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Inboard brake
Inboard brake
Inboard brake
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The McLaren M23's rear brake calipers nestle between the universal joints and the transaxle, with their calipers mounted directly to the transaxle housing

An inboard brake is an automobile brake mounted on the chassis of the vehicle, rather than directly on a wheel hub. It began with drum brakes and evolved to disks, and creates a reduction in the unsprung mass (by relocating the brake discs and calipers from the wheel hubs to rear drive components). It also improves brake performance by applying its force directly to the chassis, rather than being transferred to it through the suspension arms.[1]

Description

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Inboard brakes began as drum brakes fitted to drive components in the rear of a vehicle (such as a drive shaft, or directly to a differential or transaxle). Originally a racing feature, they also appeared on such high-performance sports cars as the iconic Mercedes-Benz 300 SL "Gullwing". With the introduction of disc brakes they were found inboard on some racers and high-end sports cars and grand turismos (notably the Jaguar E-type). Most applications have been on rear-wheel drive vehicles, although they also have been used in four-wheel drive and some front-wheel drives.[citation needed] A rare few rear wheel drive racing cars (like the Lotus 72) have also used inboard front discs, requiring a front brake shaft to be added to gain the overall unsprung mass and braking torque advantages.

Alfa Romeo 75 rear transaxle subassembly

Excepting the case of vehicles with beam axles and vehicles having no suspension, in practice it is normal for inboard brakes to be mounted rigidly with respect to the body of the vehicle, often to the differential casing. This is done to move the weight of the braking mechanism from being carried by the wheels directly as unsprung mass, to being carried indirectly by the wheels via the suspension as sprung mass. This then necessitates a means of transferring braking torque from the brake mechanism to the wheel, which is capable of operating despite the relative movement between body and wheel. Driven wheels already have shafting which serve this purpose so there is no penalty for them, but undriven wheels require adding their own brake shaft.

The benefit of such a system is primarily the reduction of unsprung mass, which improves handling and ride. Mechanically, the suspension does not have to resist twisting when the brakes are applied. The wheels don't enclose the brake mechanism allowing greater flexibility in wheel offset, and placement of suspension members. It is also much easier to protect the brake mechanism from the outside environment, and from water, dust, and oil. In addition, flexible brake lines may be replaced by rigid lines, allowing increases in brake fluid pressure, and larger disks - with greater cooling - may be fitted than within the constraints of a wheel.

The mechanical disadvantages are largely those of added complexity, and poorer cooling, resulting in brake fade and potentially requiring special ducting. Undriven wheels also require their own brake shaft. Inboard brakes also affect anti-pitch suspension geometry.

There can be practical difficulties in servicing the brake mechanism. Instead of simply removing a wheel to renew pads and discs, the entire rear of a vehicle may need to be jacked up, or the car put on a lift. Additionally renewing brake discs can require dismantling the half axle. This greatly discourages their use in motorsport, and the additional time makes for greater labour cost in servicing.

This system was more common in the 1960s, found even on the Citroën 2CV. The Hummer H1 is one of the few modern vehicles fitted with inboard brakes, to accommodate each wheel's portal gear system.

Hybrid electric vehicles may be considered to have partial inboard braking, as the motor–generator(s) used for regenerative braking are usually mounted inboard.

Examples

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Cars with inboard brakes at the driven end include:

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Inboard brake

View on Grokipedia
from Grokipedia
An inboard brake is an automotive braking system wherein the brake discs and calipers are mounted on the vehicle's chassis, typically adjacent to the differential or within the drivetrain on driven axles, rather than directly on the wheel hubs. This design applies braking torque directly to the sprung mass of the vehicle via driveshafts, distinguishing it from conventional outboard brakes that rotate with the wheels. Originally developed with drum brakes and later adapted for disc configurations, inboard systems gained prominence in the mid-20th century for their ability to minimize unsprung weight, enhancing suspension response and overall handling. Notable examples include the , which utilized inboard rear disc brakes as part of its innovative independent rear suspension introduced in 1961, and the , featuring inboard front drum brakes to support its lightweight, long-travel suspension. These applications were particularly favored in European sports cars and economy vehicles during the and , where reducing rotational mass improved efficiency and performance without compromising simplicity. Key advantages of inboard brakes include a significant reduction in unsprung mass, leading to better ride quality, improved , and protection from environmental contaminants like dust and water. They also allow for larger brake components and enhanced heat dissipation in controlled environments, making them suitable for high-performance or applications. However, drawbacks involve poorer cooling for rear brakes due to limited airflow, increased system complexity requiring robust driveshafts, and more challenging maintenance access, which often necessitates partial disassembly. In contemporary vehicles, inboard brakes remain niche but are seeing revival in electric models; for instance, the employed them across both axles, and announced its "In-Drive" system in 2024, which integrates brakes within the under development for optimized and near-lifelong durability as of 2025.

