Learning DIY (String) self alignment checks and fix if required

[garybee]

If you've changed the rake you've certainly changed the castor. That one is a given, the others less so.

 

Are you happy with the weight and sensitivity of the steering? If so there's no need to touch castor.

[CtrMint]

Last night I had a long chat with an old rally buddy I used to navigate for, he's quite well renowned with the string method, he used end up doing his own car and others in the service park pre-event. Having him explain the logic or simplistic approach to squaring the car now makes more sense.  You just need two equidistance parallel lines down either side which accounts for the track widths, rather than a perfect square.  I was wrong fixating on the square and the rotation of the car in that square.

I don't believe there is any difference between his method and what has been suggested here.  But it did leave me with the question about the track width of the 420R with 8" rears.  

The owners manual states the ;

• front track of a Seven Standard S3 chassis Sport and Track suspension = 1336 mm 
• Rear track of a Seven Standard S3 chassis Sport and Tack suspension = 1336 mm

Given the 8" wheel is optional, and not mentioned in the owners book, what additional width does the 8" add?  Surely you need to take into account the wheel offset etc. 

My plan is to run a line from dead center on the front hub to the rear, both with a nominal added value to ensure the line is equidistance.   Assuming the car has a wider track at the rear,  then the front will have a larger added value to compensation.  I just need to clarify what the rear track offset is?

Thanks

[Bricol]

I think a copy of this is needed:

https://www.amazon.co.uk/How-Make-your-Handle-HP46/dp/0912656468

If you haven't got it sussed after reading this book, pay someone . . .

[garybee]

Reply to #27 - Just take a few minutes to measure it.  Always better than trusting either what someone else tells you or what's in a manual.

Also to make the setup easier...don't you have 2 sevens?  That's 2 sets of front wheels.

[CtrMint]

I do, though the 620 has yet to get it's first service or the corrective fix for the cooling.  So I'll look to have it sorted then.  Getting it to Crawley looks to be a PITA though so the whole situation is up in the air the moment. 

As for the measurement I probably will measure, but for piece of mind should it be wider at the back with the 8" rims?

[garybee]

I think you've missed my point.  What I was suggesting was to swap the fronts of one on to the rear of the other to make the string method easier.

[BobM]

"Given the 8" wheel is optional, and not mentioned in the owners book, what additional width does the 8" add?  Surely you need to take into account the wheel offset etc. 

My plan is to run a line from dead center on the front hub to the rear, both with a nominal added value to ensure the line is equidistance.   Assuming the car has a wider track at the rear,  then the front will have a larger added value to compensation.  I just need to clarify what the rear track offset is?"

It really doesn't matter. All you do is make sure your string line is a true parallelogram by making sure you have the strings at exactly the same width front and rear. You might have a larger distance from string to hub at the front than the rear, but it should be equidistant on each side.

[7 wonders of the world]

Statically it seems the castor has little effect however you are alerting the radius of the upper and lower arms and there relationship to each other as they move (points of intersection) therefore this certainly does affect toe when you check for bump steer.

Dynamically there is much more marked effect as the castor impacts on the camber - increasing the castor ramps up the camber as lock is applied and this affect the toe - in both cases this alteration can be tunes to your advantage but must be checked to ensure its not going the wrong way and heading into the realms of adverse handling.

James - Indeed a huge can of worms, I just wanted to reiterate the point, that because one member has founds a 9mm shim works does not mean this is the case for all - you can really induce some quite interesting characteristics if you do, so bump steer MUST always be checked prior to making any adjustments. We've both been there and bought the 'chasing my tail' Tee shirt   

Whatever you doing always write down the adjustments made - so if it all goes pear shaped you can go back to your datum settings and start again.

 

[aerobod - near CYYC]

#27 Mark, the offset on the staggered 6" and 8" wheels is 24mm in both cases (for the Caterham 8-spoke 13" wheels I have), so the front and rear track stays equal with or without the staggered setup as it is measured to the centre of the tyre, although the track will change a small amount as the camber is changed. The position of the outer edge of the rim will be 1" closer to the string for the rear wheels compared with the fronts, though.

[CtrMint]

Thanks James, knowing the track stays the same is a big help.  

[7 wonders of the world]

Mark,

It doesn't stay the same though, thats the whole point of setting the car with ballast in order order that all checks and adjustments are carried out as the car would be presented whilst driving.