alignment question
04-06-2009, 06:08 PM
#16
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Mark, trying to adjust caster by measuring from the rear wheel is, uh, not quite accurate enough. Caster is either measured (difficult at best) or calculated from camber change at specific steering angles. Use slip plates in the rear, turn plates in the front, to get the steering angles just right. Measure the camber with a spirit level and drill bits per the Captain Earl method, safe easy repeatable. There are camber gauges with curved bubble levels that make this easier if you have one.
The deal with the 1/2" difference in wheelbase is that it takes a 1" move on the bottom arm with no move on the top arm. Get out your WSM's and look at the dimensions there. I'm not local to mine or I'd quote chapter and verse to you.
The 'stretched' tie rod is a geometric result of the wheel moving backwards. The center of the wheel as well as the outer end of the tie rod are now further back and therefore closer to where the end of the rack bolts to the frame. The same length of tie rod makes the toe-in appear exagerated.
Get the car on Bill's lift, and stretch a tape measure from corner suspension points to see what has moved. Do it from the bolts for the lower control arm as well as the ball joint, to some fixed point on the opposite rear corner. A little bit of measuring and some math will help you see what really moved. Or take it to a good alignment/frame shop and get the news from them.
The deal with the 1/2" difference in wheelbase is that it takes a 1" move on the bottom arm with no move on the top arm. Get out your WSM's and look at the dimensions there. I'm not local to mine or I'd quote chapter and verse to you.
The 'stretched' tie rod is a geometric result of the wheel moving backwards. The center of the wheel as well as the outer end of the tie rod are now further back and therefore closer to where the end of the rack bolts to the frame. The same length of tie rod makes the toe-in appear exagerated.
Get the car on Bill's lift, and stretch a tape measure from corner suspension points to see what has moved. Do it from the bolts for the lower control arm as well as the ball joint, to some fixed point on the opposite rear corner. A little bit of measuring and some math will help you see what really moved. Or take it to a good alignment/frame shop and get the news from them.
04-06-2009, 06:32 PM
#17
Rennlist Member
Thread Starter
that doesnt sound right does it? 1 full turn of the tie rod and you get 2 degrees movement??? thats about .75" at the tire front. what do you have some course thread tie rods or something? 
I first did 3 half turns and it only went to about 3/8"toed in. (maybe a little less after settling) when i did 3 more, it went to zero. so i backed off .5 turns and it ended up about 1/8" toed in total. (all movement on that one tie rod) .
wheel is straight , car handles well on a mountain road test and re -checked the toe. still 1/8" toed in.
The distance of the front tire to the rear of the fender looks closer and I dont know if it was always that way. I cant imagine how the caster could be pushed back, on a toe in hit. So, I measured a crude measuring point of the rear hub centers to the front hub centers, as brought down to the ground with a mark. not real accurate, but still, 1/2" difference? Ill have to do a test again.
I have a feeling that long spindle arm is what gave up the distance. after all, if it only takes 1 turn in your test for 2 degrees of toe, thats only a very small fraction of an inch of bending. (what is the pitch of the tie rod, 8 turns/inch?) of that long spindle arm where the tie rods connect.
mk
I first did 3 half turns and it only went to about 3/8"toed in. (maybe a little less after settling) when i did 3 more, it went to zero. so i backed off .5 turns and it ended up about 1/8" toed in total. (all movement on that one tie rod) .
wheel is straight , car handles well on a mountain road test and re -checked the toe. still 1/8" toed in.
The distance of the front tire to the rear of the fender looks closer and I dont know if it was always that way. I cant imagine how the caster could be pushed back, on a toe in hit. So, I measured a crude measuring point of the rear hub centers to the front hub centers, as brought down to the ground with a mark. not real accurate, but still, 1/2" difference? Ill have to do a test again.
I have a feeling that long spindle arm is what gave up the distance. after all, if it only takes 1 turn in your test for 2 degrees of toe, thats only a very small fraction of an inch of bending. (what is the pitch of the tie rod, 8 turns/inch?) of that long spindle arm where the tie rods connect.
mk
I just recently adjusted my toe, and on my car one full turn of the tie rod (360 degrees) turned out to be approximately 2.1 degrees of toe. So it is a LOT. I used a method similar to yours, Mark, except I used a long straight piece of wood to extend out the measurement to make it more accurate than just measuring using the 18" of the wheel.
