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Old 07-23-2009, 08:15 PM
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IFS design advice needed

Hey guys,

I'm CAD drawing a IFS for a sandrail buggy I'm planning building

rigth now I have pretty much all covered but I have some doubts about correct geometry of the steering mechanism.

it uses a shortened mechanical rack and pinion from a Geo Metro

I'm worried about getting bump steer or some other maladies

the double A arms wich are of the SLA (short upper, long lower a-arms) type are quite long and will have 14 inches of travel.

main concerns:

should the tie rod be parallel (horizontal wise) to the lower A arm?

should the tie rod be parallel (vertical wise) to the lower A arm or perpendicular to the centerline of the chassis?

should the tie rod be of the same length as the lower A arm?

is there any relationship that determines the length of the spindle's steering arm?

Please add all the comments you may think could help me.

thanks.

Augusto.

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Old 07-24-2009, 07:08 PM
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Augusto,
I think the best way for you to approach this project is to utilize some modeling software that will let you plug in the dimensions so you can uncover potential problems with the geometry. While there are some basic parameters to shoot for, without modeling your setup either with a computer program or with paper mockups, anything you try will be an educated guess at best in regards to camber gain, bumpsteer, roll center control etc.

Do a web search for suspension modeling software. There are several programs out there that are fairly straightforward for calculating basic suspension functions.

Andy
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Old 07-24-2009, 07:16 PM
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Steve Smith autosports has the books and software for doing that...

Sam
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Old 07-25-2009, 07:01 PM
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I have gathered lots of info about bump steer and stuff but I have this doubt still,

how many degrees of KPI (king pin inclination) is usually built into the steering geometry?

and how much is usually used for scrub radius?

I'm looking for a starting point to work my simulations from.

I have some experience with racing go karts front end geometry but since they don't use suspension they use totally different angles, most are quite exagerated if compared to cars, KPI is usually 12 to 15 degrees and scrub radius is changed moving the wheels in or out on the spindles.

thanks

Augusto.
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Old 07-26-2009, 08:54 PM
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5-7 degrees is fairly common for KPI. Less if you are using wide tires. As for scrub radius, I would shoot for 0 scrub, but 1/2" or so either way up to about 1" positive would be a good place to start.

Andy
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Old 07-27-2009, 11:17 AM
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We did my son's sand buggy entirely on Solidworks and I did some additional work with Cosmos on the stress analysis side.

The 4 wheel independent suspension works great. We spent a lot of time getting it to work especially since some of the adapted 4 wheeler stuff did not fit the movements correctly. It tool a lot of measuring and some simple fixturing to get it to work but that it does. We can go over 110 mph down a logging road 95 across a plowed corn field or just tool around on the 4 wheeler trails. The coil over shocks are the key to keeping control over the suspension. Spend $$$ here

Solid Works is great for this. Lots of R& D time but you only have to cut once and throw away none.

Last edited by bentwings; 08-01-2009 at 01:01 AM.
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Old 07-31-2009, 11:52 PM
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most IFS pictures I've seen so far and measured (using autocad) show an average of 15 degrees of KPI... seems like a lot to me but why do they use that much angle?
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Old 08-02-2009, 06:16 PM
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I have to believe that running that much KPI on a RWD car is mostly due to a compromise to get the scrub radius to a minimum when utilizing small diameter (relatively speaking) wheels. With wide and large dia wheels, I believe you would be wise to keep the KPI to at least less than 8 degrees if possible. If so you will reduce the tendency of the outside wheel to go to positive camber while cornering. If you can take the KPI to or near 0, when you dial in positive caster you might be able to achieve increased negative camber on the outside wheel when cornering.

If you haven't got it already, get the book Race Car Vehicle Dynamics by Milliken and Milliken. It will help you to answer a lot of your questions.

Andy
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Old 08-02-2009, 08:19 PM
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dune buggies use rather large front tires, 15 in rim usually, you are correct about the tire going to positive camber when the suspension is compressed, in my simulations with 15 KPI the tire goes to +9 of camber, totally useless for good handling.

if I go to zero KPI the same suspension goes to -3 of camber with the same 12 inch compression, much better.

I wonder why all the pics of buggy suspensions I've seen so far have such huge KPI.

