've shortened the lower arm and pulled the top of the wheel towards the damper (pivoting at the damper-to-knuckle joint).

This change has made my king pin axis more vertical, and put my scrub radius at +35mm. This platform in production form runs -5mm scrub.

I understand the immediate impact of a large +ve scrub radius, but I have no reference for how excessive +35mm is. Our racing surface is smooth and for the most part uniform, so I don't expect to see a lot of split-mu braking. Also the steering is EPS, with relatively high inertia and somewhat tunable damping.

Unfortunately I cannot change my wheel offset right now. We have 3 dozen 35mm offset wheels which I cannot afford to scrap.

My question to you guys is double sided:
First, is +35mm 'reasonable' for a race only application?
Second, other than the large moment arm that my longitudinal tire forces now have to break my thumbs with... is there any other performance handicaps I'm introducing with this change?

I've confirmed camber gain to be reaonable, and I expect I will be able to correct RC migration and toe curves with extended ball studs and inboard pivot height changes.

Your input is much appreciated,
-Pete

 

[GregLocock]

35 is on the high side. If you care that much you could dial it out with differential bush stiffnesses on the lower triangle, ie use toe in braking to suppress it.

For a race car I think you'll be fine.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JHarshbarger]

Whenever people talk scrub radius, this comes up because it sounds like you're increasing "scrub" on the tire. Changing the scrub radius, aside from running zero scrub radius, has no impact on the way the tire acts. The tire will generate more forces through the linkages, but especially in straight line acceleration, there will be no change in tire behavior. There are other characteristics closely associated with the scrub radius such as kingpin inclination that will change things as far as the tire goes. Running zero scrub radius could cause vibrations under zero lateral load.

Tire scrubing would result in loss in traction/speed and an increase in tire wear/heat. This is when the tire is forced laterally at the contact patchby suspension geometry.