Race chassis

[F5 Dave]

Hi, I have a question regarding tires in bikes, we race here mostly with 18" rims (classic bikes) and is there any rule of thumb how to choose tire for a bikes? For example 90-110 kg and 30-40 of hp? Is there a benefit to use for example rear 120 tire instead of 90? Any info or guidelines would be appreciated. Cheers

Always build your racebike from the tyres up. Choose the best tyres available within the rules, then go from there.
I used to run 90 section front slicks on the back of my 50. A lot of us did. But it became apparent that around a kart track at least, those with 115 could lean on them harder without falling off and were faster. I converted and benefited.

[Grumph]

Hi, I have a question regarding tires in bikes, we race here mostly with 18" rims (classic bikes) and is there any rule of thumb how to choose tire for a bikes? For example 90-110 kg and 30-40 of hp? Is there a benefit to use for example rear 120 tire instead of 90? Any info or guidelines would be appreciated. Cheers

Your weight and power figures are right where my Aermacchi replicas sit.
Most customers have used 90/90 fronts and 100/90 rears - occasionally 120 section - and been quite happy with that.
Most Classic rules specify rim widths - usually WM2 or 3 - which will limit your choice of sizes anyway.

One customer was able to buy Avons left from a test session where Schwantz evaluated a huge number of tyres using a 500 manx Norton.
The pair he's got were deemed too soft for a 500 - they're perfect for a 350 macchi.

[husaberg]

Not sure that makes perfect sense. But I'll just point out that the XB swingarm I posted earlier was mounted on the engine, which was isolated from the frame via rubber mounts and some tricky wee tie rods.

I wonder if that's why, with no linkage and nothing special by way of a rear shock that back end performs remarkably well....

As were Norton Commandos
Plenty of bikes including the RS125 NF4 the Ducati supermono have no linkages.

Husasberg and KTM dirt bikes They have a PDS shock though

Most GP race bikes went to linear rate linkages anyway, as far as i know.
Properly set up with decent dampers a twin shock can also be excellent, ALA Norton rotary race bike, But fashion dictates monoshock must be better.

[Grumph]

I wouldn't be so quick to use the Norton rotaries as an example of good twinshock practise.

AFAIK, they went twinshock because they reckoned the single damper was overheating behind the rotary. Then went back to single...

Peter Williams is reported to have laughed and said "they haven't learned anything in 20 years"

[husaberg]

I wouldn't be so quick to use the Norton rotaries as an example of good twinshock practise.

AFAIK, they went twinshock because they reckoned the single damper was overheating behind the rotary. Then went back to single...

Peter Williams is reported to have laughed and said "they haven't learned anything in 20 years"

I thought it was because Colin Seeley wanted them.
Ken McIntosh wasn't convinced that mono were better.
Those moriwaki Honda CBX750 and NS250 went well with twin shocks.

Mounted through and under the swingarm.
Can't find a pic of the 250....

[Ocean1]

As were Norton Commandos

Swingarm mounted on isolated engine you mean?

Plenty of bikes including the RS125 NF4 the Ducati supermono have no linkages.
Husasberg and KTM dirt bikes They have a PDS shock though
Most GP race bikes went to linear rate linkages anyway, as far as i know.
Properly set up with decent dampers a twin shock can also be excellent, ALA Norton rotary race bike, But fashion dictates monoshock must be better.

Aye. None of my current rides have linkages. No real preference, just coincidence. On the other hand I don't do much serious vertical shit nowadays.

One of the more practical negatives with linkages is you've got more bushes/bearings to wear, which causes a lot more hysteresis in the system. So unless you're really on top of your maintenance I doubt a casual rider would benefit. And I've rebuilt my share of completely fucked linkages that owners thought "might need a bearing" but were still considered to "feel good".

It's also quite possible to get some progressive travel without a linkage, if a bit tricky.

What I was really interested in was the observation that controlled lateral movement was the hot mumble, that the Buell system seemed to address that and seemed, (to a rank amateur) to perform accordingly.

And yes, the last thing you could want as a design engineer is a Dedicate Follower of Fashion...

[husaberg]

Swingarm mounted on isolated engine you mean?...

yip

And yes, the last thing you could want as a design engineer is a Dedicate Follower of Fashion...

