Race Engine Forum UK

In the quest to upgrade the front brakes on my Fiat Tipo I intended to use Alfa 145 front hubs. I have noticed that these hubs are slightly different. They will result in more Ackerman angle when fitted to a Tipo.

This raises a few questions. What affect will the new Ackerman angle have on handling? Will it be good, bad or insignificant? The car is of course used mainly for high speed events, not parking.

Out of interest, does anyone know how close a Tipo is to true Ackerman?

It is interesting to note, when Fiat increased the track of the Tipo for the Coupe and other later cars, they did not change the hubs. This would result in a different Ackerman angle. Fiat must have either thought the new angle was beneficial or the difference was in fact insignificant. Alfa Romeo on the other hand did not think it was insignificant as they did redesign the hubs.

Steve the easiest way to estimate if there is a true change in Ackermann is to look at the intersection of the rod end joint position to the kin pin position in relation to what essentially is the line of travel of the vehicle. If the angle is more accute with the rod end position nearer the disc the greater the ackermann will be.

Let us know if that is the case with the Alfa hub.

Rich

Hi Rich

I believe the angle will be more Accute. That is, the intersection point will be further foward from where it is standard.

Hi Steve

I can't see the setup on your picture, could you post another picture showing the complete hub?
Or do you have some technical drawing of the front suspension?

Thomas

Hi Steve,
Have you measured the bump steer effect with these two hubs. Personally I would be less concerned with the Ackerman angle at this point, much of the influence of Ackerman angle can be masked by toe adjustments anyway.

I would check that you can achieve an aceptable bump steer characteristic by shimming or otherwise altering the steering rack mounts first.

TR-Spider wrote:Hi Steve

I can't see the setup on your picture, could you post another picture showing the complete hub?
Or do you have some technical drawing of the front suspension?

Thomas

Here is another photo. The hubs are the same except the part in the picture though.

mtbr wrote:Hi Steve,
Have you measured the bump steer effect with these two hubs. Personally I would be less concerned with the Ackerman angle at this point, much of the influence of Ackerman angle can be masked by toe adjustments anyway.

I would check that you can achieve an aceptable bump steer characteristic by shimming or otherwise altering the steering rack mounts first.

No I have not. I did not think it would be significant. thanks.

How far away is the mounting for the damper?
Looks also different on both?!

Hi Steve

assuming that the location of the damper mount and balljoint are identical in both cases, then:
the Alfa one will give increased Ackermann steering, i.e. the inside wheel during cornering will run at an increased steering angle.
According to Carroll Smith's book "Engineer to win" this will reduce understeer and improove turn in (similar to running a little toe-out).

To answer the question how close your car is to true Ackermann you have to make a drawing with the suspension setup. Elongate the line connecting the steering link with the suspension/wheel rotating axis until it crosses the car's centerline. With true Ackermann steering it crosses at the back axle, with increased ackermann steering the crossing point moves towards the front.

I can't honestly make a prediction how the handling of your Tipo will change, so you will have to give it a try...


Thomas

Thanks Thomas, your assumptions are correct.

I have read a little about what might happen. No conclusions at all though and some contradictions. More turn in and less understeer sounds great so it may be worth a try.

I wonder if the Afla has this apparent different ackerman angle for a reason or its a patch for other suspension differences.

The difference with the Alfa will probably be down to wheelbase which is the main difference in achieving true Ackermann when interchanging components from different vehicles .
There is however a revised Ackermann which states that up to a 1/3rd in front or behind is acceptable.
I would be more concerned on ensuring the correct geometry of the rack to track control arm caused by a higher /lower track rod end attachment point.

Hi Kev

The wheelbase is the same between the 2 vehicles according to website infomation. The location of the rod end would sit slightly higher than a Tipo. about 10-20mm. I was concerned about that. However the front sits 50mm lower than standard without any issues so far.

I know that your car is front wheel drive but the same basic principle of alignment applies as a rear wheel drive MacPherson strut suspension as shown in the diagram.
Obviously a higher pick up point at the hub , but with the rack remaining in the same position ,would result in a misaligment of the outer triangulation . This would , at certain times during suspension travel ,result in bumpsteer.