Suspension Geometry ?'s

[mgriz]

Hi, Im building a small off road car and I am using the front suspension from a 250R. I would like to get the geometry correct but I dont have a 250R frame to measure. Would someone who has a frame handy be able to measure some key points for me. Basically I need to know how the top and bottom a-arms are mounted on the frame relative to each other. I have atatched an image of the measurements I need, I hope it is clear enough. I understand that this might be a pain so please dont get these measurements unless its easy for you, I would really aprechiate it. Thanks in advance!

[zedicus00]

man yur gunna hafta explain that better i dont get it... width of a arms an stuff like that or what?

[mgriz]

Originally posted by zedicus00
man yur gunna hafta explain that better i dont get it... width of a arms an stuff like that or what?

Haha, I thought I might...

No i have the arms and knuckle so i know widths and all that. The only thing I dont know is how they are spaced when mounted on the frame. So dimension a is the vertical distance between the front mounts of the top and bottom arms, b is the vertical distance between the rear mounts of the top and bottom arms. These dimension will tell me if the arms are parallel or not. Dimensions a and b are the same on many atvs, cars, etc but it looks to me as if on the 250r the top a arm is mounted at a slightly larger angle than the bottom a arm. c is the horizontal difference in the axis of rotation of the top and bottom arms.

[zedicus00]

cool. i think i understand. i will hafta get answers for u tomorow though... i dont think i could make it up the stairs if i tried right now...

[wilkin250r]

How accurate do you want the measurements? 1/16 inch, 1/32 inch?

[mgriz]

Thanks, I aprechiate it. Take your time, Im not in a rush.

[wilkin250r]

I took some preliminary measurements, and it's not quite as simple as you would think. I got different measurements for the front and back of each bolt, which doesn't make any sense at all.

I'm usually willing to help somebody out, but quite frankly, it's 11:30 PM, dark (obviously) and damn freezing out. I'm sorry, I just don't have the patience to sort it out.

[wilkin250r]

However, I WILL tell you that, as you have it shown, your measurement for "C" will not work, as it would require somebody to determine an exact perpendicular from the frame.

I would suggest you measure the total distance between each left and right A-arm mount. You don't need to KEEP it this distance, but these measurements will allow you to calculate your measurement for the rotational difference.

Viewed as if from the top:

[wilkin250r]

Also, I beleive that one of the things that made the 250r truly revolutionary in high speed handling was the arms were actually rotated backwards, such that the motion of the wheel as the shock compressed wasn't straight upwards, but rather upwards and a little backwards.

So along with your previous measurements, you also want a measurement of the different heights the front and rear mounts are.

The red lines would show perfect horizontal and vertical. The blue line shows the actual linear motion of the wheel as the shock compresses. As you can see, the wheel doesn't travel straight up vertically, but rather upwards and backwards.

[mgriz]

Wilkin, thanks for your time and replies. I am aware that it is a little more complicated then my drawings would indicate.

You are correct about the dimension c, the method you posted is probably the easiest way to get an accurate measurement of the difference of the rotational axis. If someone were to get those two measurements I would be able to get what I need.

Also I am aware of the rotated a arms, infact I have already built this part of my frame. So to simplify things we can think of the rotational axis of the lower a arm as parallel with the ground. The front mounts of both the lower and upper arms should be lined up almost vertically... now that I think about it, if someone could just measure the straight line distance between the front upper mount and the front lower mount then the same with the rear upper and lower, I would be have everything I needed. You could do this by measuring center to center distance of the bolt heads that hold a arms on. They dont have to be exact measurements just +/- 1/4" or so. And dont worry if everything isnt as simple as the drawings I understand the geometry more than it comes accross in my posts. I've atached another poor mspaint file...

I dont know how much of that made sence but its 5am and Ive got to get to sleep. Again, Wilkin thanks for your response and time.