Dan
'91 928GT S/C
475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft
04-06-2009, 06:38 PM
#18
Rennlist Member
Thread Starter
Thanks. I think I understand. as the wheel moves back, the tie rod is fixed but stuff moves back and is closer to the steering rack, pushing toe in. however, I dont see how a front side hit would do that,unless it stopped the wheel as I was moving forward and levered it back. Hmmm, Ill have to see on the rack as you say.
mk
mk
Mark, trying to adjust caster by measuring from the rear wheel is, uh, not quite accurate enough. Caster is either measured (difficult at best) or calculated from camber change at specific steering angles. Use slip plates in the rear, turn plates in the front, to get the steering angles just right. Measure the camber with a spirit level and drill bits per the Captain Earl method, safe easy repeatable. There are camber gauges with curved bubble levels that make this easier if you have one.
The deal with the 1/2" difference in wheelbase is that it takes a 1" move on the bottom arm with no move on the top arm. Get out your WSM's and look at the dimensions there. I'm not local to mine or I'd quote chapter and verse to you.
The 'stretched' tie rod is a geometric result of the wheel moving backwards. The center of the wheel as well as the outer end of the tie rod are now further back and therefore closer to where the end of the rack bolts to the frame. The same length of tie rod makes the toe-in appear exagerated.
Get the car on Bill's lift, and stretch a tape measure from corner suspension points to see what has moved. Do it from the bolts for the lower control arm as well as the ball joint, to some fixed point on the opposite rear corner. A little bit of measuring and some math will help you see what really moved. Or take it to a good alignment/frame shop and get the news from them.
The deal with the 1/2" difference in wheelbase is that it takes a 1" move on the bottom arm with no move on the top arm. Get out your WSM's and look at the dimensions there. I'm not local to mine or I'd quote chapter and verse to you.
The 'stretched' tie rod is a geometric result of the wheel moving backwards. The center of the wheel as well as the outer end of the tie rod are now further back and therefore closer to where the end of the rack bolts to the frame. The same length of tie rod makes the toe-in appear exagerated.
Get the car on Bill's lift, and stretch a tape measure from corner suspension points to see what has moved. Do it from the bolts for the lower control arm as well as the ball joint, to some fixed point on the opposite rear corner. A little bit of measuring and some math will help you see what really moved. Or take it to a good alignment/frame shop and get the news from them.
04-07-2009, 01:09 AM
#19
Rennlist Member
Thread Starter
found a better way to measure the wheel distances, and looks like the difference is only about 1/16th of an inch. over the 98" distanace. Everything looks like it was only the toe that changed, but Ill put it on a rack anyway. looks like if the camber went in a little, that would force the toe in as well. If this happened, it only looks like about 1/4 degree is missing from camber if anything.
04-07-2009, 12:26 PM
#20
Nordschleife Master
Mark,
If you're worried about the wheel distances, then you'll need to have the tranny cradle adjusted during the alignment. There are slots where it attaches to the body and moves fore/aft on one side.
These cars aren't made perfectly.
If you're worried about the wheel distances, then you'll need to have the tranny cradle adjusted during the alignment. There are slots where it attaches to the body and moves fore/aft on one side.
These cars aren't made perfectly.
04-07-2009, 12:55 PM
#21
Rennlist Member
Thread Starter
Im only worried if it moved substantially, which it doesnt look like it did. the cars are not perfect, i know and with the rear of the fender closer to the tire than the other side, I thougth something had moved back. doesnt appear so now
04-07-2009, 01:00 PM
#22
Race Car
The fenders on a 928 can look weird when they are hit. After someone hit&ran my passenger front fender on my former '86 while it was parked, the headlight actually rubbed on the top of the light hole even though the fender was pushed backwards. I never could quite figure out the dynamics of the shape change.
Dan
'91 928GT S/C 475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft