Prius Geometry

[Sagitar]

I recently sent a PM to Tony asking some questions related to the setting up of a Prius Gen3 T-Spirit. He asked me to put the message on the forum for general interest so I have copied it below. I have also attached a pdf showing details of the Excel file mentioned in the message.

 

 

 

 

 

Tony - I enjoyed my visit this week. It's good to get out and do something different. Apart from visiting you, I have a friend of long standing who lives in the Chalfonts, so it is a great excuse to go and see him for a couple of hours.

 

I have never paid much attention to geometry, until I had steering problems with the Lexus, but now I am trying to get a better understanding. I must admit that the changes and adjustments to the Prius are puzzling me somewhat.

 

Some of the numbers and their implications I find difficult to visualise, so I have set them out in a graphical form that I can understand more readily - see the Excel file attached. I've changed all the measurements to decimal degrees because that simplifies the arithmetic and I trust it makes sense to you?

 

Once the data are set out graphically in this way, it is easy to see that between setting the geometry on December 8th and taking new readings on September 8th there were changes in many of the positions. I suppose that is all explainable in terms of "wear and tear", but why

do some of the measurement go even further away from the mean following adjustment (the rear axle right wheel camber for example) - is this

about practical compromises or measurement limitations perhaps?

 

I don't want to pick my way through all the details because most of them appear to be trivial and well within tolerances, but there are two that

I would welcome your help in understanding.

 

The first is the front axle toe measurements. When these were set in December they were both set very close to the lower target limit. In

September they were changed again so that both were close to the mean position. It shows clearly in the Total toe measurement. I wonder why it was decided to set two different values on successive adjustments.

 

The second is the change in the SAI for the left wheel. The left side value seems to have changed markedly since the December adjustments. Is

this change in value unusual and is it also unusual to get a significant change on one side of the vehicle with no change at all on the other? Is

there anything obvious that could account for it?

 

There is no urgency about any of this, but I would be grateful for a response when you have the time. It was a bit of a shock to find that

there was a need for re-adjustment after well under four thousand miles of gentle driving and I really would like to understand what is happening.

 

Thanks and best wishes.

 

Excel - pdf

 

P.S. Sorry for the SAI typo at the foot of the spreadsheet . . . . .

[Tony]

What you are seeing is the fluid realm of chassis calibration and it's flaws.

 

One the first calibration the end front toe position would reflect visible wear on the tyre, the second calibration positions is a correction from the first adjustment due once again to the pattern of wear.

 

Here's the heavy part.... The suspension articulation has an effect on the wishbone pickup point bushings, additional effects can be drawn from air temperature and how the car was stopped on the ramp?, stopping with the foot brake will show different results than if the car was stopped by the handbrake, another factor is the fuel load at the time of each calibration.

 

The SAI has many duties but it's mainly used for diagnostics and because it's position has so many external influences there's not normally any target data.

 

Knowing the above and analyzing the past and present reports we would decide if there's reason to investigate further, from my experience nothing sinister appeared out of the ordinary.

 

In summary

Toyota have not offered a fuel load request at the time of the geometry calibration, other manufacturers do but it's not a consistent level that can apply to all cars so in essence the fuel parameter is taken into our consideration at the time of the calibration.

 

Bushing compliance/ condition will open a variable between measurements due to air temperature and the stopping inertia.

 

An argument could be had regarding the front toe measurements since for the toe there is no bushings?.... Well the steering arm connects to the rack via a grease packed ball and socket, then the steering arm connects to the track rod end which in tum connects to the hub via a grease packed ball and socket...... So collectively there are four knuckle joints all packed with grease and subject to wear and within all these wear and tear areas we have points of millimetres tolerance to work within.

 

In an ideal world everything will be new but it's not an ideal world and in fact our car live most their life in the realm of "worn", it's not new but it's not knackered either so on our part it's a hard call for every calibration.