Scrub

[dancin stu]

Hello all, ive been readin this forum for a while and have now decided to register as we are about to start prelimanary designs for our 2004 car, so expect a LOT of questions mostly suspesnsion orientated..... you have been warned!!!

anyway, for starters, ive just finished writing a very simple supension geometry package in excel, 2D, fulll bump and droop analysis, and a sort of roll analysis, assumes no scrub in roll, but having done some research ive been told this doesnt amount to that much, and the figures im getting arent to far out, and its really just to get some ideas before heading back to uni and some proper analysis software. and what i would like to know is, what kind of scrub values do people accept in their final geometries. ideally we would all like zero scrub at all times, but ive got a really nice initial set of points with regards roll and bump, but the down side is the total scrub is about 6mm on 2inches bump, this is with a 1400mm track. i know from speaking to road car designers, they look for no scrub, cos it overheats the tyres, but what about race cars, anyone care to offer any info?

[dancin stu]

Hello all, ive been readin this forum for a while and have now decided to register as we are about to start prelimanary designs for our 2004 car, so expect a LOT of questions mostly suspesnsion orientated..... you have been warned!!!

anyway, for starters, ive just finished writing a very simple supension geometry package in excel, 2D, fulll bump and droop analysis, and a sort of roll analysis, assumes no scrub in roll, but having done some research ive been told this doesnt amount to that much, and the figures im getting arent to far out, and its really just to get some ideas before heading back to uni and some proper analysis software. and what i would like to know is, what kind of scrub values do people accept in their final geometries. ideally we would all like zero scrub at all times, but ive got a really nice initial set of points with regards roll and bump, but the down side is the total scrub is about 6mm on 2inches bump, this is with a 1400mm track. i know from speaking to road car designers, they look for no scrub, cos it overheats the tyres, but what about race cars, anyone care to offer any info?

[ben]

First of all I would say a track of 1400mm is way too wide.

Secondly I think the problem with these cars is getting enough heat into the tyres rather than overheating them, so I wouldn't worry about that.

Finally, are you looking at different design targets in terms of bump vs. roll camber, scrub, etc between the front and rear suspensions, I would suggest you should be.

Ben

www.ubracing.co.uk (http://www.ubracing.co.uk)

[dancin stu]

1400mm too wide?? blimey, we ran just slightly over that on our class 1-200 car last year, what figure should i be looking closer too?

in terms of suspension design, the targets that are being aimed for are:

-roll center movement kept as realistically to zero as possible, am considering 1mm movement about a maximum.

-outer wheel vertical in roll. aim to achieve this with a fair bit of caster rather than through the geometry itelf, and run zero KPI to avoid any negative effects that lots of KPI can cause.

-maybe a bit of camber change in bump, havnt got any slip circles yet for tires, first thing on the list obviously but proving quite hard to come by, so may incorporate a small amount depends on things such as weight transfer etc

-rear geometry more inclined towards keeping the wheels upright in bump at all times to enable the power to be used to full effect coming out of the corners. havnt really invetsigated much about rear wheels in roll yet.

-thinking about the scrub change, we had the same problem getting heat into the tyres last year, we were running michelin formula renault fronts cos we got them 2nd hand from a team, think about £20 a tyre with perhaps 15 minutes of running on them. we were never able to get hardly any heat into them, even on a really hot day with very low pressures, we were trying everything...... anyway speaking to the engineers at work they try to avoid scrub to prevent the tyres overheating and graining, however with formula student this could be quite beneficial........http://fsae.com/groupee_common/emoticons/icon_wink.gif

-probably gonna keep the roll angles to 1g per degree, although that is what track testing is for......

-and a lot of work on dampers, not gonna run mountain bike shocks again, ours were too inconsistent last year, Fox Vanilla R's, we didnt have enough money to dyno them and you could notice the inconsistencies from one to the other, and later finding out that the manufacturing tolerences wernt the greatest.... plus the rear ones, because of their location behind the rad, got very hot and on one the rebound adjuster locked up, so we were stuck with them on the settings we had for the acceleration event, not good for a wet enduro..........

[dancin stu]

woops! yeah 1400 is a bit toooo big, probably nearer 1200

[ben]

I would suggest 1200mm is closer to the mark yes.

We're running 1280 front and 1200 rear this year and may go narrower than that next year following the comments of a British GT driver who tested our car, and the realisation that tyres designed for a 5-600kg car aren't desperately load sensitive on a 300kg car.

I would agree with your idea about getting camber compensation at the front from caster.

I would suggest that at the rear you want to run the wheels fairly upright (i.e. minimal negative camber) and get a lot of camber compensation in roll (at the expense of bump) to keep them upright in a corner. Maybe some anti-squat to minimise the camber change coming off the corner?

Ben

[dancin stu]

cheers for the advice on anti squat, havnt really investigated much into anti dive/squat cos i dont have the software available at the minute to model the effects of it on roll and bump and droop, my program only models front view 2d layouts, but will have access to such tools when i go back to uni.

[dancin stu]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ben:

We're running 1280 front and 1200 rear this year and may go narrower than that next year following the comments of a British GT driver who tested our car, and the realisation that tyres designed for a 5-600kg car aren't desperately load sensitive on a 300kg car.


Ben<HR></BLOCKQUOTE>

we had the same thing last year, we couldnt get any heat in the tyres, see above. the path we ended up taken was to massively pile on the front roll resistence to increase the weight transferred to the outer front, but had to stop when the rockers, their mountings and finally the dampers themselves(!, thats one of the reasons i dont intend on using them next year) started bending, realised much to late to effect a solution. doing this did increase our lap times a lot but ultimately it was a fairly crude solution, but thats what last year was for, to learn how to do it properly and be more prepared for the final year