Caterham Chassis Limitations
Discussion
analog_me said:
I have the watts linkage so i think rake is less important
The watts linkage has very little to do with how rake affects the car. The main geometry that is affected by rake is the A frame. The combination of tyre size and angle of the A frame to horizontal determines the squat/anti-squat behaviour of the car. This usually means adjusting ride height to maintain the same behaviour after a tyre size change. [edit to say this may relate to your braking balance issue] The relationship between rake and balance is also complex as the roll centre height at the rear is set by the wheel and tyre size, but less so at the front. As you change tyre sizes you will alter the balance of the car. The amount of rake you need will depend on how you use the car, but in general terms it is between 10 mm to 15 mm. But as said it will also be determined to a degree by tyre size.
In practice, it all usually starts with the preferred sump height and then worked backwards. But for track cars it may be better to run less ride height and that usually means a little less rake to get a good balance.
What I am really saying is that setting up the car is a holistic task. And there's no escaping lot of hard work, or getting some professional help from a Caterham expert.
Edited by DCL on Friday 11th August 21:28
Edited by DCL on Friday 11th August 21:31
analog_me said:
Other things being equal and the sump set at 68mm how would you say the behaviour of the car changes going from a minimum of 5 to 10 to a max 20mm rake?
Firstly this is my own opinion and not everyone will agree, but this is my understanding acquired through years of tracking a Caterham. The rake is the rear ride height minus the front ride height (at the chosen reference points). Changing it at the front is very different to changing at the rear as the geometry is very different. So there's no easy answer to your question without knowing the full geometry setup, but in generally terms it is accepted that increased rake will help reduce understeer. With 68 mm sump clearance (depending on which sump) you are on the 'road' side of setups and I suspect you should run plenty of rake to counter the 'understeery' front at that height.
But lets assume you have a perfectly balanced car i.e. the roll stiffness at each end is matched to the roll moment at each end. If you then raise the rear CoG, it will increase the roll moment at the rear. The chassis will then transfer weight from the inside rear to the outside front, both of which will (at least initially) reduce understeer/promote over-steer.
If you lower the front you will lower the CoG at the front, but more importantly, you will lower the roll centre MUCH more. So the effect is the same, but MUCH stronger.
Increasing the rake will also transfer more weight under braking as the CoG will be higher, but running a lot of rake may do the opposite as the anti squat geometry may pull the back down under braking. And on powerful cars, the anti-squat may tend to jack the car up and less rake can help improve traction on corner exits.
Edited by DCL on Saturday 12th August 11:11
Its not so simple as just fitting Nitrons. And thats not the dampers fault.
Nitrons are great as are other dampers but it is vital that spring rates match damping forces and as has been described above they can have different outcomes.
Making sure the wheel frequency is within the suitable window is the key from my perspective.
To that end you do need to know the weight of the car and its intended use. Assuming you know all other design criteria.
Nitrons are great as are other dampers but it is vital that spring rates match damping forces and as has been described above they can have different outcomes.
Making sure the wheel frequency is within the suitable window is the key from my perspective.
To that end you do need to know the weight of the car and its intended use. Assuming you know all other design criteria.
In my opinion this is the nature of the cars and you'll need to accept the ride/handling differences. An Elise (especially on Nitrons) will always handle a bumpy road better than a Caterham. Thats not to say the Caterham can't be improved a lot though. You may also be noticing an exaggerated effect because you're coming form a well-sorted Elise into a Caterham that potentially hasn't had any setup work.
I struggled with your brake lock-up issue since owning the car. The front would lockup way earlier than I felt it should, and it really dented my confidence in the car. I've now worked through various issues and the brakes are now amazing. The two changes that made the biggest difference were..
Geo setup and corner weighting from someone who knows Caterhams. My front left was especially light when I got the car. It would always lock-up first.
Secondly, following advice from Simon, I fitted the same compound brake pads front and rear to address the brake bias. Its common to fit lower friction pads on the rear but this can often be a mistake as it results in the back doing too little work.
I struggled with your brake lock-up issue since owning the car. The front would lockup way earlier than I felt it should, and it really dented my confidence in the car. I've now worked through various issues and the brakes are now amazing. The two changes that made the biggest difference were..
Geo setup and corner weighting from someone who knows Caterhams. My front left was especially light when I got the car. It would always lock-up first.
