Author Topic: off set (Read 25309 times)

Burns363R · « **Reply #30 on:** March 09, 2015, 09:47:52 PM »

Fred,

As i stated before. Bump steer is a result of the geometry relationship between the flag on the stem, spindles and A-arm mounts. Basically this will result in some amount of degrees of toe in or out as the arms cycle through the range of travel. If you use a 4:1 rim that places the center line of the rim as close to the king pin as possible, you have a certain amount of toe in or toe out with a given set of arms through the travel, some more than others. Now if you move the center line of your rim (say a 2:3) out side the king pin, and cycle the arms through the range of motion, the king pins will still move the amount of degrees as before, but because the center line of the rim is placed farther out, the rims will move more distance and faster, you will get more toe in or out because the center of the wheel is farther from the fulcrum of the king pin.

Your statement that offset does not affect bump steer is correct in the terms of geometry, has nothing to do with the inherent bump steer of a particular A-arm setup. But running more offset can make bad bump steer even worse. Or create more bump steer should i say because the center line of the rim is farther from the fulcrum. And will move more total inches in or out.

Also, because the rims have a greater leverage on the steering system since its farther from the fulcrum, it will give more feed back through to the bars. When you hit a rock or something with one wheel and not the other, you will feel more jerk in your handle bars. This again is due to the leverage or distance from the fulcrum.

I can draw a picture if you want.

Its funny how i can always be civil and respectful with you Fred, yet you cannot with me.

Burns363R · « **Reply #31 on:** March 09, 2015, 09:51:47 PM »

I just replied to this thread via Fred's tag.. After posting i see D Bergstrom posted what i was trying to say, and resaid just above. sorry for the repost.

fearlessfred · « **Reply #32 on:** March 10, 2015, 12:46:30 AM »

Quote from: Burns363R;51180

Off set of rims will exaggerate bump steer. Anyone that says other wise does not have a clue what they are talking about.

This is your idea of being civil. You have a lot of it correct,What your not understanding is that the position of the tierods and mounting points do not change when changing offsets and therefor the bump steer doesn't change either.Im going too do some pics or video to show you what I'm talking about.

Burns363R · « **Reply #33 on:** March 10, 2015, 01:32:44 AM »

Lol. I have said that it in 2 posts already Fred. The total degrees of spindle change do not change because of rims. But the farther you move your offset from the center pivot point the far the the rims move for every degree of bump steer.

No need for pics. I have already spent many hours figuring out bump steer. Spent alot of time talking to Andy Maul going through the reasons and solutions. I had a front end last year that gave me a lot of bump steer.

You seem to fail to read all my posts if you think that I haven't already said. That rim offset only exagerates bad bump steer. It's not a lot, but it does. I will try to draw something up tomorrow.

Burns363R · « **Reply #34 on:** March 10, 2015, 07:46:27 AM »

I just did a simple calc here. But this is assuming 10 degrees of bump steer. I also assumed that the center line for the hub face was 3" out from the king pings. In reality it would probably be more than that and have a great effect. But this is what I'm trying to demonstrate basically.

As you move the center line of the rim farther and farther from the king pins, the distance traveled by the center line of the rim increases in the same degrees of motion.

Heres what i was having problems with. I was running these 3:2 Rims, On some arms with incorrect spindles. At full extension i was getting serious bump steer. At ride height and through 80% of the travel they tracked great. Lol this is bad. Cant believe i tried to race this.

Full extension

Skeans1 · « **Reply #35 on:** March 10, 2015, 09:23:23 AM »

If caster is at between 4* to 7* with correct toe in and camber you'll never have bump steer as well as have steering that goes like butter. Too much caster in positive degrees the bars will not return and feel heavy, too little that's where bump steer is created, always have positive with center line of the ball joint tipped back towards the back of the bike.

Victork · « **Reply #36 on:** March 10, 2015, 09:52:09 AM »

I know this is a stupid question but i just want it confirmed. 4+1 is 1 inch narrower than 3+2 correct?

Burns363R · « **Reply #37 on:** March 10, 2015, 09:56:33 AM »

Quote from: Skeans1;51279

If caster is at between 4* to 7* with correct toe in and camber you'll never have bump steer as well as have steering that goes like butter. Too much caster in positive degrees the bars will not return and feel heavy, too little that's where bump steer is created, always have positive with center line of the ball joint tipped back towards the back of the bike.

That has not been my experience at all.

Skeans1 · « **Reply #38 on:** March 10, 2015, 09:59:15 AM »

Kyle could I ask what your front is set up as right?

Burns363R · « **Reply #39 on:** March 10, 2015, 10:20:31 AM »

I always set caster between 4-5 degrees. I dont like to much because i like my steering light. But even with that set that way, you can see the bump steer in the front end that resulted in mis matched parts. The relationship between the flag on the stem and the flag on the spindles just didnt jive at full extension. My case is extreme, but does demonstrate what happens.

I dont know if its possible to have zero bump steer. And i do understand that caster can defiantly effect bump steer, if you stay in your specified range of 4-7 degrees you wont see any effect.

My experience has shown that even when you are between 4-7 degrees, you can still have a lot of bump steer.

Burns363R · « **Reply #40 on:** March 10, 2015, 10:25:11 AM »

Quote from: Victork;51280

I know this is a stupid question but i just want it confirmed. 4+1 is 1 inch narrower than 3+2 correct?

No they would be a total of 2" narrower for the entire front end. You will have 1" less on each side.

Skeans1 · « **Reply #41 on:** March 10, 2015, 11:28:16 AM »

Kyle do you have a picture of the spindle tie rod out as well as the stem and tie rod mount and everything mounted?

Victork · « **Reply #42 on:** March 10, 2015, 12:34:06 PM »

I was just comparing 1 rim but i got my answer thanks

fearlessfred · « **Reply #43 on:** March 10, 2015, 07:54:24 PM »

To get 0 bumpsteer on the r ,the tierod position on the steering flag would have to way further outward and when you do that it goes over center and the wheels will lock when turned all the way.bumpsteer is at its min from the factory.

Victork · « **Reply #44 on:** March 11, 2015, 06:12:41 PM »

Ended up going 4+1 and 3+5 thanks for the input guys