hi there after a new set of strut top mounts the car is getting its geometery done tomorrow. the only thing is the bump steer setting i had done are in degrees and the garage i am using uses mm. any ideas of how to do the maths ?

ie what is -1.34' degree equal to in mm ?

help will be appreciated

 

John D

...err, isn't that just what Boost has said, but he did the maths as well?

...but Boost
...R can be in any units you want, as long as you measure the same unit compared with the unit that R is in
...its a bit of a simplification (I think!)- you've calculated the proportion of the circumference that is subtended by 1.567 deg. You probably want to measure the 'horizontal displacement' e.g. if angle is 90 degrees, your calc gives 0.5*pi*R (=1.57*R), but the 'horizontal displacement' is just R. But for small angles there's not much difference I suppose.

...and anyway, I'm not sure (and am more than happy to be corrrected) that trying to set up camber, castor etc angles by measuring distances instead of angles is actually practical, cos you'll never know where the centre of pivot is to any accuracy - except maybe toe-in, by setting up a wheel parallel to the centreline of the car (0 degrees toe in), the adjusting the tracking so that the 9 o'clock postion of the rim is Xmm (232 thou for a 17" wheel?) 'inside' the 3 o'clock position

Muppet maths anyone???

 

You cannot equate degrees directly with linear measurement e.g. m.m. With an angle the further away you go the wider it gets! If you ask what will be the size in m.m. when an angle of 1 degree 34 minutes is measured at, say 250 m.m. from the apex then this is simple trigonometry. Easy if you know how, a bugger if you dont! The bottom line here is - more information required!
JohnD

 

Sorry I don't really know much about suspension geometry just testing my distant memories after a few beers.

-1.34' = 1 degree 34 minutes = -1.567 degrees

circumference of a circle = 2(pi)R = 360 degrees

-1.567 degrees = -0.00435*2(pi)R

-1.34' degrees = -0.0273 R

Where R is radius in mm of the part whose movement you are measuring in mm from the centre of the circle about which it pivots.

By the way, I'vej ust remembered Radians - wot the Hell were they as opposed to degrees??

This may well be proven to be total toss.

 

thanks guys it is for the 16" standard subaru wheels. most companies that i have dealt with use mm instead of degrees now, however when i went down to brands last year i took a D tour and got the bump steer done at PS. i can't go to power station as they are 5hrs away.
i will try couple of hte formula's provides by boost and martin and see which one looks more right. but does that mean i have to meassure the distance from the centre of the wheel to the edge of the tyre ? or is there another formula for that.


cheers
sam

 

Sam

I'm getting out of my depth here - I can do the maths, but have no grasp on the reality of degrees to mm with respect to bumpsteer

I thought that bumpsteer was the packing of the steering rack on the crossmember (or wherever) so that there was no/little change in suspension settings (toe-in?) during suspension travel. As such, there wouldn't be a measurement at the wheel that would re-instate the bumpsteer settings??

But I'm getting into a big hole on this, so I'm gonna have to withdraw (oo-er) - sounds like you need to ring PowerStation

Martin

 

Not sure what the mm measurement relates to in relation to the angle, but so long as you know the radius it should be easy enough to work out.

Radians are a different way of measuring angles.
There are 2PI radians in a circle and it came about as simply being more convenient as the circumference is 2(PI)Radius.
Works quite well for integration and the like.

Ah, a maths degree, something which has turned out to have no use whatsoever in my life.

Mark - Elise/306XSi/MGBGT/(Impreza on the way and it's all BoostII's fault)



[This message has been edited by BT52b (edited 04 May 2001).]

 

Sam,
I think it will be impossible to calculate without being a suspension engineer. I don't think the radius of the wheel matters - this circle with radius R is imaginary - probably related to length of struts and stuff.

Mark/BT52b - didn't fancy BT52-II then?, I suppose I could have been Boost b. I must speak to you sometime! (John)