Secondly, following advice from Simon, I fitted the same compound brake pads front and rear to address the brake bias. Its common to fit lower friction pads on the rear but this can often be a mistake as it results in the back doing too little work.
Simon what's your experience of setting up a 620R, thinking of buying a new one an wondered what needs improving to get both good track and reasonable road behaviour. As they don't have adjustable dampers and are said to be very stiff I thought maybe changing the dampers for some 1 or 2 way adjustable as well as spring rates and geo/flat floor might help improve things?
Hi all,
Today I took the car into a garage with a perfectly flat floor and industrial finishing and did some measuring to test my geo.
I used 72kg ballast and my own bathroom scales and planks of wood of similar thickness.
I put the car on the planks of wood and rolled it until one wheel at the time was resting on the scales.
The results were better than perfect I think, and the figures were always coming up the same to the kilo :
LH RH
Front 132kg 131kg
Rear 187kg 145kg
Total 595kg full on fuel and liquids.
Clearance 67-68mm under the sump
Rake was 22mm with the bigger 205 tires at the rear.
Then i went home put my string lines around it and measured the geo;
Front toe 0.2mm toe out total or 0.02 degrees
Front camber -2.15 degrees.
Brakes are Rs14 all round, however tires are now in a BAD state 3years old and and very very close to the legal limit if not beyond.
I think so are the bilsteins...
So i think i only need some cash now to bring it to what i think is it's best state of tune.
Today I took the car into a garage with a perfectly flat floor and industrial finishing and did some measuring to test my geo.
I used 72kg ballast and my own bathroom scales and planks of wood of similar thickness.
I put the car on the planks of wood and rolled it until one wheel at the time was resting on the scales.
The results were better than perfect I think, and the figures were always coming up the same to the kilo :
LH RH
Front 132kg 131kg
Rear 187kg 145kg
Total 595kg full on fuel and liquids.
Clearance 67-68mm under the sump
Rake was 22mm with the bigger 205 tires at the rear.
Then i went home put my string lines around it and measured the geo;
Front toe 0.2mm toe out total or 0.02 degrees
Front camber -2.15 degrees.
Brakes are Rs14 all round, however tires are now in a BAD state 3years old and and very very close to the legal limit if not beyond.
I think so are the bilsteins...
So i think i only need some cash now to bring it to what i think is it's best state of tune.
I was a bit confused about your corner weights until I considered that your cars must be left-hand drive!
As you say, your car is well set up.
I would say it’s debatable whether RS14s are appropriate on the rear although if your front tyres lock up first and you aren’t experiencing any brake bias issues…
Tyres wear quite slowly on a Caterham in road use so if yours are low you can bet they’ve done a lot of work as well as being old. New tyres have to be first on the hit list then.
Improving your suspension may then be a case of ensuring that the springs are correct and your dampers are controlling them effectively.
As you say, your car is well set up.
I would say it’s debatable whether RS14s are appropriate on the rear although if your front tyres lock up first and you aren’t experiencing any brake bias issues…
Tyres wear quite slowly on a Caterham in road use so if yours are low you can bet they’ve done a lot of work as well as being old. New tyres have to be first on the hit list then.
Improving your suspension may then be a case of ensuring that the springs are correct and your dampers are controlling them effectively.
HustleRussell said:
Improving your suspension may then be a case of ensuring that the springs are correct and your dampers are controlling them effectively.
Yes LHD car.I agree with you regarding the springs. Do you know what means the writing on the springs ?
Front: 06/03 Caterham 1.61 VA
Rear: 21/01 Caterham 1.61 HA
My 2p
You may want to put those weights in a corner weight calculator such as
https://robrobinette.com/corner_weight_calc.htm
Although having the fronts the same may seem like a good idea (and does have it's applications), it will give a cross weight of 53% and that will be noticeable for the general handling. You have a difference of 40+ KG between the front/rear balance from left side to right side, and across the rear axle for that matter too. That will give a balance different between left and right cornering, and stability will be reduced.
IMHO it is better to move nearer 50% although favoring the front slightly may help braking lock-ups (ie keeping them within 20 KG or so)
The argument that fronts should be equal is flawed as the driver side of the car is heavier and means braking effort required to stop evenly is not that straight forward.