Augusto
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Old 08-02-2009, 08:56 PM
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I dunno if it is a help or a hindrance but cleaning out some papers today and I found an old set of plans for an IMCA car. For a dirt setup the specs in it call for +1.5 degrees in the left front and 3.0 degress in right front...Note: that is a circle track setup so on your buggy I would think the 1.5 degree of castor on both sides would work..Pavement setups run even less castor on the order of 0.25 to 1 degree of castor..Those cars use a modified GM front chassis section..

Sam
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Old 08-03-2009, 09:04 AM
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Sam,

definitelly circle trac numbers are very different, of course for LTO the numbers vary from side to side, see in kart racing and in my off-road karts I run +10 degrees of caster on both sides and +12deg of KPI, we have adjustable scrub radius though, we use shims to move the tires in and out in the spindles varying the front track, caster is not adjustable but it should.. maybe?

seems like the dune buggy suspension is quiete unique and use some strange geometry settings, I can't find any specific info about them, just pictures which I copy and paste over a CAD sheet and scale them accordingly to extract dimensions.

thanks a lot for helping, to you and the other guys.

if someone has experience on this matter please help me.

Augusto.
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Old 08-03-2009, 10:01 AM
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Augusto,
Found another set of plans for a dune buggy in my collection. Now this guy is showing a 58.5 inch track width and has 13 inches of suspension travel before the frame bottoms out. his alignment specs are 2 degress of castor and 1-2 degress negative camber and 1/16 inch toe-in..looks like the front frame stub which attachs the lower a-arms is only 7 inches wide..

Just a bit more info..

Sam
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Old 08-03-2009, 11:16 AM
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Check these guys out.
http://www.badlandbuggy.com/index.html
http://www.ragemotorsport.com/index.asp

My son's is very similar to the rage one. We used ATV front spindles. To me the scrub radius is all wrong but there is very little camber change thru the full travel. There is quite a bit of negative camber as you turn. Inotherwords looking at the outside wheel there is negative camber with matching positive on the inside wheel. I don't know about sprint cars but other dirt track guys seem to like this. As long as camber doesn't change much going straight ahead. We no bump steer at all. It took a lot of Solid works to get this. I can't imagine how much time and material we would have tossed had it not been for SW and Cosmos..

We have about 10 of suspension angle...first look might call it anti dive but that's not really correct. The 10 degree just allows the suspension to give at high speed as you hit things. This kart is easily capable of going over 100 mph across a plowed corn field...we've done it. To say the wheels move up and down is an understatement. haha The little shock reservoirs get mighty hot to the touch after a blast like this..

The rear a-arms are plain parallel and equal length. Work great with Porche CV joints. We made our own axle stubs after destroying several hot tip items. They are big.
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Old 08-03-2009, 07:39 PM
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Quote:
Originally Posted by Augusto
dune buggies use rather large front tires, 15 in rim usually, you are correct about the tire going to positive camber when the suspension is compressed, in my simulations with 15� KPI the tire goes to +9� of camber, totally useless for good handling.

if I go to zero KPI the same suspension goes to -3� of camber with the same 12 inch compression, much better.

I wonder why all the pics of buggy suspensions I've seen so far have such huge KPI.

Augusto
What are you using for a caster setting for your simulation?

Keep in mind that dune buggies are not particularly sensitive to minor changes in camber because of the terrain (sand or loose dirt) that they typically operate in. I would be more interested in controlling camber and toe change in bump and droop over the large range of suspension movement than a more minor amount while turning.

It is doubtful if you could achieve 0 degrees KPI without some large negative offset wheels. I would be concerned with wheel bearing loads with the large wheel offset that would be required.

Some of the other comments were regarding caster settings. KPI and caster are related, but are not the same. They need to be looked at together, but treated separately.

Andy
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Old 08-03-2009, 08:33 PM
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I'm using 10 of caster, same as used in go-karts

I'm mostly concerned on the camber variations through the suspension travel and body roll, also bump steer which has given me headaches on road cars before.

the rage buggie site has exactly what I have in mind, the dual seater looks just how the thing that buzzes inside my brain, I definitelly want one like that.

once I get most of the geometry problems sorted out in CAD I'll build a small scaled down model of the IFS to see how it actually performs, I don't want any surprises once the thing is built.

I need all the help you guys can give and deeply appreciate it.

Augusto.
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