Design - Starting point decisions

[dre22]

I am designing a buggy in my spare time, not sure I will ever build it, but it is a good mind exercise for me.

When looking at designing a buggy there are a few key decisions that have to be made early and I am trying to get my head around them all.

The first group of decisions I see as required are the following
1. Class vehicle is going to run.
2. Wheel base
3. Track
4. Wheel sizes (height and width)
5. Ride Height
6. Suspension type
7. Suspension travel required
8. Drive train

About the only thing I have decided is I want to design it around class2 in Australia which is 1650cc engine and 2 people. What do I have to consider when working out the other answers.

The vehicle is going to run a fair variety of tracks, some tight and twisty, some more open and fast so the wheelbase will have to be a compromise and it can't be too wide or it won't fit between the trees on some tracks.

What factors will affect the wheel size, what are the advantages of a smaller or bigger wheel. Wider will give more traction but can tht sometimes be a bad thing as the engine will have limited torque.

Most people seem to run Toyota 4 AGE 20 Valve engines in that class so I would use that as a starting point, gearbox decision is still in the air. Kombi box seems to be the general option there.

What are the pro's cons of A arm vs trailing arm suspension?

Any help in working out design parameters would be greatly appreciated. sorry this post is a bit all over the place...working out a starting point is proving to be fairly difficult

[Shred918]

I would say you first need to think about if you are going to race it or possibly might it the future.

Once you figure that out if you decided to go with a certain class most of the decisions are already made for you.

[dre22]

It will be designed to race. The class would be class 2 in Australia, not sure what that is equivalent to in the US, is there a listing of the US class structure anywhere?

[Shred918]

This is a rough class description:

http://www.desertrides.com/reference/racing_classes.php

I would get a rule book for who you plan on racing with for sure. Maybe start going to the races and meeting some people and asking questions. Learn as much as you can and maybe start pitting with some of the teams. There is always work to be done out there.

[BRINGTHERUCKUS]

this thread needs to keep going could be a good one!

[redmist]

First question you need to ask where you want to race? Your car will be dependant on the conditions in which you race. Do do you want to be compeditive? Then think on budget in order to achieve it. The less compeditive classes will cost SUBSTANCIALLY less to compete in as you won't need to by the latest and best components to keep up. Buying a second hand car will also cost you considerably less than building.

Your class 2 has some similarities with US class 10 or "pro buggy" (althought US have different engine restrictions based on number of seats, air cooled and Fuel Injection). Class 2 also equates to NZ class 3, although we have no restrictions in regards to number of seats.

1. Class vehicle is going to run. Budget dependant. Class 2 or 3 is probably where you want to head.
2. Wheel base. Dependant on racing envinronment between 95-110 inches.
3. Track. My Jimco is 2080 wide at both ends. Probably a little wide for a smaller class 2 or 3 car but works well in Class 1.
4. Wheel sizes (height and width). A wheel is a combination of tyre and rim. I guess you mean tyre? Fully dependant on track, class, wheel travel, surface. But for lower powered classes you want as little rotational inertia as possible. Small and light. Possibly look at rally tyres for short course and small (31inch) AT's for enduros.
5. Ride Height, dependant on travel, tyres (dont forget they act as suspension) and how much impact you want your body to take. Lower is better for CG, but limits bump travel.
6. Suspension type. Front A-Arm, rear trailing arm. If you have the money.
7. Suspension travel required. For what event?
8. Drive train. Forget the bus box, at the very least go the modified Renault. Better still a mendeola or possibly the small Albins.

What are the pro's cons of A arm vs trailing arm suspension?
A-Arm gives you substancially more front travel and can be designed to give better geometry through suspension travel. It does cost more and requires more maintenance.

In my opinion you dont want to build. Its very very costly and unless you are dedicated you won't complete the project. It's stupidly time consuming, you simply won't believe the amount of hours that goes into designing and building frame and tabing it up. And once you are completed, you won't like it anyway. You'll want to change it, alter it, or discard it because it didnt work as you had hoped or now you are racing your design isn't competidive. Buy a completed, semi compeditive car and make it compeditive. Once you have found out what works, and what doesnt on that car then go about designing your own. My 2.34c.

