Flow 1 Report post Posted January 18, 2012 Hello folks. Does any one have the measurements and method to calculate how much to change the wheel offset when increasing rolling circumference? I'm a bit new to the concept my self. Can some one tell me where to find the scrub radius specs for the 540iM-Sport with the standard wheels and tires? Many thanks Jason Quote Share this post Link to post Share on other sites
Palazzo 477 Report post Posted January 18, 2012 Google wheel offset calculator, the Miata forum one is good. Quote Share this post Link to post Share on other sites
Flow 1 Report post Posted January 18, 2012 Thanks Scott. I figured it out. Simple really. BWT I am talking about plus sizing diametrically. Sorry I should have been more specific. Correct me if I am wrong but you add tan (included angle) * radius increase to the offset in order to keep the same scrub radius. So if I ended up with tires that had a 8mm larger radius and the included angle is 13.5 degrees then I would need to change the offset from the standard 20mm ET with my 17/8 style 66 wheels to 22mm ET in order to keep the same scrub radius. For example the BBS RGR 18/8.5 ET22mm. Where do we find the scrub radiuses and included angles for BMW's? Quote Share this post Link to post Share on other sites
thorburn 121 Report post Posted January 18, 2012 (edited) I'm not following your math yet. Here is a picture because threads are better with pictures. Can you give more info on what your rolling radius was and what it is going to Going from this http://www.bmwdiy.info/alignment/index.html: steering axis inclination is: 14.8º camber is: -1.1º to -1.5º Assuming this is for your e39 Edited January 18, 2012 by bbs Quote Share this post Link to post Share on other sites
Flow 1 Report post Posted January 19, 2012 (edited) Hi James That link is where I got the included angle from. I noticed it is for a non Sport package. It would be great to find a resource that has scrub radiuses for different models and wheel combinations. This is a link that explains some of the effect of positive zero and negative scrub radiuses. If any one is interested... http://www.clubprotege.com/forum/showthrea...on-Scrub-Radius The fluid filled steering box cars can be a little vague in comparison to a rack and pinion car. It would be nice to try and get a bit more feel and response with out making it wandery. As you can see from that very good image you posted the more the diameter of the tire increases the smaller positive scrub raduises become. So there is a chance that if I plus size I will do the opposite of above. I have attached an excel spreadsheet that a guy made for an AE86. Any excel gurus want to modify it for our cars? We would need the hub face to steering axis and included angle measurements for the cars first though.... Still looking. ae86scrubradius.zip Edited January 19, 2012 by Flow Quote Share this post Link to post Share on other sites
thorburn 121 Report post Posted January 19, 2012 (edited) Might need to get under your car and start measuring. I can't find anything on stock scrub radius' for e36 or e30's so I think it's going to be hard to find anything for an e39 8mm more in tyre diameter isn't going to be all that noticeable anyway. Edited January 19, 2012 by bbs Quote Share this post Link to post Share on other sites
