I've read that many people make their 'book' chassis either longer or wider to fit their neeeds, and I was wondering if this affected the handling of the car in any way? And does the steering geometry need to be adapted for a different wheelbase?

I am no expert but I went with the assumption that it will effect front to rear roll and ackerman when I did mine. Changing the front end width would make more changes than the length of the car in my opinion . But increasing the wheelbase will take some of the responsiveness away as it will increase your turning radius and nimbleness if thats a word. I have an obsene 107ish inch wheelbase but I modled mine after the late thirties lemans racers so a regular or + 4 would definiately kick my ass on a tight course or autox. But its very unlikely my car will ever be raced asside from light to light on occasion.
Dale

I'm with Dale on this one , my car is 50 mm wider than the book and the only real difference I expect is that it will be more stable in the turns . extra length will have more of a noticable effect on the agility of the car and the amount of steering effort required for a given turn but any seven style car is going to be a whole new ball game compared to a standard car the lower gcenter of gravity is going to ensure that .
Best wishes
Iain

Track width makes no real difference from behind the wheel, but wheelbase has a bigger effect generally long wheelbase are less twitchy. With rack and pinion steering Ackerman depends almost entirely on the fore-aft locaion of the steering rack which isn't given in he book anyway and small changes in ackerman have little effect -- in any event because of tyre consideration 100% compliance to ackerman geometery is a very bad idea on anything other than a milk float.

No, but you do need some ackerman angle!!

Having said that, using typical donor bits which have come from a longish vehicle, normally the problem on a shorter car is not enough ackerman angle, a longer wheelbase would actually be slightly better!

Off topic, a fellow autograss racer was asking me whether I could set his "new" car up for him (a class 8 special) for a small fee. I said no problem, and then took a look at it in the pits............maybe not then! It had reverse ackerman and there was no way I was going to be able to get it to turn in!! I told him as much, but this was of little consequence, he crashed it into the armco on the next race and snapped most of the suspension off anyway!!!!!

I was thinking about building a +442 chassis but I want to autocross so I want to keep a narrow car.

Anyone driven a stock and a +4? I'm interested in the diffrence in handling.

My car will have a wheelbase of around 2800mm (or 110" for some) and a track width that is 160mm wider that a sierra 2wd cosworth. This is more to give more room in the cockpit, being 6'4" tall and almost the same wide I need allot of room for legs an arms .
I know the car will be heavy but I thought the most inportant thing is width to length ratio? or am i talking aload of ballcocks.
Mike

Hey mike I'm glad to see I am not the only one making a moster in size wise anyway. Are you going 4 wheel drive or what prompted the extra length- I am 6'4 and 245 lbs and going with comfortable seating I could actually use about 2 or three more inches in the leg room (going to a cushy bench seat is solving that problem- fiberglass seats would have as well except for the price) I have enough room up front to allow the rad to go infront of the engine yet behind the front suspention and hopefully still have room for the intercooler. I did leave room for a choice of engines if I decide later . I keep looking at the bay and thinking I could have built a twin turbo inline 6 and have full boost by 1500rpm.
Dale

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Originally posted by NS Dev
...a fellow autograss racer was asking me whether I could set his "new" car up for him.... It had reverse ackerman and there was no way I was going to be able to get it to turn in!!

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The truth is I haven't been following the book for size or design, I just laid my engine out on the floor of my garage and put down my diff and driveshafts and wheels and seats and then decided on where everything was going to go.
Kinda like this:
Rescued attachment PC060045p.JPG

That is incredibly clear thin tubing you are using there - I can barely see it!

Very light weight chassis that way. If going with the extended wheelbase It might be advisable to add some more trianglation in the chassis sides. I added 2 extra triangles to each side and will probably use a bolt in engine brace over top of the engine to ad some strength.
Dale

My car is standard sierra width, but measuring up I seem to have a wheelbase about 100mm shorter than a book design. This all came about fitting my front diff in.

Hopefully it should be nice and nimble, but not too much so!

Liam

quote:

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Originally posted by Rorty

quote:

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Originally posted by NS Dev
...a fellow autograss racer was asking me whether I could set his "new" car up for him.... It had reverse ackerman and there was no way I was going to be able to get it to turn in!!

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Why not?

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I agree with what you say, if a car was being driven on a loose/muddy surface as a road car would be driven. I was just curios as to NS Dev's reasoning and definition of reverse Ackerman (whether actually reverse Ackerman or modified Ackerman).
As grassers turn on overrun and steer with the throttle, it isn't really all that relevant what association the front wheels have with each other, as long as they're pointing in roughly the same direction. I've seen some truly shocking front toe and Ackerman set-ups that seemed to work perfectly well on the grass (I used to live in Yeovil, so the Sommerset and Weymouth grass clubs were very familliar to me).