[dre22]

thanks for the in depth reply redmist.

Like I said This is mainly a mind exercise for me...may not ever get built. That said I work for an engineering firm so getting help to build it may be a bit easier than it is for most. Definately want to buy a car first though, I enjoy playing with design though.

Wheel base will go for 100 inches. Is the 2080 measurement to the outside of the tires, or centreline?

What about A Arm suspension at the rear, there are a couple of buggies over here running that now. Seems like it could be a good idea.

[blurredvision1]

I think that the 2080 is the track not the wheelbase, and this would be measured from the outside of the tyre.
A Arm rear could work, but would be better utilized if in a mid engined layout. This would be so that the wheels are as close to the corners of the car as possible without the expense of loosing travel due odd angles of the CVs.
The other option could be running a 5link setup which gets the nice camber progression of an A Arm setup, with the rear located aspects of a traditional semi-trailing arm layout.
I think the main idea with a class2 (class10) is that keeping it light is important.

[redmist]

Again it depends on the course you compete on. Short course I'd go with 5 link. The old (70's design) Chenowth Magnum 5 link is still a surprisingly compeditive car given its age.
For desert racing though you can't beat trailing arm for the rear end. A-arm tends to be heavier and not able to take the knocks like trailing arm. A-Arm gives better contact patch, so long as you can design the chassis mount points around the rear end componentry without too much detrement to the geometry. But, as with the front, it will take more maintenance than traditional trailing arm.

[xterra]

I've been exercising my mind as you are doing, although I am really starting to build my car next month, and specs are almost the same as yours.
I am moving from a beam car to an a-arm car, did a lot of research and all the builders and drivers I asked said a-arm is far better set up. wheel base is going to be 110 cos most of the tracks I race in are twisty and narrow. track width 84" (2130) and I would like to have a little bit narrower, but a-arm set up doesn't leave me more choice. Tires are 33 yokohamas, because it is said they are the lightest. I have a 091 bus transmission "kombi" that I build up, I would like to go with a mendeola 5 speed, but it is out of my budget.
chassis is a probuggy, little longer of course, of the ones that have the stronger a-arm set up
plan to spend 20K since I have most of the parts

[rharriman]

Ok, Lets say I have answered all the questions above and know what basic dimensions I want and have a blank paper in front of me so to speak. What points and dimensions need to be calculated to make all the suspension and steering work optimally?

Are things such as Ackerman, camber, caster calculated or estimated using jigs or are basic rules of thumb used. From all the things I have read these all play havoc on bump steer and toe in if not done properly. I have found very view references for these for long travel applications such as TT and sand rails. Any suggestions would be good and appreciated. I have orderd some books which I expect will give me some guidance but will fall WAY short in specifics for long travel applications.


Thanks

[Shred918]

Ok, Lets say I have answered all the questions above and know what basic dimensions I want and have a blank paper in front of me so to speak. What points and dimensions need to be calculated to make all the suspension and steering work optimally?

Are things such as Ackerman, camber, caster calculated or estimated using jigs or are basic rules of thumb used. From all the things I have read these all play havoc on bump steer and toe in if not done properly. I have found very view references for these for long travel applications such as TT and sand rails. Any suggestions would be good and appreciated. I have orderd some books which I expect will give me some guidance but will fall WAY short in specifics for long travel applications.


Thanks

Honestly if you cant answer those questions on your own you may not want to tackle building a chassis from scratch. There are a lot of thread in these forums discussing those issues if you want to do the leg work.

[rharriman]

I have some help as I am learning but was more curious to find out if anyone had a written reference or basic guidelines to follow. Thanks for your honesty and will read up and dig through some older threads.

[redmist]

For a start in some basic concepts I'd read
"race and rally car source" by Allan Staniforth. The basic precepts are applicable to offroading and the string computer may assist you in offroad use.
I've attempted to use some of the A-Arm geometry programs, however they tend to "out of bounds" for the length of travel we commonly use.