Overview

Definition

An inboard brake is a braking system in which the brake components, such as discs and , are mounted near the vehicle's centerline, typically on the , differential housing, or a dedicated brake shaft, rather than at the wheel hubs. The braking force is transferred to the wheels through intermediate elements like driveshafts or half-shafts. A primary characteristic of inboard brakes is the relocation of heavy brake elements from the rotating assemblies to stationary parts of the or , which significantly reduces the unsprung mass of the . This design choice minimizes the weight that must accelerate and decelerate with the wheel's unsuspended motion, potentially improving handling and ride quality. In contrast to outboard brakes, which are directly mounted on the hubs for immediate actuation of the rotating , inboard brakes position the braking mechanism centrally within the structure. This structural difference allows inboard systems to avoid integrating brake hardware into the 's rotating components.

History

Earlier examples include inboard drum brakes, such as those on the front of the Citroën 2CV starting in 1948, which helped reduce unsprung mass in lightweight designs. Inboard brakes emerged during the 1950s and 1960s as part of broader advancements in disc brake technology, transitioning from earlier drum systems mounted on drive components. British automaker Jaguar played a pioneering role, introducing inboard rear disc brakes on production vehicles with the 1961 E-Type, which featured four-wheel disc brakes as standard. This design was popularized in British sports cars like the Jaguar E-Type and subsequent models for performance advantages, including reduced unsprung mass in the suspension. The technology reached peak usage from the 1960s through the 1980s, becoming widespread in high-end production vehicles such as Jaguar's XJ series (from 1968) and XJ-S (1975–1993), where it complemented independent rear suspension systems. In racing, inboard brakes were notably adopted in Formula 1 prototypes during the 1960s, with teams like BRM employing a single inboard rear on the P48 model (1959–1960) to suit rear-engine layouts and optimize weight distribution. Lotus continued this trend into the 1970s with inboard setups on cars like the Type 72, enhancing handling in competitive environments. By the , inboard brakes began to decline in favor of outboard designs, driven by improvements in processes that prioritized simplicity and integration with evolving suspension technologies. Jaguar transitioned away from inboard rears starting with the XJ40 in 1986 and fully on the XJ-S by mid-1993. Today, inboard brakes are rare in production automobiles but persist in niche racing applications where unsprung mass reduction remains critical.

Design and Operation

Key Components

Inboard brake systems primarily consist of brake rotors and mounted centrally on the vehicle's , typically near the differential housing or transmission output shaft, rather than at the wheel hubs. The brake rotors, often larger in diameter than equivalent outboard rotors for enhanced heat dissipation, are fixed to the differential or gearbox shaft to rotate with the . Adjacent to these, the brake , which house pistons and pads, are secured to the via specialized mounting brackets or hubs that minimize vibration and ensure stable positioning. These apply clamping force to the rotors through hydraulic pressure supplied by lines connected to the . Force transmission from the inboard braking elements to the wheels relies on drive shafts or half-shafts, which link the central rotors to the hubs on driven axles. These shafts, often splined for secure connection to a or spool, allow the braking torque to act on both wheels simultaneously in rear-axle setups. Hydraulic actuators, integrated directly into the framework, include the and fluid reservoir to generate and distribute efficiently to the . Mounting brackets further support this integration, bolting the firmly to the while accommodating the components like the propshaft in rear configurations. Variations in inboard designs include disc-based systems, as described above, and historical drum configurations where brake drums encase shoes mounted on the driveshaft or differential for friction application. Rear-axle implementations are more prevalent due to easier access via the driveshaft, whereas front-axle setups are rarer and typically require additional shaft adaptations. Support elements such as cooling ducts may be incorporated around the rotors in high-performance variants to aid heat management, though they are not universal.

