knuckle risers

[Divano Veloce]

Hello,

there seems to be two ways to raise the front roll center on a 105, raising the top ball joint with a spacer welded to the knuckle or to lower the lower ball joint by either cutting and extending the knuckle or by using something like the alfaholics extender. The top spacer is cheap and easy and as its less stressed  than any of the lower ball joint mods (which transmit wheel loads to the chassis via the lower spring pan) is probably stronger and safer.

Im after some technical opinions/advice.

Cheers,

Matt

[Andrew Bose]

Try Vin Sharp at Pace his details will be on the sponsors page

[RobDee]

This diagram explains what's going on: http://www.autocrossitalia.it/ROLLCTR.GIF

It would be difficult to raise the roll centre by more than a few millimetres unless you make some serious mods to the geometry.

[Duk]

The top spacer is cheap and easy and as its less stressed  than any of the lower ball joint mods (which transmit wheel loads to the chassis via the lower spring pan) is probably stronger and safer.

What do you mean by this?
The spring pan takes the vertical forces. Changing roll centre height shouldn't have any effect on the spring pans vertical forces.

Be aware that any change in roll centre height will cause bump steer and it can become quite severe (quite a few millimeters of toe angle change with bump or droop travel).

But the important aspect isn't just the raised roll centre height but the moving of the 'Instantaneous Centre' or the 'Virtual Swing Arm Pivot Point' to the other side of the car. This will allow the wheel to gain negative camber with bump travel versus the original set up that has the wheel moving very vertically and eventually gaining positive camber with bump travel.
This camber gain with bump travel helps to maintain the wheel in much more perpendicular relationship with the road for much more tyre grip.   

[Colin Byrne]

My dad and i have tried both methods in our rally cars, and i'm actually onto a third method which is moving the inboard points of the top arms down, bit this is more to do with the rules of the class we compete in.

As has been said, the knuckle risers are probably the easiest, and the safest as that joint doesn't have to contend with the forces of the spring like the lower joint does.  The Alfaholics lower joint mod does have the advantage of some additional caster, however we've had problems with ground clearance to the bottom arm during rallies and also internal clearance with a 14" rim, the other issue is there is a fair amount of mucking around with spring/spacer to get back to the correct ride height, where as there is no ride height effect by changing the top arm

As far as I know both these mods are good for raising the roll center by around an inch, I haven't actually mapped out the pickup points on the front of the car myself yet, but these are the values we were given to us and seem to make sense with the spring rates we've ended up with.

I'm interested in the comment about bump steer, as far as I can see a change in the Roll centre shouldn't have any effect on bump steer characteristics.  Bump steer is the relationship between the length of arc of the steering arm compared to that of the upright.  The roll centre point or the following roll moment has no direct effect on this geometry this is why bump steer can be checked/adjusted by isolating one side of the vehicle and just moving the upright through its bump/droop travel, but am interested to hear your thoughts?   

[Duk]

As far as I know both these mods are good for raising the roll center by around an inch, I haven't actually mapped out the pickup points on the front of the car myself yet, but these are the values we were given to us and seem to make sense with the spring rates we've ended up with.

By lowering the bottom ball joint, you should be able to both lower the centre of gravity (lower the car) and raise the roll centre at the same time and gain a benefit that way (shorter roll couple), but obviously it is the more involved and expensive way to do it.

I'm interested in the comment about bump steer, as far as I can see a change in the Roll centre shouldn't have any effect on bump steer characteristics.  Bump steer is the relationship between the length of arc of the steering arm compared to that of the upright.  The roll centre point or the following roll moment has no direct effect on this geometry this is why bump steer can be checked/adjusted by isolating one side of the vehicle and just moving the upright through its bump/droop travel, but am interested to hear your thoughts?   

If you check out the Alfaholics set up you will see the big drop leg they used for the outer rod end.
You will see different solutions to bump steer, if you look at the drop spindles made by Alfa7/Richard Jemmison on AlfaBB and MD's upside down ball joints for his Alfetta track cars.
I have Pace Engineering Long Shank Top Ball Joints for my 75 and when you take the springs out and work the suspension through its travel, you can easily see toe change (toe in with bump, toe out with droop).
The fix is (again) to lower the outer tie rod, but my failed attempt at just bending the steering arms showed that I didn't need to lower the outer tie rod any where near as much as I 1st thought. The LSTBJ's moved the top pivot point up by maybe 30mm, but the effect on the bump steer requirements aren't as dramatic as the they would have been if I had lowered the bottom ball joint by 30mm.
Bump steer characteristics will have to be checked after geometry changes are made.

[Paul Byrne]

Good discussion and, sticking to 105s, Colin has covered our experience in the front roll centre area. Just a couple of things to chuck in.
The design of the lower wisbone link from the UK supplier does involve the concern that the lower swivel joint bolt is in tension, compared to the original Alfa joint wich is in 'fail safe' compression.

Re bump steer, once the 'parallelness' of the top and bottom arms is changed by changing the location of either the top or bottom arm chassis pickups or outer ends, the steering track rod 'parallelness' required for minimal bump steer also changes. Luckily, if the UK supplier bottom joint is the mod adopted, the bump steer can be minimised by replacing the 2L 105 upswept steering arms with earlier 105 'straight' steering arms. For other mods, height adjustable track rod ends are probably needed.

[Divano Veloce]

Thanks to everyone for your response to my question. I am currently rebuilding the front suspension and will resist the urge to add modifications at this stage. Instead I have measured up the components and built a rough model. Attached is a preview. I need to add the chassis, hub, wheel and steering arm. Then i can start with some analysis of options.

I am in the process of doing the same with the rear end.....

[Divano Veloce]

Some progress, still need to model the hubs/wheels and steering arms and set up a motion study