You may want to put those weights in a corner weight calculator such as
https://robrobinette.com/corner_weight_calc.htm
Although having the fronts the same may seem like a good idea (and does have it's applications), it will give a cross weight of 53% and that will be noticeable for the general handling. You have a difference of 40+ KG between the front/rear balance from left side to right side, and across the rear axle for that matter too. That will give a balance different between left and right cornering, and stability will be reduced.
IMHO it is better to move nearer 50% although favoring the front slightly may help braking lock-ups (ie keeping them within 20 KG or so)
The argument that fronts should be equal is flawed as the driver side of the car is heavier and means braking effort required to stop evenly is not that straight forward.
Edited by DCL on Friday 18th August 14:41
MikeE
I have a 620R with upgraded dampers from Simon.
I have the Penske 3 way adjustable with 250lb springs front and 150lb rear (as opposed to 350lb standard).
I had it flat floored by Andy Jupp at PGM when the dampers were swopped over.
The car is much improved IMHO and the ride feels a lot smoother now although I'm still getting to grips with all the adjustments and I'm not sure I have it quite right just yet.
I have a 620R with upgraded dampers from Simon.
I have the Penske 3 way adjustable with 250lb springs front and 150lb rear (as opposed to 350lb standard).
I had it flat floored by Andy Jupp at PGM when the dampers were swopped over.
The car is much improved IMHO and the ride feels a lot smoother now although I'm still getting to grips with all the adjustments and I'm not sure I have it quite right just yet.
DCL said:
My 2p
You may want to put those weights in a corner weight calculator such as
https://robrobinette.com/corner_weight_calc.htm
Although having the fronts the same may seem like a good idea (and does have it's applications), it will give a cross weight of 53% and that will be noticeable for the general handling. You have a difference of 40+ KG between the front/rear balance from left side to right side, and across the rear axle for that matter too. That will give a balance different between left and right cornering, and stability will be reduced.
IMHO it is better to move nearer 50% although favoring the front slightly may help braking lock-ups (ie keeping them within 20 KG or so)
The argument that fronts should be equal is flawed as the driver side of the car is heavier and means braking effort required to stop evenly is not that straight forward.
Hi David and thanks for you input.You may want to put those weights in a corner weight calculator such as
https://robrobinette.com/corner_weight_calc.htm
Although having the fronts the same may seem like a good idea (and does have it's applications), it will give a cross weight of 53% and that will be noticeable for the general handling. You have a difference of 40+ KG between the front/rear balance from left side to right side, and across the rear axle for that matter too. That will give a balance different between left and right cornering, and stability will be reduced.
IMHO it is better to move nearer 50% although favoring the front slightly may help braking lock-ups (ie keeping them within 20 KG or so)
The argument that fronts should be equal is flawed as the driver side of the car is heavier and means braking effort required to stop evenly is not that straight forward.
Edited by DCL on Friday 18th August 14:41
I did have my elise set up with the diagonal weights equal but feel the same is not so beneficial with the dedion rear axle of the caterham. Having said that i am far from a race driver or with such experience. I just enjoy it when it all works to my standards of perfection and enjoy tuning it to reach this point. But have little experience in comparison to the UK caterham race team standards.
As you mentioned with the current setting i do feel the lighter right side and the wheel spins (or lifts) quite often on tight right hand side corners.
I just thought since i am not after best times breaking consistancy might be more important but now i tend to believe there is a compromise between the two.
Most probably it is not optimal to equalise the front to the last kilo and loose out on the diagonals.
How much difference would you allow the front to have so not to effect the breaking.
Are you suggesting than 50% on the diagonal with <20kg different left to right on the front is a better compromise?
Is there an "ideal" "perfect" weight distribution to try?
May be i should try Simon Roger's standard (no scales) setting and see what figures this gives so to get some more experience and feel on the car's handling.
If you have access to a good flat floor, then I'd set it up as per Simon's instruction and it won't be far off, as correct ride heights will generate correct cross weights (just less accurate that way).
Personally, if you can measure corner weights then I'd go a little closer to 50% cross weights. That usually keeps the fronts on my car within 20 Kg of each other. But you're not far off, and if equal fronts work for you, then that's all that matters, and it's not a big deal.
Personally, if you can measure corner weights then I'd go a little closer to 50% cross weights. That usually keeps the fronts on my car within 20 Kg of each other. But you're not far off, and if equal fronts work for you, then that's all that matters, and it's not a big deal.
Edited by DCL on Friday 18th August 21:14
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