Mechanism of Operation

Inboard brakes operate through a hydraulic system where generated by the , in response to the driver's pedal input, is transmitted via lines to the inboard-mounted . This forces the caliper pistons (with fixed to the ) to clamp the brake pads against the rotating discs, which are mounted on the components, creating frictional resistance that generates braking on the rotating , rather than directly on the hubs. This braking is transferred from the rotating discs through half-shafts or drive shafts connected to the hubs, and in differential setups, distributed to both on the . To accommodate suspension articulation and steering movements, these shafts incorporate universal joints or constant velocity (CV) joints, ensuring smooth transmission without binding. Compared to outboard brakes, where act directly on wheel-mounted rotors, inboard systems feature a longer force path from the to the wheels via the shafts, which can introduce slightly higher rotational due to the in the . However, this centralized actuation allows for more compact brake placement near the differential, facilitating easier integration with suspension components and potentially improved heat dissipation.

Advantages and Disadvantages

Advantages

One primary advantage of inboard brakes is the significant reduction in unsprung mass, as the heavy components such as rotors and —typically comprising a substantial portion of wheel-end weight—are relocated from the wheel hubs to the vehicle's or differential, converting them to sprung mass. This relocation enhances suspension responsiveness by allowing the wheels to react more quickly to irregularities, resulting in improved ride quality and greater contact with the surface for better traction. In high-performance applications, such as electric vehicles, this can yield total unsprung mass savings of approximately 91 kg across the vehicle, though typical reductions in sports cars are on the order of several kilograms per depending on brake size. Inboard brakes also provide handling enhancements through optimized weight distribution and minimized dynamic loads on the suspension during braking. By applying braking forces directly to the chassis rather than through suspension arms, they reduce wheel hop and improve overall vehicle stability, particularly at high speeds or during sudden maneuvers. This configuration centralizes mass closer to the vehicle's center of gravity, lowering the moment of inertia and enabling more precise control, which is especially beneficial in performance-oriented designs. Additionally, the setup facilitates larger brake sizes without compromising wheel clearance or increasing rotating mass at the wheels, further boosting braking efficiency and modulation. From a perspective, inboard offer greater flexibility for integration with advanced suspension systems, such as independent setups common in modern vehicles. This mounting approach simplifies packaging around wheel arches and allows for innovative features like active braking controls in applications, where space constraints and demands are acute. The shaft-based transmission of braking in these systems also supports seamless adaptation to driven axles without adding complexity to wheel-end components.

Disadvantages

One significant limitation of inboard brakes is their cooling challenges, stemming from the reduced to the brake components due to their placement near the vehicle's or differential rather than at the wheels. This inboard location often results in higher operating temperatures, increasing the risk of during prolonged or intense use, particularly in rear applications where aerodynamic effects are limited. For instance, in classic models with inboard rear brakes, the confined underbody space contributes to notoriously poor cooling, necessitating auxiliary ducting or upgrades to vented rotors to dissipate heat effectively and prevent performance degradation. Inboard brake systems also introduce increased mechanical complexity compared to traditional outboard designs, as they require additional drive shafts, universal joints, and linkages to transmit braking from the chassis-mounted to the wheels. These elements create potential failure points, such as shaft misalignment or under load, and demand stronger components to handle the extended torque path, which can shift and complicate overall . Maintenance is further hindered by the need for precise alignment and the difficulty in accessing components without major disassembly, like lowering the rear differential. From a practical standpoint, inboard brakes incur higher and service costs owing to their intricate and specialized parts, making them less economical for mass-produced . Pad replacement and are notably more labor-intensive than with wheel-mounted systems, often requiring elevated tools or suspension removal, which elevates ownership expenses and discourages widespread adoption in modern automobiles. Additionally, the proximity to the raises risks of , where brake-generated or particulates can affect nearby seals and components, potentially leading to premature wear in the differential assembly.

Applications and Examples

Automotive Applications

Inboard brakes find their primary application on the rear axles of and luxury cars, where they contribute to handling optimization by minimizing unsprung mass and allowing for larger brake components without compromising suspension geometry. This configuration enhances ride quality and tire contact under dynamic loads, particularly in vehicles prioritizing performance over everyday utility. Front axle implementations remain uncommon due to the added complexity of integrating them with mechanisms, which requires specialized constant-velocity joints and increases rotational during turns. In racing contexts, inboard brakes were employed in 1970s Formula 1 cars to centralize mass and improve aerodynamic efficiency, with designs like those from Lotus exemplifying their role in weight tuning and polar moment reduction. They also appeared in various racing setups, where reducing unsprung weight at the wheels aids in quicker suspension response and better traction, though challenges with cooling and concerns (such as driveshaft failures) led to a preference for outboard systems. Today, their use persists in niche areas such as vintage racing restorations and custom performance builds, often via specialized kits that adapt them for contemporary . Beyond passenger vehicles, inboard brakes serve non-passenger applications in heavy-duty trucks and off-road vehicles, particularly those with solid rear axles, enabling centralized braking that protects components from environmental debris and facilitates easier maintenance in rugged conditions. This setup is advantageous for fleet operations requiring durability under high loads and varied terrain. Overall, while inboard brakes have become rare in post-2000 production vehicles due to advancements in lightweight outboard discs and packaging constraints, recent revivals in electric vehicles demonstrate their continued viability through aftermarket performance upgrade kits tailored for enthusiasts seeking optimized dynamics.