[bradley300]

what kind of racing is this? is it a flat track or an offroad racer that needs lots of travel? for a smooth track racing where susp. travel isnt an issue as much as boddy rol, the 250r frame might not be the best. for a real smooth turning track, the want the left and right a-arms mounted as far away as possible with a real short a-arm, thus reducing the wheels availiable travel. (thanks to the short range of motion from the short a-arms) wich will help body roll. for instance, say you want a 46 inch front end and a stock 250r is 44. for flat racing, make your frame 2 inches wider in the front and use a 300ex length a-arm (wich is an inch narrower or so). for a rough track where wheel travel is a must, you would want the frame maybe, 2 inches narrower than a stock 250r, with plus 2 a-arms. the two set ups are the same width, but each woud do better than the other in its element. the 250 a-arms do have about 25-30 degrees of rake, pretty sure thats about right. the rake, combined with the shocks being tilted back at the tops is what makes a 250r (or any honda sport quad for that matter) ride so good.

hope i have helped make any sence at all, i know what i mean in my head, just not sure if i got it typed,lol

[bradley300]

BTW, especialy for flat track type stuff, you might want more rake on the a-arms. why? when the wheels go thru there motion, the wheelbase will get shorter because the wheels are moving up as well as back. so, under heavy braking the wheels will start going thru thier motion causing the wheelbase to get shorter, thus quicker turning

[beerock]

Originally posted by wilkin250r
Also, I beleive that one of the things that made the 250r truly revolutionary in high speed handling was the arms were actually rotated backwards, such that the motion of the wheel as the shock compressed wasn't straight upwards, but rather upwards and a little backwards.

So along with your previous measurements, you also want a measurement of the different heights the front and rear mounts are.

The red lines would show perfect horizontal and vertical. The blue line shows the actual linear motion of the wheel as the shock compresses. As you can see, the wheel doesn't travel straight up vertically, but rather upwards and backwards.

yea thats from the rake on the frame itself.

[mgriz]

Originally posted by bradley300
what kind of racing is this? is it a flat track or an offroad racer that needs lots of travel? for a smooth track racing where susp. travel isnt an issue as much as boddy rol, the 250r frame might not be the best. for a real smooth turning track, the want the left and right a-arms mounted as far away as possible with a real short a-arm, thus reducing the wheels availiable travel. (thanks to the short range of motion from the short a-arms) wich will help body roll. for instance, say you want a 46 inch front end and a stock 250r is 44. for flat racing, make your frame 2 inches wider in the front and use a 300ex length a-arm (wich is an inch narrower or so). for a rough track where wheel travel is a must, you would want the frame maybe, 2 inches narrower than a stock 250r, with plus 2 a-arms. the two set ups are the same width, but each woud do better than the other in its element. the 250 a-arms do have about 25-30 degrees of rake, pretty sure thats about right. the rake, combined with the shocks being tilted back at the tops is what makes a 250r (or any honda sport quad for that matter) ride so good.

hope i have helped make any sence at all, i know what i mean in my head, just not sure if i got it typed,lol

Yeah, I know what you mean. All valid points, I need a lot of wheel travel as it will be raced off road. Im planning on getting +3" a arms which will put the front of my car just shy of the max allowable width. I was planning on using 450r hardware for the front suspension but the 250r setup became available to me so I went with it. The 450r would have been simpler for two reasons, I have access to one to make measurements and the a arms are mounted parallel to each other unlike the 250r. Which by the way am I correct in my drawings that the top a arm has more rake than the bottom? These dimensions are crucial because they will end up controlling the behavior of the wheel during suspension compresion. If the spacing on the car is too great I will gain posotive camber in compression if it is too small I will gain negative camber (a little of this is wanted). I could just wing it and go with my own spacing based on the knuckle dimensions but I figured the Honda engineers knew more than I and that thier geometry probably works very well. Thanks for everyones replies.

[beerock]

heres what you do, make a jig to pin point the r front end points, then extend the jig to the width you would want.


make the jig something like this

////..............//// -top arm
//................//
//................//
//................//
//................//
///................/// -bottom arm
// ................//
//................// -
//////////////// -extend it here, it will keep the geometry you want but widen the frame for less body roll on pavement.

depending on how much rake you put into the frame will alter the dynamics.

[mgriz]

Yes I plan on jigging up the geometry, the only problem is I dont know what the geometry is. The three measurements would give me enough information to get the geometry close enough.