Scrub, KPI, spindle balance

[colinmseries]

Hello. I’ve re-read the relevant bits of RCVD and looked through my pile of prints from your discussions. Let me tell you what I think is true and I hope you will give me your thoughts.

My car has SLA front and rear I am lowering it by moving both the front and rear, top and bottom outboard pivots down vertically relative to the hub centres. I will do this by remaking the front and rear uprights. My concern is mainly with the effects this will have at the front.

Currently it uses: 3deg castor, 9deg KPI, 100mm spindle length and has 53mm positive scrub radius, all measured wheels straight ahead. Due to the proximity of the lower outboard front pivot to the brake disc, then as the pivot moves down so either the spindle length must grow, or the KPI reduce and scrub radius increase.

To maintain a constant spindle length, I calculate the KPI must reduce to 5.6deg and the positive scrub radius will increase to 71mm.

I have gathered from my readings that a 2:1 KPI:castor is a good guide, I wonder if for a road car you would agree?

What do you all think about the balance of these existing features and the best choice of the ones to move to?

Thank you, Colin.

 

[Toddr8541]

If you are designing and building a new upright I would build the spindle length to maintain current track width (wheel mounting distance from each other or center of car) unless there is a interference problem. That way you dont have to worry about making all of those changes. It's alot easier to just adjust the spindle length than to correct all of the other issues.

 

[colinmseries]

Thank you for your reply. I intend to maintain the track, it was the choice between more spindle length or less KPI that I'm hung up on. The upright will be remade anyway to lower the car.
My interest is in how others viewed the balance of srub, KPI and spindle as now and how they could be moved.
I have tried without much success to find data for a range of current road cars to see where castor and KPI sit.

 

[Toddr8541]

I doubt you will get that type of answer because there are too many factors involved to determne a good KPI/ Scrub. Every different car will be based on tire, roll rates type of usage, etc. A good starting point is Zero scrub and adjust your KPI to work with the other attributes of the suspension to give you the camber curves you need for your application requirements. Sorry I couldn't have been more helpful.

 

[colinmseries]

Hey, please dont apologise, I'm grateful for your input.
I had thought about tyre roll as the car uses old style tall tyres and may well, for all I know, loose its srub when cornering hard.
I'm sure I've seen either people on here or somewhere similar talk about this 2:1 ratio of KPI:castor, can't remember where I picked it up from though.
My inclination at the moment is to keep the spindle length as is and reduce the KPI and let the scrub grow. Not had the jig made yet so still time to change mind, will see.
Thanks again, Colin.