Notable Vehicle Examples

The , produced from 1961 to 1975, featured rear inboard disc brakes as part of its advanced independent rear suspension system, which helped reduce unsprung weight for improved handling and ride quality in this iconic 1960s . This design, derived from Jaguar's racing heritage, contributed to the car's balanced performance and high-speed stability during its era. The Jaguar XJ-S, manufactured from 1975 to 1996, utilized rear inboard disc brakes in its early models to complement the independent rear suspension and optimize weight distribution in this luxury . This setup provided benefits in suspension articulation and reduced rotational , aligning with 1970s-1990s priorities for refinement and , though it was later switched to outboard brakes in 1993 for simpler maintenance. The Citroën 2CV, produced from 1948 to 1990, featured inboard front drum brakes to support its lightweight, long-travel suspension, aiding efficiency in this economy vehicle. The , produced from 1992 to 2006, employed inboard brakes across both axles, providing protection from off-road debris and contributing to its rugged durability. In modern applications, introduced the "in-drive" braking system in 2024, integrating brakes within the for optimized weight distribution and enhanced durability in electric vehicles. In niche and aftermarket contexts, inboard brake kits from Wilwood Engineering have been adapted for Chevrolet and kit cars, offering upgraded conversions that mount calipers closer to the vehicle's centerline to minimize unsprung weight in custom or performance builds. These kits cater to enthusiasts seeking era-specific benefits like enhanced suspension response in restored classics or lightweight racers.

References

  1. https://www.irjmets.com/uploadedfiles/paper//issue_4_april_2024/51809/final/fin_irjmets1712142898.pdf
  2. https://www.irjet.net/archives/V10/i4/IRJET-V10I4229.pdf
  3. https://macsmotorcitygarage.com/video-engineering-the-jaguar-e-type/
  4. https://blog.garritys.org/2009/09/inboard-brakes.html
  5. https://www.carthrottle.com/features/inboard-brakes-what-they-are-and-how-they-work
  6. https://www.greencarreports.com/news/1145099_mercedes-in-drive-moves-braking-system-inside-ev-s-electric-motor
  7. https://media.mbusa.com/releases/pioneering-innovations-for-the-car-of-the-future-mercedes-benz-provides-exclusive-insights-into-research-activities-and-future-technologies
  8. https://automotivedictionary.org/inboard_brakes
  9. https://www.irjet.net/archives/V5/i5/IRJET-V5I5960.pdf
  10. https://www.collinsdictionary.com/us/dictionary/english/inboard-brakes
  11. https://www.jcna.com/jaguar-model-guides-e-type
  12. https://www.motortrend.com/features/c12-0601-jaguar-e-type-series
  13. https://www.jagbits.com/jaguarxj6.html
  14. https://www.ultimatecarpage.com/car/3030/BRM-P48.html
  15. https://medium.com/formula-one-forever/the-most-important-car-in-formula-1-history-lotus-72-64237441d098
  16. https://www.ijsrd.com/articles/IJSRDV6I20499.pdf
  17. https://www.sae.org/periodicals/mercedes-proposes-drive-brakes-better-evs-sae-ma-07506
  18. https://www.jag-lovers.org/xj-s/book/InboardBrakeUpgrade.html
  19. https://engineering.stackexchange.com/questions/14528/why-are-automotive-brakes-set-inside-the-wheels-and-not-on-the-axles
  20. https://www.goodwood.com/grr/f1/the-nine-best-f1-cars-of-all-time/
  21. https://www.wilwood.com/BrakeKits/BrakeKitSearch?wid=3&appid=2
  22. https://www.hemmings.com/stories/jaquar-e-type-1961-1967/
  23. https://www.hemmings.com/stories/buyers-guide-1961-1970-jaguar-e-type-series-1-and-2/
  24. https://www.hemmings.com/stories/jaguars-posh-affordable-xj-s/
  25. https://www.wilwood.com/Corvette/Index
  26. https://www.wilwood.com/BrakeKits/BrakeKitLanding
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