There is the rub if you are a designer if what you design isn't considered fashionable you might not have a job. uless its for a museum
I admire clever stuff the most, thinks that works great and is also pretty to look at. as a whole complete form

I could spend hours looking at the details on a works Ducati race bike or a 1950's Ferrari.

[Ocean1]

yip

Wonder if carefully though out rubber swingarm bushes might not be a better idea. Far lower maintenance and real easy to change for tuning.

There is the rub if you are a designer if what you design isn't considered fashionable you might not have a job.

My old man was a product design engineer. He always said there was nothing more redundant than a successful development engineer.

The other thing about competition design is that the real objective isn't just to win, it's to win using bikes that look like next years production model. And the accountants have a lot more influence on that second bit than any engineer does on the first bit.

[Frits Overmars]

Properly set up with decent dampers a twin shock can also be excellent.

Until you fit pressurized twin shocks in the old-fashioned position near the rear axle; the gas pressure alone already gives too much preload.

It's also quite possible to get some progressive travel without a linkage.

Yep, you can achieve up to 15% progressiveness that way, which is enough for road racing.
Omitting the linkage saves weight and components, and avoids wear and extreme forces acting on the frame.

What I was really interested in was the observation that controlled lateral movement was the hot mumble

Try searching for "Suspension Smith". I haven't got a link handy but I will post it when I find it.

[husaberg]

Wonder if carefully though out rubber swingarm bushes might not be a better idea. Far lower maintenance and real easy to change for tuning.



My old man was a product design engineer. He always said there was nothing more redundant than a successful development engineer.

The other thing about competition design is that the real objective isn't just to win, it's to win using bikes that look like next years production model. And the accountants have a lot more influence on that second bit than any engineer does on the first bit.

Not only that but the sales guys have a lot of influence hence Honda pro-arm on a NSR250R and NSR500V.
On the other side KTM with its mainly black bikes with steel frames and non linkages being made in odd displacements made themselves a huge niche.

What I was really interested in was the observation that controlled lateral movement was the hot mumble

Sorry i missed that. i think its only an issue of bikes and demi gods at the limit on very very sticky tyres at big lean angles.
its all explained in the links i posted.
https://www.kiwibiker.co.nz/forums/s...post1131044358

[Ocean1]

Yep, you can achieve up to 15% progressiveness that way, which is enough for road racing.
Omitting the linkage saves weight and components, and avoids wear and extreme forces acting on the frame.

Try searching for "Suspension Smith". I haven't got a link handy but I will post it when I find it.

I will. In the meantime I found this interesting...

http://dirtbikemagazine.com/ktm-susp...ds-vs-linkage/

[Ocean1]

Sorry i missed that. i think its only an issue of bikes and demi gods at the limit on very very sticky tyres at big lean angles.
its all explained in the links i posted.
https://www.kiwibiker.co.nz/forums/s...post1131044358

So I saw, hence the comment.

And yes, my assessment is necessarily that of an amateur. Nonetheless my experience with those Buells is that they seem to offer better feedback than conventionally mounted rears.

[husaberg]

So I saw, hence the comment.

And yes, my assessment is necessarily that of an amateur. Nonetheless my experience with those Buells is that they seem to offer better feedback than conventionally mounted rears.

I am a PRO, Googler

[Frits Overmars]

I am a PRO Googler

Huh? You google, dressed in miniskirt and high heels? Does it help?

[Michael Moore]

A certain amount of rear suspension design is dictated by packaging/mass centralization concerns. If there's a more important component that gets a higher priority then the suspension can be relocated, and with linkages many different configurations can give very similar suspension curves. Having linkages may be of more use on long travel dirt bikes or heavy tourers where you want more progression than a RR bike might need.

Yep, you can achieve up to 15% progressiveness that way, which is enough for road racing.
Omitting the linkage saves weight and components, and avoids wear and extreme forces acting on the frame.

The person who got me started building frames remarked that if the subframe was strong enough to support the rider it was probably strong enough for twin dampers located in the conventional position. A conventional swing arm can be just stiff enough for the loads coming up through the tires and the rest of the chassis and not need to be hugely stiffer because of large bending loads being put in by a heavily leveraged suspension.